Efficient XML Interchange Measurements Note

W3C Working Draft 18 July 2006

This version:
Latest version:
Greg White, Stanford University
Don Brutzman, Web3D Consortium
Stephen Williams, High Performance Technologies, Inc.


This Working Group Note presents the first measurement results of various high-performance XML interchange encoding formats and their associated processors, made by the Efficient XML Interchange (EXI) Working Group. The measurements have been conducted following the recommendations of the XML Binary Characterization (XBC) Working Group. In particular, this draft covers measurements of the properties of "compactness" and "processing efficiency", as defined by the XBC WG. We start by describing the context in which this analysis is being made, and the position of an efficient format in the landscape of high performance XML strategies. Then we describe the measured quantities in detail and the test framework in which they were made. A short description of each format is included. Since the measurement and analysis effort is still in progress, for this first draft the raw results and preliminary findings are given elsewhere (see respectively the EXI Measurements Results Preview and the Analysis of the EXI Measurements).

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This First Public Working Draft presents initial measurement results of the EXI Working Group's tests of various high performance XML interchange encoding formats. This draft concentrates on the properties of "compactness" and "processing efficiency". Some formats and alternative technologies to which we wish to compare results, are not yet in the measurement framework (notably ASN.1 PER, and XML Screamer [Screamer], though these are expected very soon). Additionally, the preliminary results are being used to further optimize some candidates. Therefore, results given in the Analysis of the EXI Measurements are subject to change in the immediate future. Following drafts of this note will add further analysis of the results and successively address more of the property requirements, as determined by the XML Binary Characterization Working Group. While compactness will not generally vary from implementation to implementation, processing efficiency may vary widely between implementations of a given format. The current framework characterizes processing efficiency of specific implementations, rather than attempting to evaluate any nominal theoretical processing efficiency limit of each format itself. In addition, we have tabulated the W3C requirements met by each format (as identified in the XBC Characterization document), though each format's development is ongoing and therefore those results, and possible implications for performance, are likely to change. For these reasons, the dynamical results and analysis are given elsewhere, see the EXI Measurements Results Preview and the Analysis of the EXI Measurements).

This document was developed by the Efficient XML Interchange (EXI) Working Group. A complete list of changes to this document is available.

Comments on this document are invited and are to be sent to the public public-exi@w3.org mailing list (public archive). If substantive comments are received, the Working Group may revise this Working Group Note.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

1. Objectives

The objective of this document is to provide an analysis of the expected performance characteristics of a potential "Efficient XML Interchange" (EXI) encoding format. A successful EXI format will include facilities for helping computers encode and decode the entities of XML documents efficiently and in a compact form. The purpose of such a format would be to enable the use of tools and processing models in the current XML technology stack, in environments where the costs of producing, exchanging, and consuming XML are currently high or prohibitive. Additionally, such a format may well enable an expansion of the use of web technologies to new applications or industries, which presently find some facilities of XML attractive, but are limited by some hard constraints on encoded length or some aspect of computational efficiency.

This first-draft Note presents measurement data for the processing and compaction performance properties of a number of existing alternative XML encoding format technologies. These formats are considered representative "proxies" of an eventual EXI technology. One or more of these formats may be selected as the basis of a standard EXI format. Successive drafts will add a structured analysis by use case, measurements of other candidates, and a more comprehensive quantitative analysis of other properties, as outlined by the XML Binary Characterization (XBC) Working Group's document XML Binary Characterization Properties [XBC properties]. Based on these efforts, a single format may be proposed to become a Recommendation of the W3C.

2. Methodological Context

The Extensible Markup Language (XML), is an application profile of Standard Generalized Markup Language (SGML). It provides a well defined syntax and encoding by which structured, textual, data can be examined by people, and exchanged and interpreted by computers. The well defined basis of XML, and its simple syntax that permits ready semantic interpretation, has resulted in a very broad range of successful uses. There are a number of use cases for which these advantages are tempered by inefficiencies which stem from the textual encoding of XML. These cases, for instance, require a compact form, such as in small portable devices like mobile phones, or an encoding which avoids the time spent on floating point conversion, such as in scientific and engineering fields. Many use cases, and the properties of XML to which they are sensitive, were documented extensively by the XML Binary Characterization Working Group, in [XBC Use Cases] and [XBC properties].

The XBC WG recommended that work on an Efficient XML Interchange format proceed, since alternative formats which appeared to meet some or all of the property criteria already existed. However, the XBC WG did not perform a quantitative analysis of those formats, nor define measurable thresholds of performance which should be achieved for each use case and property. This Note provides measurement data regarding some of the quantitative Properties of XML relevant to the creation of an efficient format for the interchange of XML, including its encoding and processing. In general, we do not here review the simple-valued properties of each format, such as whether they are "Royalty Free".

This draft concentrates on the two most critical properties of XML for a potential efficient interchange format: "compactness" and "processing efficiency". These are important in that they are both significant drawbacks with existing XML solutions in many use cases, and they are difficult to accurately model. Subsequent drafts will address more of the XBC property requirements.

In order to characterize the performance of the candidates with respect to these properties, the EXI working group has assembled a significantly larger set of test data than was collected by the XBC. The group has also created a testing framework, based on Japex, which enables efficient testing of different candidate formats and implementations against a variety of different XML data. The wide set of test data is intended to highlight any binary-encoding format which achieves good results at the expense of being narrowly focused, and to help determine the generality of each format. The inclusion of many candidate algorithms improves the likelihood of identifying superior solutions which might ultimately inform an ideal single-format EXI recommendation.

Results are included for each of these candidates versus common XML solutions. Since processing efficiency is a measure of both format and implementation combined, this effort will include at least two different, already existing, high-performance XML parsers to obtain baseline measurements of XML processing efficiency. Presently we have xals, and XML Screamer will be integrated shortly. The EXI group has also issued a public call for additional fast XML implementations, in order to avoid any possibility of drawing misleading conclusions.

3. High Performance XML Strategies

Submitted: May, PH. Integrated: 23-May, GW. Edited: 27-May, GW: Collapsed intent from OG's viability arg and Peter's HP XML parsing/screamer arg, into smaller space. June 4: SW: rewrote introduction, edited, included condensed thoughts from PH's note.

Editorial note: The group feels that this section is important, but there is not yet consensus as to appropriate content, points, and form.

XML standards and related industry practice provide numerous benefits. This has been very valuable, but there is significant consensus that XML can be improved in several ways, mostly in terms of efficiency. Less clear to some is whether the cost of various improvement strategies outweighs the cost of change required to support strategies that involve a new encoding format. Additionally, existing XML processing implementations are often not optimal. These points of view lead to the main high performance XML strategy dichotomy: improve parsing technology for existing XML encoding or design an improved format that is more efficient overall.

There are numerous parsers of various types available for XML. A representative member of commonly used parsers is being used for comparison to possible EXI candidates. The XBC working group determined a comprehensive set of properties that are desirable in an EXI candidate. A faster parser technology might address some of these, but certainly not others, principally compactness. The EXI working group made a public request for fast XML parser implementations. The only example submitted was [XML Screamer]. XML Screamer is a proof-of-concept project that makes use of generated and compiled special-purpose parsers that are created for each vocabulary to be exchanged. This architecture does not fit all of the XBC-identified use cases, however reported results are instructive to both the evaluation of EXI candidates and for the EXI viability decision. Other methods exist for improving processing stacks that are centered around the separation of roles, or separation of metadata from data, where the data which must be efficiently encoded is moved out of line (SOAP with MTOM and XML-binary Optimized Packaging [XOP], GRID's Web Services Resource Framework [WSRF], etc). Another class of techniques is that of collapsing the protocol stack to remove inefficiencies at their boundaries, to bind them more efficiently, and to use more than one basic optimization method (bSOAP, XML Screamer). Another, quasi-solution, is to wait until the prevailing cost of the required network or computing speed makes a particular use case practical with XML.

A number of efforts used in various XML communities address the problem by optimizing the format itself. There are many options in the design of an encoding format that can be compatible with and still improve upon XML. Some existing efforts retain the basic textual XML encoding but use the facility of XML Schema's base64Binary type to encode binary insertions, some replace the whole bit-stream. In all cases the encoding process can use knowledge about the data, either explicitly via a schema, or implicitly by using assumptions inherent in the use case. As described in XBC Working Group documents, different methods externalize various amounts of information in various ways. They encode the data either more compactly, or in a way which avoids expensive computational overhead, or both. For instance, in scientific and engineering data exchanges floating point encoding from text to binary and back to text can account for 90% of the wall-clock time for large, numerically saturated exchanges [Engelen], [Chui]. Some of the more general purpose efforts are described in this document (see the section Contributed Candidate EXI Implementations and the XBC Working Group documents though there are many others (bnux [NUX], [BXML], bSOAP [Abu-Ghazaleh], [gSOAP], [BXSA]).

One might say that each of these solution strategies are indicated in some circumstances. For instance, collapsing the protocol stack can be done in largely proprietary situations and textual parser optimization seems better when "view source" is important. EXI is an effort to find a standardizable solution for that significant group of communities for which the indicated solution appears to be to optimize the format itself. Ideally, an EXI candidate will have a flexible and robust design that answers all important properties and efficiencies for most use cases.

4. Test Methodology

4.1. Notional XML-EXI Measurement Processing Pipeline

The following diagram illustrates the various steps whereby XML documents are typically converted into a binary encoding and then decoded back into the original text-based XML encoding. This is the nominal processing pipeline that was instrumented by the measurement framework used to make the performance measurements.

The EXI end-to-end processing pipeline

The following steps are typically performed when converting an XML document into a binary-encoded document.

The following corresponding steps typically occur when converting a binary-encoded document into an XML-encoded document.

4.2. Data Characteristics and Complexity

Submitted: 25-Apr by GW. Integrated: 1-May, GW. Status: Edited 1-May, GW Forward Reference: Note that the property of a format that it can handle large documents will probably not be satisfied by those which require to load the whole document into memory!

The XBC Measurements Document [XBC Measurements] (Measurement Considerations) says that processing efficiency is related to size and to complexity. Additionally, it discusses data complexity in the context of a large number of scenarios and property profiles of use cases. However, how to quantitatively determine the complexity of a single document, in order to gauge the response of a format's processing characteristics to complexity, is not discussed.

A list of the continuous components of complexity which might be tractable for characterizing a format's processing efficiency with respect to complexity, for the tasks for which the group measured each candidate (see Measured Tasks of Processing Efficiency and Measured Compactness), could be:

Components of XML Complexity

  1. size
  2. total number of elements
  3. number of unique elements
  4. tree-height (nesting)
  5. number of unique external references
  6. total number of attributes
  7. number of unique attributes
  8. regularity, or "self-similarity"

This list, derived from Determining the Complexity of XML Documents ([DET-COMPLEXITY]), includes size. It can be argued that since size is largely orthogonal to the remaining characteristics, it ought to be treated separately. However, over the whole XML community, "size" will vary on a large interval, such as a few 10s of bytes to say 10 Gbytes (MicroArray Gene Expression descriptions in MAGE-ML, 3d modelling in X3D, MEDLINE/Pubmed etc). So this range is now about 9 orders of magnitude, and quite possibly will be greater in the future. Therefore, size will dominate any requirement for the expected response of a format to the (continuous) characteristics of documents the format might be required to process.

From previous work, the group is also aware that the simple ratio of content data to markup, is a blunt but effective predictive variable for XML processing time. This quantity is called "Information Density" in the Analysis of the EXI Measurements.

Therefore, as a simplification, to reduce the number of continuous independent variables over which the formats must be measured, we looked at each candidate's performance only with respect to size and to information density. The response of formats to the remaining continuous components above may be evaluated in proceeding drafts of this document.

One important non-continuous property that is immediately related to size though, is generality. Specifically, any format that cannot process very large documents due to the format's need to process entirely from main memory, cannot pass the generality requirement for an EXI format.

The tests additionally characterize each format with respect to the fidelity with which it reproduces the information in documents, according to a scale analogous to various XML data models. Those systematic measurements are described in the Fidelity Considerations section below.

We will be doing an analysis of complexity in future work.

4.3. Figures of Merit

Submitted: 25-Apr, GW. Integrated: 1-May, GW. Edited: 1st pass 1-May, GW, for syntax. Needs content review.

For this draft of the Note we measured two of the three Algorithmic Properties described in the XML Binary Characterization Properties, for each contributed format; Processing Efficiency and Space Efficiency. Additionally, the tests measure the Format Property of Compactness. This section describes the quantities used to characterize those properties in each potential EXI format that has been measured.

4.3.1. Characterization of Processing Efficiency

This subsection describes the quantities we use to evaluate each format's Processing Efficiency.

In addition to characterizing the speed of processing, the XBC Measurement Methodologies [XBC Measurements] document, section 6.2.1 Processing Efficiency Description, delineates four properties of a format's algorithm which need to be analyzed for processing efficiency; Incremental Overhead, Standard APIs vs Abstract Properties, Processing Phases, and Complexity [of the algorithm itself, not of the input data]. Of these, really only incremental overhead is amenable to empirical testing. Incremental Overhead refers to whether a format "allows and supports the ability to operate efficiently so that processing is linear to the application logic steps rather than the size of data complexity of the instance" [emphasis added]. That is, it is reasonable to expect the processing time of an application using our format to be dominated by the complexity of the application, not by the processing in our format. So the desirable characteristic of a format is that its processing time be (only) linearly or sublinearly dependent on the input data complexity, i.e. that it's O(n).

For most users of XML, linearity over a broad range is likely to be a secondary concern. Of primary interest will be expected wall-clock elapsed time to process "most" typical examples of an individual's use cases, in their own scenarios. From the perspective of the XML community however, we require good performance over a very broad spectrum of document complexities. Therefore, as described above in Data Characteristics and Complexity, the complexity requirement as a whole is dominated by size; that is, the above expected processing time function f(n)=O(g(n)) must hold for a large range of document sizes (n).

A number of the use cases described by the XBC in XML Binary Characterization Use Cases, present a 3rd requirement for the processing of a format, that of minimal memory consumption. These are, for instance, use cases Web Services for Small Devices, Multimedia XML Documents for Mobile Handsets and Sensor Processing and Communication. The memory requirements of a format were captured by two of the XML Binary Characterization Properties: Small Footprint and Space Efficiency. These respectively describe the expected memory requirement of the XML processor itself and the amount of dynamic memory required in execution. The memory usage requirements of a format can be characterized in some depth. For example, the tests might note behavior with different amounts of bounded memory and check for the extent of an expected inverse relationship with respect to processing speed. So far, however, the tests measure only total unbounded memory consumption [Ed: At the time of writing, figures for memory consumption are being withheld from the results pages until some anomalous values are better understood]. Components of Processing Efficiency

Putting the above together then, the important continuous measurable components of processing efficiency to test are:

  1. Speed - expected wall clock elapsed execution time
  2. Linearity - the extent to which, empirically, the algorithm implementation's execution time is O(n), and over how great a range of sizes n does this hold
  3. Memory Consumption - unbounded memory consumption during execution.

Once figures for memory consumption are consistently measured across the various format processing implementations, they will be correlated with document size and processing efficiency. Measured Tasks of Processing Efficiency

The above components were measured for each of the following processing tasks, derived from the XML Binary Characterization Measurement Methodologies document, part 6.2.2:

  1. Encoding from a SAX event store
  2. Encoding from typed-API event callbacks - intended to enable measurement without the added inefficiencies imposed by a proscribed API, and to inform the encoder about the data's type, which it can then use to optimize the encoding
  3. Decoding to a SAX store - the time it takes to parse a format, decode and serialize to SAX events
  4. Decoding to a typed-API.

Note that, at the time of writing, we intend that the measurement for ASN.1 BER and ASN.1 PER formats' encode and decode, will be to bound data structures, when those formats are included in the measurement framework (see Native Language Implementation Considerations). Additionally, the current measurements are not yet using a typed-event API.

In further work we may also add processing for the DOM API (see Appendix C: Further Work).

Formats which include schema-aware encoding methods have been allowed to use them in cases where schemas exist. Since some formats require a schema, where no schema existed for a test group, a naive XML Schema (root of xsd:anyType) instance was generated for purposes of format performance comparison. Where a schema or some other meaningful shared state might be exchanged, timing tests do not include the time it would take to share the state, such as exchanging the schema.

4.3.2. Characterization of Compactness

In the next version of this document, characterization of application classes and use cases based on XBC will be added to this section.

A figure of merit for compactness used in the literature, is the normalized compactification rate = 1-(c/l), (that's one minus c over l) where l is the length of the original XML document (say utf-8 encoded on disk), and c is the length using some compactification scheme. The factor can be multiplied by 100 to get a percent compaction. This formalism results in higher +ve values (0..1-eps; or 0..100-eps%) the more compaction there is (and -ve values when the output is bigger than the input). In order to provide more intuitive figures we have chosen to use, at least initially and for presentation of results, simply c/l, or (c/l)*100%. That is, smaller is better.

Note that the XBC Measurement Methodologies document [XBC Measurements] describes the property "Compactness", of which one method is "compression". "Compression" here means "document compression", which is taken to mean loss-less compression algorithms - those which use redundant information in a document to encode its data in a smaller space. Roughly speaking, compactness includes compression, but may also derive from other methods.

The XBC Measurements Methodology document says that the compactness property for a given format can be characterized separately for each of the following methods from which overall compactness may be derived ([XBC Measurements], section 6.1.2):

  1. Tokenization - if possible, use of the format's encoding without the use of any of the methods below. In the Japex reports this is called "Neither"
  2. Schema - the use of schema-based encoding methods
  3. Compression - by data analysis. In the Japex reports this is called "Document"
  4. Schema and Compression ("Document") combined
  5. Deltas - use of a template, parent or earlier message (no formats use this, so not measured)
  6. Lossy - use of some compression scheme where accuracy is traded for compression (no formats uses this, so not measured).

In accordance with the XBC Measurements Methodology document, each of the elements in this 6-vector was to be measured and evaluated numerically, or else characterized as follows "N/S (not supported) if the method is not supported, or N/A (not applicable) if the method does not apply" ([XBC Measurements] part 6.1.2 para 1). The evaluation of lossy compression itself was to be characterized by a vector, being the amount of compression obtained as a function of the lossiness implied by each (if more than one) combination of lossy compaction schemes used by a format (possibly in combination with an input "permitted lossiness parameter").

For these measurements of compaction we have taken two simplifying steps. First, the tests do not attempt to characterize the compactification that may be gained from delta encodings and lossy encodings (5 and 6 above). None of the formats submitted so far utilize deltas in any integral way. If a non-trivial schema was not provided for the test group, the format is not permitted to look for one "elsewhere" (like at a URI). Just as for the processing efficiency measurement, if there is no non-trivial schema, the format's processor is permitted to use a pre-generated trivial schema (root of xsd:anyType).

Also none of the formats we considered include a lossy compression scheme.

At this stage in the process of evaluating the viability of an EXI, and for comparison of candidates, measurements of each candidate were permitted to be made with any number of combinations of that format's parameters. So there may be more than 4 compactness results for any one candidate (for a given test group). However, for each configuration used, both compactness and processing efficiency must be reported for that same configuration. Measured Compactness

The figure of merit for compactness is then (c/l)*100%, and is an overall measure of lossless compactness achieved by each format. We will give the following aggregations in a following draft:

  1. Measured compactification for each document in the test suite
  2. Arithmetic mean compactification of all documents in a test group (e.g. RDF)
  3. Arithmetic mean for a use case (e.g. test groups UBS and RosettaNet combined into one result for the Inter/intra Business use case).

We can clearly identify which formats are using schema-based encoding on the compactness results, and characterize the advantage of schema to non-schema based compression and PE (if there is in fact any, cf Screamer) in the analysis section.

If we do ultimately characterize compression in the presence or non-presence of schema, and if relevant, for various amounts of lossiness, we would have to agree on how such measures would be combined for a figure of merit. For instance, is compaction derived purely from (loss-less) compression, to be given any more weight than that from schema compression?

4.4. Measurement Framework

In order to make consistent measurements of all candidates over the whole test suite set, we used a framework based on Japex. Japex is a simple tool that is used to write Java-based micro-benchmarks. It can also be used to test native language (e.g. C/C++) systems via the Java Native Interface (JNI), which we used to test native candidate implementations.

Japex is similar in spirit to JUnit in that it does most of the repetitive programming tasks necessary to make a measurement. These tasks include loading and initializing the required drivers, warming up the VM, forking multiple threads, timing the inner loop, etc.

The input to Japex is an XML file describing a given test. The file's primary constituents are the location of a test data group (for example all of our examples of X3D XML files), and references to one or more "drivers". These drivers interface Japex to the code which is under test. Each implements some well defined micro-benchmark measurement, such as "encoding with schema".

By default, the micro-benchmark measurement metric is throughput in transactions per second (TPS). However, it is possible to customize a test to either calculate throughput using a different unit, such as milliseconds, or KBs per second, or to make a different kind of measurement, such as latency, memory usage or message size. We used such customizations to make our various measurements for the Figures of Merit above.

The output of Japex is a timestamped report in XML and HTML formats. The HTML reports include one or more charts generated using JFreeChart. The output gives, for each observable under test, results for each data file, plus aggregated results for the test group.

4.4.1. Framework's Measurement of Processing Efficiency

Recall that to characterize processing efficiency, we measure speed, linearity, and memory consumption, for each of the processing tasks listed above.

To measure the "Speed" component of a candidate's Processing Efficiency, we measured aggregate throughput for each of the processing tasks.

For our purposes, the term "throughput" is defined to be work over time. Japex is designed to estimate an independent throughput for each of the tests in the input. Throughput estimation is done based on some parameters defined in the input file. There are basically two ways to specify that: (i) fix the amount of work and estimate time, or (ii) fix the amount of time and estimate work. We fixed the time and measured work. In addition to each test's individual throughput, aggregate throughtputs in the form of arithmetic, geometric and harmonic means of results for all files in the test suite, are computed for each test. Additionally, each test was run a number of times to estimate variance. The arithmetic mean of those aggregates was used in the comparison of results (not harmonic mean - although TPS is a "rate", the number of seconds given to each run was invariant).

The "Linearity" component was characterized from the same measurement data as that for Speed.

The "Memory Consumption" component was measured, for Java based candidates, by Peak Heap estimation. The Java virtual machine has a heap for object allocation and also maintains non-heap memory for the method area and the Java virtual machine execution. The Java virtual machine can have one or more memory pools. Each memory pool represents a memory area of one of the following types: heap or non-heap. The peak heap memory used by each driver (which implements a single test), was estimated by the Japex framework using the following procedure:

  1. Force collection by calling System.gc() a number of times, until the available heap stabilizes
  2. Set peak heap memory usage to current memory usage (i.e., reset peak usage)
  3. Accumulate peak memory usage across all memory pools
  4. Execute all runs for the driver under test
  5. Accumulate peak memory usage across all memory pools
  6. Assign the output japex.peakHeapUsage parameter by subtracting (3) from (5).

This simple algorithm for estimating peak heap memory usage is experimental and subject to change. Note that non-heap memory is currently not estimated because it includes the virtual machine memory which cannot be easily distinguished from a candidate's implementation.

See the Japex Manual [JAPEX] for further information about this tool.

4.4.2. Measurement Process

Each measurement began by parsing the XML-format test document into an in-memory representation. Two such representations were used: one was a direct sequence of the SAX events produced when parsing a document (1 and 3 of the measured tasks above), and the other was a similar sequence, but extended with an event type (2 and 4). These typed-content events were used when a schema was available, to indicate to the encoder that the content of the element was of some data type. The encoder might then use that to optimize the process of encoding, and the encoded form.

The measurement consists of feeding the events of the in-memory representation, one by one, to the measured encoder, and then using the measured decoder to parse the produced bytes back into the in-memory representation. Processing time was measured separately for the encode and decode phases.

So far we have run the measurements on only a single computer system (see Appendix A: Measurement Details). The test system was carefully prepared to ensure reproducible measurements. It was disconnected from the network and any other peripherals that might cause interrupts, and only the measurement framework was permitted to run during the measurement timing.

This use of a single system, is not unreasonable for a test purely composed of comparison of performance amongst technologies. However, when in further work, we measure the absolute performance of promising candidates, that is, their processing efficiency in bytes processed per second, we will expand the test hardware so as to isolate systematic effects (such as different caching mechanisms, different implementations of important methods, etc).

4.4.3. Reproducibility Criteria, Warm-up and Caching

In our tests, measurements were made by a Java-based micro-benchmark framework, both for processors implemented in Java and those implemented in native languages. Since a Java Virtual Machine (JVM) typically performs a just-in-time compilation of the running code, the first run does not usually reflect actual application performance. Therefore, the actions of each experiment were repeated as a warm-up, without measurement, until sufficient cycles had passed to ensure that the measurements were stable.

This process has the intended effect of approximating and benchmarking the performance of a warmed-up application. However, systematic effects may still arise from caching (or warming-up) the input data, in addition to the code. For example, if the benchmarking framework repeatedly reads a copy of the input data from a fixed location in memory while warming up the code, the processor will have the input data in high-speed cache memory, allowing access times typically over 25 times faster than a real application might encounter. One way to address this problem is by sequentially processing multiple copies of the input data laid out end-to-end in memory. However, simplistic versions of this memory access pattern might cause the results to be skewed by pre-fetch caching hardware. Although these issues can be addressed, care must be taken to tailor buffers, gaps, and total data so that cache levels are cleared while avoiding virtual memory paging.

The cache cannot be removed or defeated for an empirical assessment because caching behavior has a significant positive impact on the performance of modern systems. For example, an algorithm designed to have a good locality of reference to take advantage of caching and pre-caching architectures would perform significantly worse if the cache is disabled than it would in actual practice. In addition to data cache effects, modern processors are affected significantly by branch prediction. Modern branch prediction is based on local and global historical branch activity, meaning that warming up the processor on data that is significantly different than the test data can result in some performance difference. Most of the performance of modern systems relies on caching, branch prediction, and related capabilities.

Our primary interest is to understand the performance of EXI algorithms independently of I/O related factors. Therefore, the benchmarking framework attempts to accurately model the code paths and memory access patterns of real applications to the extent possible while minimizing the cost of I/O, blocking, context switches, etc. To factor out the majority of the I/O, blocking and context switching costs associated with network I/O, we use a local loopback interface. The local loopback reads and writes all data through local memory instead of a physical network, essentially modeling a memory-speed network. This approach uses the same buffering algorithms, code paths and memory access patterns as a real-world application, reproducing realistic caching effects without warming-up the input data. At the same time, since it's memory-speed, the interface will probably provide data faster than an EXI algorithm can consume it. This way, the algorithm remains CPU bound throughout the duration of the test and is not subject to I/O related blocking or context switching. The resulting benchmarks show the processing efficiency of the EXI algorithm isolated from the speed or I/O effects of a particular network.

The benchmarks that are likely to most accurately reflect the performance of real EXI applications are those that include I/O to some external media, like a network or file system. EXI is about interchange, so most EXI use cases will read or write EXI documents to and from devices that are often slower than main memory (i.e., networks or storage media) although there are important cases of memory to memory interchange. Therefore, the benchmarks that are likely to most accurately model real-world performance for many cases will be those that read and write EXI documents to and from networks and storage devices. Careful scrutiny is needed, especially when not using a loopback interface, to detect bandwidth bottlenecks that are smaller than the throughput of algorithms being tested. These benchmarks are expected to accurately model the interaction between the caching algorithms employed by modern computing architectures and the memory access patterns of buffering algorithms employed by typical device drivers. They also are expected to accurately model the subtle, but significant performance implications of blocking and context switching that occur as algorithms shift repeatedly between CPU and I/O. For a given platform and use case, the average time spent doing context switches and being I/O-bound vs. CPU-bound will differ from one EXI format to another. It will be influenced by a variety of factors, including the average throughput of the EXI algorithm, the average throughput of the I/O device, and the compactness of the EXI format. It must be noted however that this interaction with operating system characteristics may not be deterministic. The determinism of a trial may be subject to particular buffer sizes or other details that are not obvious. Therefore, for higher confidence, a comparison might be needed between a network-intermediated trial and a cache-clearing, memory to memory trial.

To look more closely at those effects of the network, in a real-world- like setting, the framework has been extended to measure the performance of EXI algorithms reading and writing from any TCP/IP based network (e.g., Wired LAN, WI-FI LAN, Internet, GPRS, etc.). This will enable us to collect benchmarks using the same network media, buffering algorithms, device drivers, and code paths employed by EXI use cases. As such, it replicates the memory access patterns, blocking patterns, context switches, etc. of, at least, TCP/IP based real-world applications. However, although the group has developed this part of the framework, it has not yet been used to take any official measurements (see Appendix C: Further Work - Framework Network Driver).

4.5. Native Language Implementation Considerations

Submitted: 11-may, ED. Integrated: 13-May, GW. Status: 1st edit, some questions and rewordings need confirmation.

Some of the candidates currently being considered as a basis for an EXI standard only have C or C++ implementations, as opposed to Java, available at this time. In order to test the processing efficiency of these candidates using Japex, the framework was augmented to provide a driver API through the Java Native Interface (JNI), that allows non-Java applications to be benchmarked.

The JNI is used in such a way as to isolate the performance of the C/C++ implementation from the Java processing in the Japex framework. This is done by doing all of the timing on the native (i.e. C/C++) side of the interface. Clearly, there will be some residual systematic effect, since at least a JVM is cohosted, but we believe that effect is negligible.

However, the processing efficiency timing results that are produced cannot not be directly compared with results for Java candidate implementations for several reasons:

  1. There is no "JVM effect" in C/C++. One would expect processing times to be somewhat faster since no byte code interpretation is required and there are fewer overhead tasks such as garbage collection (though of course an algorithm implemented in native code will also have to do housekeeping). The actual amount of this speedup is not known in detail, since we do not yet have any candidate with isomorphic implementations in both Java and C/C++. In further work, we may try to evaluate whether this fraction, the amount of speedup, is what we would expect for comparable applications (see Appendix C: Further Work)
  2. The methodology for getting events into the native applications for processing is different. Since it is not practical to feed Java Events across the JNI boundary for processing, and no existing implementation of a SAX-like typed-event API could be found, an alternative to the SAX and typed-SAX APIs which are used for Java was required. One method that is currently being used is to construct a DOM tree from an XML source and then to use this tree to populate objects for serialization into the candidate binary format. For deserialization, the time to deserialize from the binary format to objects is measured. Two of the native formats are those based ASN.1 and the objects are those of classes generated by compiling the ASN.1 schema.
  3. Different implementation types. At least two known candidates that currently have C/C++ implementations use data binding technologies as opposed to the more standard interpretive XML technologies. Data binding produces tightly coupled, very performant applications by building information directly from schemas into the compiled native code. The XML Screamer application [Screamer] has used this technology to produce impressive performance results with XML.

For these reasons, it might not be highly accurate to compare native implementations with their Java counterparts for processing efficiency. Instead, a comparison is done with existing C/C++ based XML implementations. One such implementation is the open source libxml2 library (http://xmlsoft.org), a highly performant implementation available in most Linux distributions. Libxml2 doesn't resolve all issues; for instance, it does not provide data binding. [ we need to point to the libxml2, and the native candidate implementations, in the japex results; or if libxml2 is not measured in this 1st draft, either remove above reference or say that libxml2 measurements will be added]

SW (w/PS): Write APIs of Primary Interest section

This has to be explained; why are we using the typed-event API [to remove the inefficiencies of SAX]? What is it? How is it implemented? DOM too, at least for parsing? PS has given some good text for this in http://lists.w3.org/Archives/Member/member-exi-wg/2006Mar/0148.html.

4.6. Fidelity Considerations

Submitted: 8-May, PS. Integrated: 10-May, GW. Edited: 1st pass 10-May, GW. Some items needed: 1. Short introduction - what is fidelity and why is it needed, or at least why does it need to be evaluated. 2. Explanation of why this section is included in the document. We don't at the moment evaluate fidelity - so why include the taxonomy on which it would be measured?

In this section we also describe a taxonomy with which the group can evaluate the extent to which each EXI format candidate satisfies the lexical reproducibility requirements of the examples in the test suite .

The degree to which a candidate can accurately reproduce the information represented by a test group is determined jointly by the following:

  1. properties of the candidate. For example, candidates may not be able to support the preservation of certain information or support the preservation of information with only a particular fidelity
  2. properties of the test group and more broadly the use cases to which the test group belongs. For example, test groups that contain signed information (as specified by W3C XML-Signature Syntax and Processing [XML Signature]) may require higher fidelity to the original than test groups that contain no signed information.

To characterize the extent to which each candidate preserved the information necessary for a test group, each test group was annotated with what information must be preserved, and with what accuracy, to satisfy the particular requirements of that case.

A "round trip support" measurement property of the test framework will be used to determine whether the candidate accurately reproduced the information represented by each test group, according to the annotation. If the result of this measurement was that the candidate did not reproduce the information with the required fidelity, or it is predetermined to fail without running the test, then the candidate was determined to have failed the round-trip support measurement property for that test group, and additionally failed for all other measurement properties that utilize the test case.

The fidelity requirements were communicated to the test framework, and to each candidate under test, through japex parameters. Those parameters prescribed what did and did not need to be preserved. Candidates were free to examine the parameters to optimize their encoding. For example, if preservation of comments and processing instructions was not required, as is the case for SOAP message infosets, then a candidate might utilize that knowledge to encode SOAP message infosets more efficiently than if it could not make such a narrowing assumption.

4.6.1. Preservation of information represented by test groups

The following information represented by a test group does not need to be preserved by a candidate:

  1. the XML declaration
  2. the use of single or double quotes characters for quoted strings
  3. markup white space.

It is not anticipated nor expected that candidates will accurately reproduce information on a syntactic or byte-per-byte basis [see fidelity scales 3 and 4].

The following subsections present the distinct sets of information that may or may not be preserved by a candidate. Preservation of white space

If a white space needs to be preserved then all non-markup white space information [see 2.10 White Space Handling, XML 1.0] represented by the test group MUST be preserved. Otherwise, all non-ignorable white space information MUST be preserved. Such non-ignorable white space information may be determined from a schema, if present with the test case, and/or by inspection of the test group. Preservation of comments

If comments need to be preserved then all comment information represented by the test group MUST be preserved. Otherwise, comment information need not be preserved. Preservation of processing instructions

If processing instructions need to be preserved then all processing instruction information represented by the test group MUST be preserved. Otherwise, processing instruction information need not be preserved. Preservation of namespace prefixes

If namespace prefixes need to be preserved exactly then all namespace prefix information represented by the test group MUST be preserved verbatim. Otherwise, namespace prefix information need not be preserved identically.

The verbatim preservation of namespace prefixes is important when a test group contains signed information or prefixes as part of a qualified name information in element content or attribute values. Preservation of namespace prefixes is important in any case to maintain document validity and correctness. Preservation of lexical values

If lexical values need to be preserved then all character data (see Extensible Markup language (XML) 1.0 [XML 1.0] section 2.4 Character Data and Markup) information represented by the test group must be preserved. Otherwise, lexical values need not be preserved.

Lexical values may not be fully preserved if a candidate chooses to encode a lexical value into a more efficient lexical value, or in binary form, and the original lexical value cannot be reproduced. For example, the decimal lexical value of "+1.0000000" might be converted to the more efficient canonical decimal lexical value "1.0". Similarly, the boolean lexical value "1" might be converted to binary form as one bit of information and it may not be determinable from the binary form whether the original lexical value was "true" or "1". Preservation of the document type declaration and internal subset

If the document type declaration and internal subset need to be preserved then the document type declaration and internal subset information represented by the test group must be preserved. Otherwise, document type declaration and internal subset need not be preserved.

4.6.2. Fidelity Scale

Candidate formats, and test groups, can be classified according to a "scale of fidelity". For formats, the scale is a metric of the extent to which information is preserved with respect to various XML data models. For test groups the same scale is used to classify the requirement the of the test group for round-trip accuracy. We defined it to go in order of increasing fidelity. A candidate which has been determined to score at a certain level on the scale may still preserve some, but not all, information specified at higher levels.

Note that this does not imply that documents produced by candidates have an isomorphic representation of the information represented by certain XML data models, rather that a candidate stores enough information to be able to reproduce the preserved information.

Table 1: The Fidelity Scale for Classification of EXI Candidates
Level Preserves
-1 Preserves only a subset of the XPath data model

Level -1 defines the class of candidates that preserve a subset of the XPath 1.0 data model. For example, a candidate might not preserve namespace prefixes.

0 Preserves the XPath data model

Level 0 defines the class of candidates that preserve the XPath 1.0 data model.

Such preservation includes the root node, elements, text nodes, attributes (not including namespace declarations), namespaces, processing instructions and comments.

Unexpanded entities, the document type declaration and the internal subset are not preserved.

This level corresponds to a common denominator in XML processing, it is equivalent to the SOAP XML subset with the addition of processing instructions and comments.

1 Preserves the XML Information Set

Level 1 defines the class of candidates that preserve the XML Information Set data model.

The XML Information Set preserves more of the information represented in the document type declaration and internal subset than the XPath 1.0 data model but not all information, such as attribute and element declarations, is preserved.

The [all declarations processed] property of the Document Information Item is not, strictly speaking, part of the Infoset [XML Infoset] and therefore does not need to be preserved.

NOTE: A candidate can fully support level 1, level 2, and the preservation of the complete internal subset by including and encoding the internal subset as a string, thereby leaving it up to the decoder to optionally process it.

2 Preserves the the XML Information Set and all declarations

Level 2 defines the class of candidates that preserve the XML Information Set data model, as in level 1, in addition to all information that is not purely syntactic. Such additional information will include the attribute, element and entity declarations.

This level covers the preservation of all information represented by an XML document but does not accede to supporting purely syntactic constructs.

NOTE: A candidate can fully support level 2 by including and encoding the internal subset as a string, and thus leaving it up to the decoder to optionally process it.

3 Preservation of syntactic sugar of the XML document

Level 3 defines the class of candidates that preserves all information represented by an XML document, as in level 2, in additional to some but not all syntactic information.

NOTE: Full syntactic information is supported by level 4.

Such preservation includes but is not limited to:

  1. CDATA Sections
  2. resolved external entity reference boundaries (when an entity is resolved a candidate will flag the part in the produced infoset where the information included from it starts and ends)
  3. the use of single or double quotes characters for quoted strings
  4. white space in markup
  5. the difference between empty element variants
  6. the order of attributes.
4 Preservation of bytes

Level 4 defines the class of candidates that preserve the XML document byte-per-byte.

NOTE: This is the simple level supported by generic encodings such as gzip.

5. Contributed Candidate EXI Implementations

This section provides a technical description of the basic architecture of each of the contributed XML formats whose performance characteristics were measured (see Results). The statements in these descriptions are those of representatives of each format and not necessarily agreed upon statements by the working group.

5.1. X.694 ASN.1 with BER

Submitted: 25-Apr, ED. Integrated: 25-Apr, GW. Edited: partly edited for content 25-Apr, GW.

The X.694 ASN.1 BER candidate submission uses a set of standards that have been in place for many years for binary messaging (ASN.1 itself since the early 1980s). There are three parts to the candidate:

  1. The Abstract Syntax Notation 1 (ASN.1), from the International Telecommunications Union - Telecommunications Group (ITU-T), and the International Standards Organization (ISO), is a schema much like the XML Schema for describing abstract message types
  2. The ITU-T/ISO Basic Encoding Rules (BER), is a set of encoding rules to be used with ASN.1 to produce a concrete binary representation of a set of values described using an ASN.1 schema
  3. The ITU-T/ISO X.694 standard provides a mapping of XML Schema (XSD) to ASN.1, and allows the use of ASN.1 and its associated encodings in XML applications.

In this format candidate, BER encoding was selected in preference to PER (Packed Encoding Rules - see X.694 ASN.1 with PER below) because it was felt that the flexibility in the tag-length-values (TLV) that it provides, is closer in spirit and functionality to XML than the tightly coupled encodings produced by PER. The general principle is that each element construct in XML;

<tag>textual content</tag>

is replaced by a similar binary encoding consisting of a tag-length-value descriptor.

Efficiencies are gained from two properties of this format: 1) the tags and lengths are binary tokens that are in general much shorter than XML textual start and end tags, and 2) the content is in binary instead of textual form.

Some advantages of using ASN.1 BER to encode XML are that the TLV format is similar to XML's start-tag / content / end-tag pattern; encoded length can be 5 to 10 times less than textual XML, and encoding and decoding is very efficient - no compression or other CPU intensive algorithms are used. Additionally, ASN.1 BER is mature and stable, and its security has been studied in-depth.

However, it is schema-based - an XSD or similar schema is needed to encode and decode, and some fraction of the XML Information Set can not be represented (for example, randomly occurring comments and Processing Instructions).

See references [USE-OF-ASN.1], [ASN.1], [BER], [X.694].

Submitted: 15-May, PT. Integrated: 16-May, GW. Edited: 1st pass. Needs to be checked by Paul, changed quite a lot.

5.2. X.694 ASN.1 with PER

Similarly to the X.694 ASN.1 with BER candidate format described above, this one uses X.694 to map from an XML Schema document to ASN.1. In this variation, we add the use of two further ITU-T standards, X.693 and PER. These additions allow direct output in textual XML, and a somewhat more compact binary encoding.

PER, which stands for Packed Encoding Rules [PER], is one of the ASN.1 Encoding Rules published by the ITU-T, IEC, and ISO. PER was specifically designed to minimize the size of messages needed to convey information between machines. It has been widely adopted in critical infrastructure where bandwidth is limited. It originated from work on an efficient air-to-ground communication protocol for commercial aviation, and has since been used in many areas including cell phones, internet routers, satellite communications, internet audio/video, and many other areas.

Another of the ASN.1 Encoding Rules, Extended XML Encoding Rules, or "EXTENDED-XOR" [X.693], defines a set of encoding rules, that can be used in a way analogous to XML Schema, and applied to ASN.1 types to produce textual XML.

Since the schema notation used by XML Schema is quite different from the ASN.1 notation, ITU-T Rec. X.694 | ISO/IEC 8825-5 was created to give a standard mapping from schemas in XML Schema to ASN.1 schemas in such a way that XML Schema aware endpoints, could exchange documents with ASN.1 aware endpoints, using the EXTENDED-XER Encoding Rules.

With the ASN.1 schema generated using X.694 from an XML Schema instance, one can generate not only XML documents (using the ASN.1 engine with EXTENDED-XER), but binary encodings with any of the ASN.1 Encoding Rules, of which PER is the most compact.

Submitted: 25-Apr, JK. Integrated: 29-Apr, GW. Edited: style & content 29-Apr.

5.3. Xebu

The Xebu format is a product of research into XML messaging on small mobile devices. The central XBC Properties considered in its design were; Streamable, Small Footprint, and Implementation Cost.

Xebu models an XML document as a sequence of events, similarly to StAX or SAX. The event sequence is serialized one by one. The basic Xebu format, applicable to general XML data, includes mappings from strings in the XML document to small binary tokens. These mappings are discovered dynamically during processing.

Xebu also includes three optional techniques for when a schema is available:

  1. Pretokenization; this populates the token mappings beforehand, based on the strings appearing in the schema
  2. Typed-content encoding; this results in a more efficient binary form for certain data types, than can be achieved without a schema
  3. Event omission; this leaves out events from the sequence if their appearance and placement can be deduced from the schema.

The main advantages of Xebu are; its support for general XML with varying levels of schema-awareness, a direct correspondence with well-understood XML data models, so making XML compatibility easy to achieve, and that it's simple and straightforward to implement.

Xebu is though presently research software; the precise capabilities and properties of the automata used for event omission are not well understood, their generation is not straightforward, and in general Xebu's present implementation is not fully mature.

See reference [XEBU1]

5.4. Extensible Schema-Based Compression (XSBC)

Submitted: 29-Apr, DM. Integrated: 29-Apr, GW. Edited: partly edited for content 29-Apr, GW. Edited 3-may. GW: 27-May, removed editorial comments flagging editorial changes in prep for publication - so should be checked.

Extensible Schema-Based Compression (XSBC) is a system for encoding XML documents that are described by schemas into a binary format. The result is more compact, faster to parse, and has better databinding performance than textual XML.

XSBC preprocesses the schema that describes an XML document, and creates lookup tables consisting of integers that index the string values of element names. The schema's type information about marked-up data, if any, is captured in one lookup table. In the same way, element attribute names are added to a lookup table, along with their type information. The size of the integers that correspond to the element and attribute names is chosen so that if only a few names are in the document, smaller numbers are used.

Once the schema has been processed and the lookup tables have been populated, the XML file is transcoded into binary format. Text element start and end tags are replaced by the binary format whole numbers to which they correspond in the element lookup table. Additionally, if any marked-up text data can be represented in an equivalent binary format, such as floating-point, it is replaced.

Element attributes are added after the binary element start tag. Attribute representations may be only the attribute start tag, followed by the attribute data, in binary format, if the schema type information makes this possible.

The marked-up data in binary format, for either element data or attribute data is then passed through data compressors. In the case of simple, fixed-length integers and floats, the standard IEEE 754 format can be used. Variable length data, such as strings, can be represented by a starting value that includes the length, followed by the actual data. The data compression system is easily extended to handle data other than that represented by the standard data compressors, and in fact can be extended dynamically. For instance, it's possible to write custom compressors for sparse or repetitive matrices, floating point data which is representable in only a certain number of significant digits, or other specialized data types.

Encoding and decoding a textual XML document to an XSBC document is quite straightforward. A simple finite state machine can be constructed to decode XSBC documents. An XSBC decoder can step through the document easily because of the structure of the document.

An XML Schema instance is required to encode the document. In future, the XSBC team plans to implement a feature in which a document without a schema can be "pre-preprocessed" and a stand-in schema generated for it. This stand-in schema will be untyped (all data will be represented as strings) but this would allow an arbitrary XML document without a schema to be represented.

XSBC's virtue is its simplicity. It is, essentially, every programmer's first idea of how to represent an XML document in binary form. There is a strong correspondence between the textual XML representation and the binary representation.

5.5. Fujitsu XML Data Interchange Format (FXDI)

Submitted: 1-May, TK. Integrated: 3-May, GW. Status: Edited, some clarification of encoders needed. 27-May GW; removed editorial comments for publication. Should be checked. References needed. 29-May: TK resubmitted with clarified distinction between encoders. Text says that diff encoders are used for compactness and PE, although my recollection is that was to be not allowed - all properties results should be presented for all configurations. emailed group to confirm.

The Fujitsu XML Data Interchange (FXDI) format was designed to serve as an alternative encoding of the XML Infoset, which allows for more efficiency, both in the exchange of data between applications and in the processing of data at each end-point. FXDI's primary design goals were document compactness, and to enable the implementation of fast decoder and encoder programs, which run in a small footprint, and without involving much complexity.

FXDI is based on the W3C XML Schema Post Schema Validation Infoset (PSVI), though some of the format features derive from the XPath2 Data Model. Although FXDI performs much better when schemas are prescribed before documents are processed, it is capable of handling schema-less documents and fragments through its support for Infoset tokenization.

At the core of FXDI is the "compact schema". FXDI uses Fujitsu Schema Compiler to compile W3C XML Schema into a "schema corpus". A schema corpus contains all the information expressed in the source XML Schema document plus certain computed information such as state transition tables. A compact form of the schema is then computed from a schema corpus by distilling only those information items which are relevant to the function of FXDI processors.

There are two types of FXDI processors. Firstly, those that are used to generate FXDI documents. These are called "FXDI Encoders". Conversely, FXDI Decoders, decode FXDI documents into data usable by programs.

FXDI supports two different methods to create FXDI documents:

In the EXI Test Framework tests, the validating encoder was used in the compactness tests. The validating encoder carries out schema-validation as part of encoding process and logs any errors in test case XML documents. This is useful for diagnosing performance anomalies caused by test case documents that deviate from their associated XML Schemas. On the other hand, the non-validating encoder is used in the encoding tests for maximizing processing efficiency.

FXDI works well with conventional document redundancy-based compression such as gzip. That facilitates use cases that need the additional compression and can spend the additional CPU cycles.

See W3C EXI WG and Fujitsu for more information.

5.6. Fast Infoset

Fast Infoset is an open, standards-based binary format based on the XML Information Set [XML Infoset]. ITU-T Rec. X.891 | ISO/IEC 24824-1 (Fast Infoset) [FI] is also itself an approved ITU-T Recommendation as of 14 May 2005. An ISO ballot has been initiated (and is near completion) that will result in ISO/IEC 24824-1 being available for free when published.

The XML Information Set specifies the result of parsing an XML document, referred to as an "XML infoset" (or just an "infoset"), and a glossary of terms to identify infoset components, referred to as "information items" and "properties". An XML infoset is an abstract model of the information stored in an XML document; it establishes a separation between data and its representation that suits most common uses of XML. An XML infoset (such as a DOM tree, StAX events or SAX events in programmatic representations) may be serialized to an XML 1.x document or, as specified by the Fast Infoset specification, may be serialized to a Fast Infoset document. Fast Infoset documents are generally smaller in size and faster to parse and serialize than equivalent XML documents.

The Fast Infoset format has been designed to jointly optimize the axes of compression, serialization and parsing, while retaining the properties of self-description and simplicity. The approach has been to find, when not taking advantage of advanced features, a "sweet spot" where moderate compression can be achieved but not at the undue expense of creation, processing performance and simplicity.

The use of tables and indexing is the primary mechanism by which Fast Infoset compresses many of the strings present in an infoset. Recurring strings may be replaced with an index (an integer value) which points to a string in a table. A serializer will add the first occurrence of a common string to the string table, and then, on the next occurrence of that string, refer to it using its index. A hash table can be used for efficient checking of strings (the string being the key to obtaining the index; every time a unique string is added to the hash table, the index of the table is incremented. A parser will add the first occurrence of a common string to the string table, and then, on the next occurrence of that string, obtain the string by using the index into the table.

Fast Infoset is a very extensible format. It is possible, via the use of encoding algorithms, to selectively apply redundancy-based compression or optimized encodings to certain fragments. Using this capability, as well as other advanced features, it is possible to tune the "sweet spot" for a particular application domain. An example of this is the use of a built-in encoding algorithm to directly encode binary blobs without the need for any additional encoding, in a way similar to the Message Transmission Optimization Mechanism(MTOM), but with the binary blobs encoded inline as opposed to as attachments. Other built-in algorithms can be used to efficiently encode arrays of primitive data types like integers and floats, a feature often used by scientific applications to reduce message size and increase processing efficiency.

Additional features of Fast Infoset include support for restricted alphabets (for better compactness) and for external indexing tables, for those cases in which a tighter coupling is acceptable in the interest of achieving better performance.

5.7. Efficient XML

Submitted: 11-May, JS. Integrated: 13-May, GW. Status: Moved 1st 2 paras to end and removed assertions that might reasonably be made of all formats without supporting data. Also removed assertion about w3 requirements, since that is what the data is intended to show, and again the point is to show this for all formats.

Efficient XML is a general purpose interchange format that works well for a very broad range of applications. It was designed to optimize performance while reducing bandwidth, battery life, processing power and memory requirements. It is the only format currently being tested that supports all of the features specified by the minimum binary XML requirements defined by the W3C XBC group XML Binary Characterization [XBC Characterization].

The encoding is schema "informed", meaning that it can leverage available schema information to improve compactness and performance, but does not depend on accurate, complete or current schemas to work. It will work very effectively with partial schemas or no schemas at all. It also supports arbitrary schema extensions and deviations and allows dynamic schema negotiation, discovery and acquisition.

Efficient XML achieves broad generality, flexibility, and performance, by unifying concepts from formal language theory and information theory into a single, relatively simple algorithm. The algorithm uses a grammar to determine what is likely to occur in an XML document and encodes the most likely alternatives in fewer bits. The fully generalized algorithm works for any language that can be described by a grammar (e.g., XML, Java, HTTP, etc.); however, Efficient XML is optimized specifically for XML languages. The built-in Efficient XML grammar accepts any XML document or XML fragment and may be augmented with productions derived from XML Schemas, RelaxNG schemas, DTDs or other sources of information about what is likely to occur in a set of XML documents. The Efficient XML encoder uses the grammar to map a stream of XML information items onto a smaller, lower entropy, stream of tokens. The encoder then encodes the stream of tokens using a Huffman tree derived from the grammar or, if additional compression is desired, passes the stream of tokens to a more sophisticated XML compression algorithm that replaces frequently occurring token patterns to further reduce size. When schemas are used, Efficient XML also supports a user-customizable set of datatype CODECs for efficiently encoding typed values and provides typed streaming APIs for efficiently accessing typed values.

The binary form of Efficient XML is very compact. It is competitive with hand-optimized formats and is consistently smaller than both ASN.1 PER and gzipped XML. Even on very large, repetitive documents where gzip works best, it is not uncommon for Efficient XML to be 2-5 times smaller than gzipped XML.

The Efficient XML implementation used for W3C tests is a non-production implementation designated for evaluating W3C-proposed changes to Efficient XML. In addition to implementing the minimum W3C requirements, it includes the format features required to support advanced requirements, such as random access, accelerated sequential access and digital signatures. As a non-production version it has not been fully optimized. For example, the SAX API used for W3C performance tests sits above a translation layer that copies every character information item in a XML document from the String format generated by the underlying StAX API to char[] arrays required by SAX. Future versions of this implementation will improve its performance.

Production implementations of Efficient XML have been integrated into a broad range of platforms, including mass market mobile phones, PDAs, application servers, web servers, high-volume message routers, pub-sub systems, vehicles, aircraft, and satellite broadcast systems. High quality, commercial implementations are available for Unix, MS-Windows and a wide variety of mobile devices running both Java and Microsoft.NET.

See Theory, Benefits and Requirements for Efficient Encoding of XML Documents [EffXML] for more information.

6. Summary and Analysis of Test Results

Since this work is still in progress, the intermediate measurement results and preliminary analysis are to be found in the Analysis of the EXI Measurements) page. At the time of publication of this first draft, those results are subject to change at any time.

6.1. Compactness

A summary of the results of overall compactness, for each format, will be given here, for each of the notional configurations given in the above section on Characterization of Compactness. These results will be compared to the compactness thresholds defined by the XBC Measurements Methodology document.

6.2. Processing Efficiency

A summary of the results will be given here for the speed of processing, for each format, in each of the aggregations above. Additionally, we will note the Linearity of performance of each format, and its memory usage. See the section Characterization of Processing Efficiency for the definition of these figures.

If results clearly correlate significantly with components of complexity other than size or information density (see the Components of XML Complexity), we will characterize that relationship.

7. Conclusions

At the time of writing of this first draft, it is too early to give conclusions drawn from the test results. This draft Note is being published to encourage review and comment as this work continues.

8. Bibliography

[XBC Use Cases]
XML Binary Characterization Use Cases, Mike Cokus, Santiago Pericas-Geertsen editors, World Wide Web Consortium, 31 March 2005. http://www.w3.org/TR/xbc-use-cases/.
[XBC Properties]
XML Binary Characterization Properties, Oliver Goldman, Dmitry Lenkov editors, World Wide Web Consortium, 31 March 2005. http://www.w3.org/TR/xbc-properties/.
[XBC Measurements]
XML Binary Characterization Measurement Methodologies, Stephen D. Williams, Peter Haggar editors, World Wide Web Consortium, 31 March 2005. http://www.w3.org/TR/xbc-measurement/.
[XBC Characterization]
XML Binary Characterization, Oliver Goldman, Dmitry Lenkov editors, World Wide Web Consortium, 31 March 2005. http://www.w3.org/TR/xbc-characterization/.
XML Screamer: An Integrated Approach to High Performance XML Parsing, Validation and Deserialization, Margaret G. Kostoulas et al, proceedings of the 15th International World Wide Web Conference, May 2006. http://www2006.org/programme/item.php?id=5011.
SOAP Message Transmission Optimization Mechanism, Martin Gudgin, Noah Mendelsohn, Mark Nottingham, Hervé Ruellan editors, World Wide Web Consortium, January 2005. Latest version http://www.w3.org/TR/soap12-mtom/.
XML-binary Optimized Packaging, Martin Gudgin, Noah Mendelsohn, Mark Nottingham, Hervé Ruellan editors, World Wide Web Consortium, January 2005. Latest version http://www.w3.org/TR/xop10/.
Web Services Resource Framework (WSRF), Globus Alliance, OASIS, April 2006.
Pushing the SOAP Envelope With Web Services for Scientific Computing. Robert van Engelen, ICWS 2003.
The gSOAP Toolkit for Web Services and Peer-To-Peer Computing Networks. Robert van Engelen, Kyle A. Galliva. Proceedings of the 2nd International Symposium on Cluster Computing and the GRID, 2002.
Investigating the Limits of SOAP Performance for Scientific Computing. K. Chiu, M. Govindaraju, and R. Bramley. Proceedings of 11th HPDC, pp 246-254. July 2002.
NUX - Efficient and Powerful XML Processing Made Easy. Distributed Systems Department, Lawrence Berkeley National Laboratory. http://dsd.lbl.gov/nux/
BXML-CWXML Home page. CubeWerx. http://www.cubewerx.com/main/cwxml/.
Differential Serialization for Optimized SOAP Performance, Nayef Abu-Ghazaleh, Michael J. Lewis, Madhusudhan Govindaraju. Proceedings of the 13th HPDC, pp 55-64, June 2004. Describes bSOAP.
A Binary XML for Scientific Applications. Kenneth Chiu, Tharaka Devadithya, Wei Lu, Aleksander Slominski.
Determining the Complexity of XML Documents, Mustafa H. Qureshi, M. H. Samadzadeh, ITCC'05. http://ieeexplore.ieee.org/iel5/9755/30769/01425179.pdf.
The Use of ASN.1 Encoding Rules for Binary XML, Ed Day, Objective System Inc., http://www.obj-sys.com/docs/ASN1forBinXML.pdf.
Information technology — Abstract Syntax Notation One (ASN.1): Specification of Basic Notation [The ASN.1 Standard (ITU-T Rec. X.680)], International Telecommunication Union (ITU), July 2002. http://www.itu.int/ITU-T/studygroups/com17/languages/X.680-0207.pdf.
Information technology — ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER) [The ASN.1 BER Standard (ITU-T Rec X.690)], International Telecommunication Union (ITU), July 2002. http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf.
Information Technology - ASN.1 Encoding Rules: Specification of Packed Encoding Rules (PER) [The ASN.1 PER Standard (ITU-T Rec X.691 | ISO/IEC 8825-2)], International Telecommunication Union (ITU), July 2002. http://www.itu.int/ITU-T/studygroups/com17/languages/X.691-0207.pdf.
Information technology — ASN.1 encoding rules: Mapping W3C XML schema definitions into ASN.1 [ITU-T Rec X.694], International Telecommunication Union (ITU), January 2004. http://www.itu.int/ITU-T/studygroups/com17/languages/X694.pdf.
Information technology — ASN.1 encoding rules: XML Encoding Rules (XER) [ITU-T Rec X.693], International Telecommunication Union (ITU), December 2001. http://www.itu.int/ITU-T/studygroups/com17/languages/X.693-0112.pdf. This work is also standardized by ISO/IEC 8825-4 (with Amendment 1).
Xebu: A Binary Format with Schema-based Optimizations for XML Data, Jaakko Kangasharju, Sasu Tarkoma, and Tancred Lindholm. In 6th International Conference on Web Information Systems Engineering, Lecture Notes in Computer Science 3806, Springer-Verlag, November 2005. http://dx.doi.org/10.1007/11581062_44.
[XML Signature]
XML-Signature Syntax and Processing, Donald Eastlake, Joseph Reagle, David Solo editors, World Wide Web Consortium, 12 February 2002. Latest version http://www.w3.org/TR/xmldsig-core/.
[XML 1.0]
Extensible Markup Language (XML) 1.0, Tim Bray et al editors, World Wide Web Consortium, 4 February 2004 (Third Ed). Latest version http://www.w3.org/TR/REC-xml/.
[XML Infoset]
XML Information Set, John Cowan, Richard Tobin editors, World Wide Web Consortium, 4 February 2004 (Second Ed). Latest version http://www.w3.org/TR/xml-infoset/.
ITU-T Rec. X.891 | ISO/IEC 24824-1 (Fast Infoset), International Telecommunication Union (ITU), May 2005. http://www.itu.int/rec/T-REC-X.891/.
Theory, Benefits and Requirements for Efficient Encoding of XML Documents, AgileDelta, Inc. http://www.agiledelta.com/EfficientXMLEncoding.htm.
Japex Manual, Santiago Pericas-Geertsen, java.net, April 2006. https://japex.dev.java.net/docs/manual.html.
HTTP/1.1 Content Negotiation, R. Fielding et al, World Wide Web Consortium, June 1999. http://www.w3.org/Protocols/rfc2616/rfc2616.html.
W3C EXI WG and Fujitsu, Takuki Kamiya, Fujitsu, June 2006. http://software.fujitsu.com/en/interstage-xwand/activity/xbrltools/indexFXDI.html.

9. Appendices

9.1. Appendix A: Measurement Details

Until the measurements stabilize, please refer to the Analysis of the EXI Measurements for up-to-date status of the measurements system implementation, and the status of each candidate under test.

9.2. Appendix B: Description of Measurements Test Suite

Submitted: 21-May, RB. Integrated: 23-May, GW. Edited: 1st pass 23-May Some items needed: SyncML and XMPP Instant Messaging have no test cases. Some fidelity evaluations are missing.

The test data suite used for the measurements described in this document, is fully described in an external addendum - the Analysis of the EXI Measurements. The test suite is organized into "test groups". Each group consists of XML document examples which pertain to the same vocabulary, or to related applications of XML. For each group, information is provided on the type of data it contains, and on the use cases from XML Binary Characterization Use Cases [XBC Use Cases] to which it relates.

The test suite presently contains more than 10,000 documents.

9.3. Appendix C: Further Work

  1. Scenarios: A "scenario" is the systematic implementation of some XML system, congruent to a given use-case. That is, it is the system architecture, environment, and choice of optimizing parameters of an implementation of an EXI format in a given situation. To characterize the performance of various textual XML and alternative binary formats, as they would be used in each specific use-case environment, we would closely define the scenario for each use case. For our testing purposes, a scenario would be encapsulated by a japex driver and a set of japex parameters and their values.
  2. Streaming: We intended to measure textual XML and binary formats in streaming scenarios.
  3. Native Implementation Comparison: After looking at results for more properties, if we have to evaluate whether a given natively implemented format and processor are algorithmically superior to a format for which we only have a Java implementation, we would seek other work which has made isomorphic comparisons of Java to "C/C++" for applications with the same CPU and I/O distribution, and compare the difference in performance in that work.
  4. Framework Network Driver: Take measurements over various networks using the network drivers in the measurements framework.
  5. Evaluate Advisability by Use Case: For each (generalized) use case, use the cross-reference of each test group to each use case, given in the Analysis of the EXI Measurements, plus the analysis of the results for each test group, to determine whether or not the improved results would indicate the advisability of an EXI format for that use case. For example, the use case Metadata in Broadcast Systems is presently represented by 2 test groups, BCAST and CBMS. If an aggregate relative compactness measurement over the test groups is not competitive with any hand-coded binary formats now used in Broadcast Systems, then improved compactness alone would be unlikely to justify an EXI recommendation from the perspective of the Broadcast Systems use case. This numerical evaluation might be combined with external factors such as industry pressure for a standard format.
  6. Evaluate Fidelity: Evaluate candidates on the metric of fidelity outlined in Table 1 to evaluate the XBC property of Roundtrip Support.

9.4. Appendix D: Scenario for Interoperability of XML and EXI using HTTP

This appendix describes a simple scenario for the transfer of a new application level file format over HTTP. We illustrate the case for a binary file format whose content corresponds to equivalent XML (which we'll call here 'EXI'), as this document describes, but the description is not necessarily confined to that objective. Nor is this intended to be a normative account. However, this is the scenario presently used in commercial production situations by some of the candidate format contributions described above.

In a heterogeneous distributed system consisting of clients and services that produce and consume textual XML, it would be important when introducing an EXI format to ensure that XML interoperability is not unduly affected. Since at any time, some but not all of the clients or services might support an EXI format, we would like that in a single HTTP interoperation between two nodes, they use EXI if they both understood it, and their configurations allowed it, but to use textual XML otherwise.

The HTTP 1.1 protocol specifies a feature called "agent-driven negotiation" (See [CN], part 12.2). This feature may be used to determine whether an HTTP client and service are capable of communicating using XML or an EXI format, and furthermore to make the interoperation based on the optimal capabilities of each.

Agent-driven negotiation is driven from the client. The client request informs the service of its capabilities and the service will respond according to its own capabilities which are compatible with the client.

The "Accept request-header field" ([CN], part 14.1) can be used to specify certain media types which are acceptable for the response.

If a client that did support XML and EXI formats performed an HTTP GET, it would include in the GET request an Accept request-header field containing the XML mime type and an EXI format MIME type. The service processing the request knows that the client accepts XML and the EXI format, and that either is acceptable for the response.

If a client that did support XML and EXI formats performed an HTTP POST, then there would be two possibilities.

First, the client might assume that the service did support the EXI format, and send the request in the EXI format. If the service did not support the EXI format then it would return an HTTP 415 error, unsupported media type, and the client would fall back to XML for a second and subsequent requests.

Second, the client can assume the service did not support the EXI format. It would then send the request in XML, and include an Accept request-header field containing the XML mime type and the EXI format MIME type. The service then knows that the client supports the EXI format and if it were capable of EXI then it would reply using the EXI format. Otherwise the service would reply using XML. For the second and subsequent requests the client would use the format in which the server replied to the first request, so if the server replied using the EXI format then the second and subsequent requests would be sent using the EXI format.