This specification intends to meet the following requirements:

While the general pattern of asynchronous, event-based messages is extremely common, different applications often require different notification delivery mechanisms. For instance, in some cases a simple asynchronous message is optimal, while other situations may work better if the event consumer can poll for notification in order to control the flow and timing of message arrival. Some consumers will require notifications to be wrapped in a standard "event" SOAP envelope, while others will prefer messages to be delivered unwrapped. Some consumers may require notifications to be delivered reliably, while others may be willing to accept best-effort event delivery.

In order to support this broad variety of event delivery requirements, this specification introduces an abstraction called a Delivery Mode and an abstraction called Delivery Format. These concepts are used as an extension points, so that event sources and event consumers may freely create new delivery mechanisms that are tailored to their specific requirements. This specification provides a minimal amount of support for delivery mode and format negotiation by allowing an event source to provide a list of supported delivery modes and formats in response to a subscription request specifying a delivery mode or format it does not support.

When the Delivery Format feature is engaged the formatting of the going events occurs after any filtering. This ensures that regardless of what type of formatting might occur, the same Filter dialect/expression can be used to subset the event stream.

This specification defines a single delivery mode, Push Mode, which is simple asynchronous messaging.

This specification specifies two delivery formats: wrapped and unwrapped. Use of wrapped format indicates that notification messages should be contained in a wrapper element. Use of unwrapped format indicates that notification messages are not wrapped.

In some scenarios the event source itself manages the subscriptions it has created. In other scenarios, for example a geographically distributed publish-and-subscribe system, it may be useful to delegate the management of a subscription to another Web service. To support this flexibility, the response to a subscription request to an event source will include the EPR of a service that the subscriber may interact with to manage this subscription. This EPR should be the target for future requests to renew or cancel the subscription. It may address the same Web service (Address and ReferenceParameters) as the event source itself, or it may address some other Web service to which the event source has delegated management of this subscription; however, the full subscription manager EPR (Address and Reference Parameters) must be unique for each subscription.

We use the term "subscription manager" in this specification to refer to the Web service that manages the subscription, whether it is the event source itself or some separate Web service.

lists a hypothetical request to create a subscription for storm warnings.

Lines (07-09) in indicate the message is a request to create a subscription, and Line (16) indicates that it is sent to a hypothetical event source of ocean events.

While Lines (13-15) indicate where a reply should be sent, Lines (20-29) indicate where and how notifications should be delivered; there is no requirement that these match. The absence of a Mode attribute on Line (20) indicates that notifications should be delivered using Push mode; that is, they should be asynchronously sent as SOAP messages to the endpoint described in lines (21-28). Note that Lines (25-27) illustrate a typical pattern where the event sink lists a reference parameter (Line 26) that identifies the subscription and will be included in each notification.

lists a hypothetical response to the request in .

Lines (07-09) in indicate this message is a response to a request to create a subscription, and Lines (10-12) indicate that it is a response to the request in . Lines (17-26) provide the subscription manager EPR for this subscription, and Line (27) indicates the subscription will expire in 30 hours unless renewed.

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 .

This specification uses the following syntax to define normative outlines for messages:

In addition to Message Information Header properties , this specification uses the following properties to define messages:

These properties bind to a SOAP Envelope as follows:

The elements defined in this specification MAY be extended at the points indicated by their outlines and schema. Implementations MAY add child elements and/or attributes at the indicated extension points but MUST NOT contradict the semantics of the parent and/or owner, respectively. If a receiver does not recognize an extension, the receiver SHOULD ignore that extension. Senders MAY indicate the presence of an extension that has to be understood through the use of a corresponding SOAP Header with a soap:mustUnderstand attribute with the value "1".

Extension elements and attributes MUST NOT use the Web Services Eventing namespace URI.

The XML namespace URI that MUST be used by implementations of this specification is:

lists XML namespaces that are used in this specification. The choice of any namespace prefix is arbitrary and not semantically significant.

The working group intends to update the value of the Web Services Eventing namespace URI each time a new version of this document is published until such time that the document reaches Candidate Recommendation status. Once it has reached Candidate Recommendation status, the working group intends to maintain the value of the Web Services Eventing namespace URI that was assigned in the Candidate Recommendation unless significant changes are made that impact the implementation or break post-CR implementations of the specification. Also see http://www.w3.org/2001/tag/doc/namespaceState.html and http://www.w3.org/2005/07/13-nsuri .

An implementation is not compliant with this specification if it fails to satisfy one or more of the MUST or REQUIRED level requirements defined herein. A SOAP Node MUST NOT use the XML namespace identifier for this specification (listed in ) within SOAP Envelopes unless it is compliant with this specification.

Normative text within this specification takes precedence over the XML Schema and WSDL descriptions, which in turn take precedence over outlines, which in turn take precedence over examples.

All messages defined by this specification MUST be sent to a Web service that is addressable by an EPR (see ).

To create a subscription, a subscriber sends a request message of the following form to an event source:

The following describes additional, normative constraints on the outline listed above:

Other message information headers defined by WS-Addressing MAY be included in the request and response messages, according to the usage and semantics defined in WS-Addressing.

Other components of the outline above are not further constrained by this specification.

If included within the Subscribe request message, the wse:NotifyTo and wse:EndTo SHOULD have some cursory validity checking performed before the Subscribe response is returned. While not all errors can be detected prior to sending a message to those EPRs, some obvious ones can be detected. For example, an unsupported transport specified within the wsa:Address IRI. Detecting these errors during Subscribe processing will lessen the chances of the subscriber creating an unusable subscription. If this check is performed and a problem is detected then the event source MAY generate a wse:UnusableEPR fault rather than returning the SubscribeResponse message.

If the event source accepts a request to create a subscription, it MUST reply with a response of the following form:

The following describes additional, normative constraints on the outline listed above:

Other components of the outline above are not further constrained by this specification.

If the event source chooses not to accept a subscription, the request MUST fail, and the event source MAY generate a wse:EventSourceUnableToProcess fault indicating that the request was not accepted.

lists another hypothetical request to create a subscription.

Like the request in , Lines (07-09) of indicate the message is a request to create a subscription. Line (19) indicates that it is sent to a hypothetical event source of ocean events.

Lines (13-18) indicate where to send the response to this request, Lines (23-30) indicate where to send a SubscriptionEnd message if necessary, and Lines (31-34) indicate how and where to send notifications.

Line (41) indicates the event sink would prefer to have the subscription expire on 26 June 2004 at 9:07 PM Pacific time.

Lines (42-45) indicate the event sink only wants weather reports where topic is storms, using a custom filter dialect.

lists a hypothetical response to the request in .

Like the response in , Lines (08-10) of indicate this message is a response to a request to create a subscription, and Lines (11-13) indicate that it is a response to the request in . Lines (14-17) indicate the response is sent to the event sink indicated in Lines (13-18) of . Lines (21-30) provide the address of the subscription manager for this subscription; note that this particular response uses the global wse:Identifier element defined by this specification. Finally, Line (31) indicates the subscription will expire on 1 July 2004 unless renewed; there is no requirement that this time be necessarily longer or shorter than the requested expiration (Line (41) of ).

To update the expiration for a subscription, subscription managers MUST support requests to renew subscriptions.

To renew a subscription, the subscriber sends a request of the following form to the subscription manager:

Components of the outline listed above are additionally constrained as for a request to create a subscription (see ). Other components of the outline above are not further constrained by this specification.

If the subscription manager accepts a request to renew a subscription, it MUST reply with a response of the following form:

Components of the outline listed above are constrained as for a response to a subscribe request (see ) with the following addition(s):

If the subscription manager chooses not to renew this subscription, the request MUST fail, and the subscription manager MAY generate a wse:UnableToRenew fault indicating that the renewal was not accepted.

Other components of the outline above are not further constrained by this specification.

lists a hypothetical request to renew the subscription created in .

Lines (07-09) indicate this is a request to renew a subscription. Lines (19-21) contain the reference parameter that indicates the subscription to be renewed is the one created in . Line (25) in indicates the request is to extend the subscription until 26 June 2004 at 9:07 PM Pacific.

lists a hypothetical response to the request in .

Line (17) in indicates the subscription has been extended only until 26 June 2004 at noon.

To get the status of a subscription, the subscriber sends a request of the following form to the subscription manager:

Components of the outline listed above are additionally constrained as for a request to renew a subscription (see ). Other components of the outline above are not further constrained by this specification.

If the subscription is valid and has not expired, the subscription manager MUST reply with a response of the following form:

Components of the outline listed above are constrained as for a response to a renew request (see ). Other components of the outline above are not further constrained by this specification.

lists a hypothetical request to get the status of the subscription created in .

Lines (07-09) indicate this is a request to get the status of a subscription. Lines (16-21) indicate that the request is sent to the subscription manager for the subscription created in .

lists a hypothetical response to the request in .

Line (17) in indicates the subscription will expire on 26 June 2004 at noon.

Though subscriptions expire eventually, to minimize resources, the subscribing event sink SHOULD explicitly delete a subscription when it no longer wants notifications associated with the subscription.

To explicitly delete a subscription, a subscribing event sink sends a request of the following form to the subscription manager:

Components of the outline above are additionally constrained only as for a request to renew a subscription (see ). For example, the faults listed there are also defined for a request to delete a subscription.

If the subscription manager accepts a request to delete a subscription, it MUST reply with a response of the following form:

Components of the outline listed above are not further constrained by this specification.

lists a hypothetical request to delete the subscription created in .

Lines (07-09) in indicate the message is a request to delete a subscription. Lines (16-21) indicate that the request is addressed to the manager for the subscription created in .

lists a hypothetical response to the request in .

If the event source terminates a subscription unexpectedly, the event source SHOULD send a Subscription End SOAP message to the endpoint reference indicated when the subscription was created (see ). This endpoint reference MUST refer to an endpoint that supports the SubscriptionEndPortType portType. The message MUST be of the following form:

The following describes additional, normative constraints on the outline listed above:

Other message information headers defined by WS-Addressing MAY be included in the message, according to the usage and semantics defined in WS-Addressing.

Other components of the outline above are not further constrained by this specification.

lists a hypothetical SubscriptionEnd message corresponding to an early termination of the subscription created in .

Line (08) is the action URI for Subscription End. Lines (10-13) indicate the message is sent to the EndTo of the subscribe request (Lines (23-30) in .). Line (17) indicates the event source is shutting down, and Lines (18-20) indicate that the event source was going off line.

The following element is used to convey additional information in the faults.

This fault is sent when a Subscribe request specifies a delivery mode that is not supported by the event source. Optionally, this fault may contain a list of supported delivery mode URIs in the Detail property.

This fault is sent when a Subscribe request specifies an expiration time that is in the past or an expiration duration of zero.

This fault is sent when a Subscribe request specifies an expiration time and the event source is only capable of accepting expiration durations; for instance, if the event source does not have access to absolute time.

This fault is sent when a Subscribe request contains a filter and the event source does not support filtering.

This fault is sent when a Subscribe request specifies a filter dialect that the event source does not support. Optionally, this fault may contain a list of supported filter dialect URIs in the Detail property.

This fault is sent when the event source is not capable of fulfilling a Subscribe request for local reasons unrelated to the specific request.

This fault is sent when the event source is not capable of fulfilling a Renew request for local reasons unrelated to the specific request.

If a request message does not comply with the corresponding outline listed above, the request MUST fail and the event source or subscription manager MAY generate the following fault indicating that the request is invalid:

This fault is sent when a Subscribe request specifies a delivery mode that is not supported by the event source. Optionally, this fault may contain a list of supported delivery mode URIs in the Detail property.

This fault MAY be generated when an Event Source detects a wse:Subscribe request containing a filter that, for whatever reason, will never evaluate to true.

This fault MAY be generated when an Event Source detects that the wse:NotifyTo or wse:EndTo EPR is unusable.

It is strongly RECOMMENDED that the communication between services be secured using the mechanisms described in WS-Security . In order to properly secure messages, the body and all relevant headers need to be included in the signature. Specifically, any headers identified in the <wse:NotifyTo> element and standard messaging headers, such as those from WS-Addressing , need to be signed with the body in order to "bind" the two together. For messages with empty bodies, the <s12:Body> element should be signed so content cannot be added in transit.

Different security mechanisms may be desired depending on the frequency of messages. For example, for infrequent messages, public key technologies may be adequate for integrity and confidentiality. However, for high-frequency events, it may be more performant to establish a security context for the events using the mechanisms described in WS-Trust and WS-SecureConversation .

It should be noted that if a shared secret is used it is RECOMMENDED that derived keys be used to strengthen the secret as described in WS-SecureConversation.

The following list summarizes common classes of attacks that apply to this protocol and identifies the mechanism to prevent/mitigate the attacks:

It is important for event sources to properly authorize requests. This is especially true for Subscribe requests, as otherwise the ability to subscribe on behalf of a third-party event sink could be used to create a distributed denial-of-service attack.

Some possible schemes for validating Subscribe requests include:

Other mechanisms are also possible. Note that event sources that are not reachable from the Internet have less need to control Subscribe requests.