Tim Berners-Lee
Originally created: 2002-10, last change:
$Date: 2009/08/27 21:38:10 $
Status: Rough amalgam of input from generally reliable sources. Editing status: draft. This is really a set of pointers, as the web services architecture is being elaborated by a whole communit, with the WS-Arch group playing a specific role. Started as notes from W3C Web Services Workshop [@@].

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Web Services

Program Integration across Application and Organization boundaries

Introduction

Web Services mean many things to many people. In the end, there will be a set of standards which allow us to do things we could not do before, but in the mean time different people and companies approach them from different positions, and with different expectations. In 2001-2, Web Services have also been a buzzword used repeatedly and claimed to be one of the hot new technologies. The common themes are:-

The Web in Web Services is, from the first point, a misuse: the term Internet Services would be more appropriate. The Web comes from the second point - the use of the HTTP and XML is already in use as a well-understood and well-debugged set of protocols which support the Web, and so it makes sense to reuse them in providing remote operations and those things connected with them. The third point is what makes web service requirements so different from a local RPC system. The fact that data is exchanged for business purposes and between different social entities means that accountability is required, rather than just reliable transmission.

The architecture of Web Services is the scope of the W3C Web Service Architecture Working Group.

Philosophy

Other articles have dealt with the fundamental architectural difference between remote operations and the architecture of the information space, and the mappings between the two.

The Web Services architecture group has produced various drafts:

The architecture uses the following diagram for the highest level:

Basic Web services architecture graphic

However, the essential part of Web services is the Interact relationship between a Service provider and Service requestor. This is the Web Service. Discovery agencies need not be used - they will in some cases but not in others. The discovery agencies are well represented as a cloud, rather than being a well-defined module in the web services architecture. They will become interface to a huge world of data and query services which provide data about web services as well as many other things.The Interact between between requestor and provider is the essential defining element to web services. As we shall see, the metadata about web services

Technologies within the Web Servcies umbrella

There is a mass of different pieces being bolted onto the foundations of Web Services provided by WSDL and SOAP 1.2 and th diagram implies things considerably. The management layer is a supervisory layer allowing the conrol of the many agents involved in a web services-based operation. The "Application semantics" layer indicates the necessity, for any useful interoperability, to have

Stcak based on XML, and HTTP has WSDL and SOAP 1.2 as WS foundation.

Run Time messaging

The design work of web services is divided between the run time protocols and the descriptions of services.

The W3C work at runtime based on HTTP transport of XML-encoded messages, using the SOAP protocol. (Here by SOAP we mean SOAP 1.2, previous versions including early proprietory submissions which are not standards or guaranteed to interoperate) . There is a bifurcation in the design at this point, as SOAP operates basically in two modes.

In one, the XML message is used to encode the parameters to a remote operation in much the same way as remote method invokation in for example, Corba, DCOM, or RMI. In this mode, XML is used as the marshalling style, but the system is a distributed using remote procedure call in a fairly traditional way.

In the other mode, SOAP carries an XML document, and the task of the receiver is seen more as a document processing operation. This is less rigid than the RPC style.

The document mode of SOAP seems to be getting the most traction in the ecommerce stack. This is not an accident. The XML mode is more flexible than the RPC mode. It is easier in principle to extend an XML-based message system to include more information as a system grows. In fact, RDF is especially powerful in this area, as new information can be parsed into an entity-relationship form by old agents, and it becomes logically clear which parts can be ignored by those who do not understand them.

Functionality which has been mentioned as required above the basic layer at runtime includes:

Description

The descriptions of services are made at various different models and different levels of abstraction in different specs proposed as part of the stack, though there is agreement on WSDL as the modelling of the lowest level, the message or request/response interaction, and the binding to the specific HTTP (say) port at which it happens.

Lots of concepts interconnected

Higher layers in the description above WSDL are known variously as coordination, orchestration, choreography, composition.

They involve (compared with basic WSDL), for example:

Composability and Choreography

Composability of web services refers to the building, from a set of web services, of something at a higher level, typically itself exposed as a larger web service.

Choreography refers more abstractly the part of the description of web services which defines a way, or the ways, in which a acyual invokations to various web services work together. (Peltz uses Choregraphy when it involves multiple parties, and Orchestration when it is internal to one party. Thus the former crosses application boundaries, the latter also crosses organization boundaries. here we use Choregorahy in general for both.)

There is so small amount overlap here, which has led to some confusion. To be general, one might say, for example, that a flight confirmation must involve an already reserved flight. This the actual constraint. One can describe a particular choregraphy (a particular dance, if you like) in which a flight query service is called, and produced a list of flights, and then a reservation service is called to reserve the flight, that is successful, and the resulting reservation is passed to the confirmation service. It may be that there are other ways -- other choreographies -- in which one could have achived a reserved flight. The engineer has the choice of modelling the many possible ways all in one choreogrpahy, or of making several choreographies.

Web services can be combined in such as way that messages are passed around in a very random fashion. However, a particular design techniqe is for a master process to delegate to other services in a recusive tree-like manner, as has been de rigueur in programming languages since Pascal. For example, if the consumer asks the travel agent and the the travel agent books a hotel, the hotel will reply to the travel agent, not to the consumer. This makes everything orderly.

WSCI, BPML and BEPL take this approach to choreography. This is a programming language approach with

WSCI has the empahsis on description, and BPEL on being able to compile to an executable agent. As neither is intended to do the actual calculations or business rules, it would be closer to compare themm with scripting shells such as bash which handle concurrency and synchronization but actually call programs (or rather web services) to do the real work.

See:

@@ Different attitudes - top down program design, or bottom-up agent design, bottom up document design.

Message-oriented choregraphy

The Paper Trail concept is that the state of a mult-agent multi-process system can be looked at, sometimes rather effectively, as a function of the documents which have been transmitted.

The process-oriented attitude to a bank-customer relationship may be "In parallel, the customer writes checks, merchants pay in checks, credit card transactions happen, all month. Then, the charges, interest are assessed and a bank statement sent from the bank to the customer". The document-, or message-oriented one is more like "Every month a bank balance lists valid transaction dated that month. A cleared incoming check in a valid transaction. A cleared outgoing check is a valid transaction. A validated credit card debit is a valid transaction. A check is cleared if it is incoming and there is a matching transfer from the payee bank", and so on. This builds the relationships up in a bottom-up, weblike way. The process-oriented attitude suggests the bank be written as a procedure in a top-down way using for example WSCI and BPL. The document-oriented attitude suggests the use of business rules systems triggered by the receipt of new information -- new documents, in this case new web services messages.

(Web service messages are of course documents just like documents sent in email. Messages are particular in that they have a particular time of transmission, and their document content sdo not change. They do of course generally have identifiers, and even though they can only be accessed by sender and explicit receivers, they can still be regarded as part of the web by those parties.)

Whether the design process is a top-down process-oriented one or a bottom-up document-oriented one, the design will have to be translated into a set of agents and their responses to incoming messages. This manipulation can of course be done automatically.

A concern in all this frantic design is it evolution with time. A BPEL script sets out to be a description of a a business process at a high level. The critical values which decide on conditional execution, or which correlate a particular process with a given transaction, are expressed as parts of the structure of the XML messages. This may lead to what has been called "DTD fragility". What happens which you change the DTD? The design of the message types with XML schema is the sort of thing which is difficult to get everyone in a company to agree on, and tents to change with time. There are many arbitrary choices made as to how the knowledge in the message is serialized as XML. Moving to RDF may, by removing a layer of arbitrary design, reduce that fragility and allow web sevice choregraphy to evolve with time within and outside a company.

Process modeling

When considering a business system with multiple agents and multiple concurrent processes, one would like to have an automated way of checking some fundamental questions.

and so on.

The pi calculus and other calculi derived from it are formal ways of modeling systems with multiple agents and multiple processes. They can do some way to answering these questions. Rule-based systems can also be designed so that proofs can be found of these sort of conjecture. This is a good reason for keeping the languages involved as simple as possible It may be the design reason for the limitations on computational power in WSCI and BPEL.

See petri nets (IBM; and stadford), Pi calculus @@ refs.

Much of the functionality is seen in terms of tying web services down to well-known functionality such as exiting transaction processing systems, PKI trust infrastructure, and so on.

Discovery

In a large number of applications, web servcies will be provided by on one hand and used by on the other peers who have established relationships. Indeed, until a trust infrastructure is fairly developed it is not reasonable to expect computers to do automatic comparison shopping for very many services. Web services will probably (like the web in 1993, and the Semantic Web in 2003) spread first within the corporate firewall, where security problems are minor and mistakes less embarassing than inter-enterprise or publically. However, the goal is that so many web services should be available that it will be important to be able to find them in all kinds of ways.

The UDDI project and the related work on description and query systems is aimed at this. A positive aspect of UDDI is the definition of an ontology for web services. Problems with it are that it is centralized by design, both in the single-tree ontology, and in the design based fundamentally on a central registry, with inter-registry operation as a secondary thing.

From the semantic web point of view, web services are simply one aspect of the many things which will be searched for. Indeed, the fact that a web service is provided may in fact be rather incidental to the essential nature of the business item which is discovered -- a trader in stocks, a seller of lawnmowers, and so on. The semantic web aims to describe any aspect of anything, including the catalogs, parts, materials, services organization, relationships and contracts. A query system which addresses web services only makes sense when smoothly integrated with the rest of the web of enterprise knowledge.

Web Services and Semantic Web

The question of the relationship between these two activities is constantly in the air.

The argument against integration of the technologies is mainly social. It is costly to coordinate very large groups. It is much more effcient to develop WS and SW independently. Neither side has a great incentive to take on the learning required to absorb the needs and potentials fo the other. Using technology in preparation by another group takes a great leap of faith, and does really add to the development time. These are real issues. So while it may take more effort in the long run, it is a better parallelization of the design task to allow web services and semantic web to proceed in together without a mandated link. (This was the apparent consesnsus of the W3C AC meeting in Nice, 2000/11)

That said, wherever overlap of expertise between the technologies occurs, those who form a bridge should do their best to make the conceptual differences as small as possble. There is a Semantic Web Services group, connected to the ______

Service design tools

Most modern software design differentiates strongly between the design of an interface, and the design of the software which implements it.

Web services are required to be composable - you should be able to make a web service implmentation by building it out of component web services. At the low level, think of making a latittude/longitude to state code converter composed from a latittude/longitude to postal code converter and a postal code to state code converter. At a high level, think of a making a vacation being composed of resrvations of flights, hotels and entertainment.

Runtime System management

Real web services have multiple agents running commerical environments, in which downtime is expensive, and incorrect operation could be disasterous. The running, monitoring, provisioning and upgrading of such systems clearly requires tools, but their design is out of scope for this overview.


References, Fodder

http://www.bpmi.org/

web ser vices orchestrationa review of emerging technologies, tools, andstandards, http://devresource.hp.com/drc/technical_white_papers/WSOrch/WSOrchestration.pdf

Related activites elsewhere

RPC history

See: @@@ Web Services Specs

From W3c Web Services Workshop

IBM, Microsoft: "Web Services Framework"

http://www.w3.org/2001/03/WSWS-popa/paper51

Other white papers

@@ HP's ESpeak papers - Web Services roots

From IBM:

http://www-106.ibm.com/developerworks/library/w-ovr/

From Microsoft:

http://msdn.microsoft.com/webservices/default.aspx?pull=/library/en-us/dnglo bspec/html/ws-securitypolicy.asp

http://msdn.microsoft.com/library/en-us/dnglobspec/html/ws-trust.asp?frame=true

http://msdn.microsoft.com/webservices/understanding/whatsnext/default.aspx

Fodder

@@@@@

BPSS From FAQ: Q. What is the relationship between WSCI and ebXML BPSS? A. There is no direct relationship between BPSS and WSCI. They are used for different purposes and have different design centers. BPSS is used for defining the semantics of commercial collaboration between businesses. Its design center is the commercial transactions between two business partners, and as such it provides full commercial semantics, and is designed to work in with conjunction with the ebXML Collaboration Partner Profile/Agreement (CPP/CPA). In contrast, WSCI is used for describing a Web service and the operations performed by that Web service. Its design center is the WSDL service definition, and it describes the relationship between multiple WSDL operations that are performed by a given Web service. Because WSCI is generic to all Web services, it does not provide any explicit commercial semantics, nor does it have any notion of a trading collaboration partner agreement. ]]] -- http://wwws.sun.com/software/xml/developers/wsci/faq.html

@@ concept diagram

Other areas one can contemplate include:


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Tim BL