XML Reference Guide

Mar 14, 2003

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WS-Notifications is a specification -- or rather, a group of specifications -- that enables you to set up a system in which one part of your application can request to be notified when a particular event occurs. The standardization of this process enables you to create a publish-subscribe system, easily adding or removing "topics" for which a component wishes to receive notifications.

For example, we have previously talked about the Web Services Resource Framework (WSRF), which gets around the problem of the stateless nature of Web services by creating WS-Resources, or Web service requests that include information to identify a specific resource. One common use for WS-Notifications is for one resource to subscribe to receive notifications when changes occur in another resource. For example, you might have a service that monitors a database. That service will need to be notified in the event that the databases log files fill up, or if a particular threshold is reached.

In its simplest form, the system involves a notificationProducer, which sends notifications to a notificationConsumer. These notifications get sent in response to specific "situations", but only when the consumer has been subscribe to a particular "topic". For example, I might have a service that subscribes to the topic "diskFull":

When you come down to it, a NotificationConsumer is just a Web service designed to receive specific messages. For example, we might define it with this WSDL file:

<?xml version="1.0" encoding="UTF-8"?>
<definitions name="DatabaseAdministrator" targetNamespace="http://example.com/database"
    xmlns="http://schemas.xmlsoap.org/wsdl/" xmlns:tns="http://example.com/database"
    xmlns:wsa="http://schemas.xmlsoap.org/ws/2004/03/addressing"
    xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
    xmlns:wsntw="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification-1.2-draft-01.wsdl"
    xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/">
    
    <wsdl:import namespace="http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ResourceProperties-1.2-draft-01.wsdl"
        location="WS-BaseN.wsdl" />

    
    <types>
        <xsd:schema targetNamespace="http://example.com/database" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
            <xsd:import namespace="http://schemas.xmlsoap.org/ws/2004/03/addressing" schemaLocation="WS-Addressing.xsd" />
        </xsd:schema>    
    </types>
    
    <binding name="ConsumerSoapBinding"  type="wsntw:NotificationConsumer">

        <soap:binding style="document" transport="http://schemas.xmlsoap.org/soap/http"/>
        <operation name="Notify">
            <input>
                <soap:body use="literal"/>
            </input>
            <output>

                <soap:body use="literal"/>
            </output>
        </operation>
    </binding>
        
    <service name="ConsumerService">
      <port name="ConsumerPort" binding="tns:ConsumerSoapBinding">

          <soap:address location="http://example.com/databaseMessageConsumer"/>
      </port>
    </service>

</definitions>

When you come right down to it, this is a pretty simple service. It has only one operation, Notify, that gets defined in the imported WS-BaseN.wsdl document, part of the WS-Notification specification.

To subscribe to a topic, the NotificationConsumer sends a message to the NotificationProducer:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"
       xmlns:db="http://example.org/databaseSystem"
       xmlns:wsa="http://www.w3.org/2005/02/addressing"
       xmlns:wsnt="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification-1.2-draft-01.xsd">
    <SOAP-ENV:Header>
       <wsa:Action>
           http://docs.oasis-open.org/wsn/2004/06/WS-BaseNotification/Subscribe
       </wsa:Action>
       <wsa:To SOAP-ENV:mustUnderstand="1">http://example.com/database</wsa:To>

       <db:DatabaseId>CINDB</db:DatabaseId>
    </SOAP-ENV:Header>
    <SOAP-ENV:Body>
       <wsnt:Subscribe>
          <wsnt:ConsumerReference>

              <wsa:EndpointReference>
                 <wsa:Address>http://example.com/databaseMessageConsumer</wsa:Address>
              </wsa:EndpointReference>
          </wsnt:ConsumerReference>
          <wsnt:TopicExpression Dialect="http://docs.oasis-open.org/wsn/2004/06/TopicExpression/Simple">

               db:diskFull
          </wsnt:TopicExpression>
          <wsnt:UseNotify>true</wsnt:UseNotify>
          <wsnt:InitialTerminationTime>
              2007-12-31T12:00:00
          </wsnt:InitialTerminationTime>
       </wsnt:Subscribe>

    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

The Header includes the fact that we are trying to create a subscription, as well as the resource to which we are attempting to subscribe, the database identified as CINDB. The actual subscription requests to includes the And pointReference for the consumer, which in this case is a simple URL. It also includes the topic or topics to which we want to subscribe (in this case, the diskFull topic), whether we want to use the Notify form (the alternative, raw messages, is much more difficult to specify in a WSDL file) and how long we want the subversion to continue. Note that this is the InitialTerminationTime for the subscription, and can be extended at any time.

The producer sends back a confirmation message:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"
        xmlns:db="http://example.org/databaseSystem"
        xmlns:wsa="http://www.w3.org/2005/02/addressing"
        xmlns:wsnt="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification-1.2-draft-01.xsd">
    <SOAP-ENV:Header>

       <wsa:Action>
           http://docs.oasis-open.org/wsn/2004/06/WS-BaseNotification/SubscribeResponse
       </wsa:Action>
       <wsa:To SOAP-ENV:mustUnderstand="1">
            http://example.com/databaseMessageConsumer
       </wsa:To>
    </SOAP-ENV:Header>

    <SOAP-ENV:Body>
       <wsnt:SubscribeResponse>
          <wsnt:SubscriptionReference>
              <wsa:Address>
                  http://example.com/databaseSubscriptions 
              </wsa:Address>
              <wsa:ReferenceProperties>

                  <db:ResourceID>39983872</db:ResourceID>
              </wsa:ReferenceProperties>
          </wsnt:SubscriptionReference>
       </wsnt:SubscribeResponse>
    </SOAP-ENV:Body>

</SOAP-ENV:Envelope>

The response message includes the ResourceID for the subscription itself, so the client can extend or cancel it at any time. So engines can also be paused and resumed.

The actual notification comes in the form of a SOAP message, the body of which might look something like this:

<wsnt:Notify>
    <wsntw:NotificationMessage>

       <wsnt:Topic Dialect="http://docs.oasis-open.org/wsn/2004/06/TopicExpression/Simple">

           db:diskFull
       </wsnt:Topic>

       <wsnt:ProducerReference>
           <wsa:EndpointReference xmlns:wsa="http://www.w3.org/2005/02/addressing"
               xmlns:db="http://example.org/databaseSystem">
              <wsa:Address>http://example.com/database</wsa:Address>
              <wsa:ReferenceProperties>

                  <db:ResourceId>RES1228</db:ResourceId>
              </wsa:ReferenceProperties>
           </wsa:EndpointReference>
       </wsnt:ProducerReference>

       <wsnt:Message>

           <db:DiskFullMessage>
              <db:MessageTitle>
                  Disk Full
              </db:MessageTitle>
              <db:MessageContent>
                  The Q: drive, on which the database stores its log files, is now at 95% of capacity.  Please take action to resolve this issue.
              </db:MessageContent>

           </db:DiskFullMessage>
       </wsnt:Message>

    </wsntw:NotificationMessage>
</wsnt:Notify>

The message starts by specifying the topics for which this message contains notifications. A NotificationProducer may send notifications for more than one topic at a single message, if the consumer allows it. Next, the message specifies the NotificationProducer from which the notification is being sent. The ResourceId referenced in the EndpointReference refers to the actual subscription, making it straightforward for the consumer to cancel (or destroy) its subscription if necessary. (Don't you wish spam messages came with such a capability?) Finally, we have the message itself, which includes the message title and content.

Available topics are defined in a topicSpace, which is a collection of one or more topicTrees. For example:

<wstop:topicSpace name="DatabaseTopics" 
     targetNamespace="http://example.com/database" 
     xmlns:db="http://example.org/database"
     xmlns:wstop="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-Topics-1.2-draft-01.xsd">
   <wstop:topic name="Systems">
       <wstop:topic name="ResourceLevels">
          <wstop:topic name="diskFull"/>
          <wstop:topic name="CPULockedUp" />

          <wstop:topic name="networkSwamped" />
       </wstop:topic>
       <wstop:topic name="Errors">
          <wstop:topic name="unexpectedShutdown" />
          <wstop:topic name="diskFull" />
          <wstop:topic name="driveCorruption"/>

          <wstop:topic name="DOSAttack"/>
          <wstop:topic name="alienAttack"/>
       </wstop:topic>
  </wstop:topic> 
  <wstop:topic name="Databases">
       <wstop:topic name="Active">

           <wstop:topic name="EmployeeDB"/>
           <wstop:topic name="InventoryDB"/>
       </wstop:topic>
       <wstop:topic name="Inactive">
           <wstop:topic name="VacationRequestsDB"/>
       </wstop:topic>

  </wstop:topic>
</wstop:topicSpace>

So in this case, a consumer could actually choose from Systems/ResourceLevels/diskFull and Systems/Errors/diskFull.

The actual NotificationProducer is just a Web service with additional ResourceProperties and operations. (For a refresher on Resource Properties, see the guide entry on WSRF.) For example, the service might have the following Resource Property:

<wsn:NotificationProducerRP xmlns:wsn="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification-1.2-draft-01.xsd">

    <wsn:Topic>diskFull</wsn:Topic>
    <wsn:Topic>systemReset</wsn:Topic>
    <wsn:FixedTopicSet>false</wsn:FixedTopicSet>
    <wsn:TopicExpressionDialects>

       http://docs.oasis-open.org/wsn/2004/06/TopicExpression/Simple
    </wsn:TopicExpressionDialects>
</wsn:NotificationProducerRP>

In this case, the producer is advertising that it sends notifications on the diskFull and systemReset topics, but that a consumer can subscribe to additional topics, and the NotificationProducer will send them notifications applicable. The producer has also specified that it expects subscription topics to come in the "simple" dialect, meaning that all topics are at the root of the topicSpace.

Defining a NotificationProducer itself involves making sure the appropriate operations and elements have been defined:

<?xml version="1.0" encoding="UTF-8"?>
<definitions name="Database"
    targetNamespace="http://example.com/database"
    xmlns="http://schemas.xmlsoap.org/wsdl/"
    xmlns:tns="http://example.com/database"
    xmlns:wsa="http://schemas.xmlsoap.org/ws/2004/03/addressing"
    xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
    xmlns:wsrp="http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ResourceProperties-1.2-draft-01.xsd"
    xmlns:wsrpwsdl= "http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ResourceProperties-1.2-draft-01.wsdl"
    xmlns:wsrl="http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ResourceLifetime-1.2-draft-01.xsd"
    xmlns:wsrlwsdl= "http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ResourceLifetime-1.2-draft-01.wsdl"
    xmlns:wssg="http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ServiceGroup-1.2-draft-01.xsd"
    xmlns:wssgwsdl= "http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ServiceGroup-1.2-draft-01.wsdl"
    xmlns:wsnt="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification-1.2-draft-01.xsd"
    xmlns:wsntw= "http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification-1.2-draft-01.wsdl"
    xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/">

    
    <wsdl:import namespace="http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ResourceProperties-1.2-draft-01.wsdl"
              location="WS-ResourceProperties.wsdl" />
    
    <wsdl:import namespace="http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ResourceLifetime-1.2-draft-01.wsdl"
        location="WS-ResourceLifetime.wsdl" />
    
    <wsdl:import namespace="http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ServiceGroup-1.2-draft-01.wsdl"
        location="WS-ServiceGroup.wsdl" />

    <wsdl:import namespace="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification-1.2-draft-01.wsdl"
        location="WS-BaseN.wsdl" />
    
    <types>
        <xsd:schema 
           targetNamespace="http://example.com/database"
           xmlns:xsd="http://www.w3.org/2001/XMLSchema">

            
            <xsd:import 
                 namespace="http://schemas.xmlsoap.org/ws/2004/03/addressing"
                   schemaLocation="WS-Addressing.xsd" />

            <xsd:import namespace="http://docs.oasis-open.org/wsrf/2004/06/wsrf-WS-ResourceLifetime-1.2-draft-01.xsd"
             schemaLocation="WS-ResourceLifetime.xsd" />

            <xsd:element name="createDatabase">
                <xsd:complexType/>
            </xsd:element>

            
            <xsd:element name="createDatabaseResponse">
                <xsd:complexType>
                    <xsd:sequence>
                        <xsd:element ref="wsa:EndpointReference"/>
                    </xsd:sequence>
                </xsd:complexType>

            </xsd:element>
            
            <xsd:element name="GenericDatabaseProperties">
              <xsd:complexType>
                <xsd:sequence>
                   <xsd:element ref="wsrl:CurrentTime" minOccurs="1" maxOccurs="1"/>
                   <xsd:element ref="wsrl:TerminationTime" minOccurs="1" maxOccurs="1"/>

                   <xsd:element ref="wsnt:Topic" minOccurs="1" maxOccurs="unbounded"/>
                   <xsd:element ref="wsnt:FixedTopicSet" minOccurs="1" maxOccurs="1"/>
                   <xsd:element ref="wsnt:TopicExpressionDialects" minOccurs="1" maxOccurs="unbounded"/>
                   <xsd:any/>
                </xsd:sequence>
              </xsd:complexType>

            </xsd:element>            
            
        </xsd:schema>
        
    </types>
...
    <portType name="DatabasePortType" wsrp:ResourceProperties="tns:GenericDatabaseProperties">
...
        <wsdl:operation name="Subscribe">

            <wsdl:input message="wsntw:SubscribeRequest" 
               wsa:Action="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification/Subscribe"/>
            <wsdl:output message="wsntw:SubscribeResponse" 
               wsa:Action="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification/SubscribeResponse"/>
            <wsdl:fault  name="ResourceUnknownFault" 
                message="wsntw:ResourceUnknownFault" />
            <wsdl:fault  
                name="SubscribeCreationFailedFault" 
                message="wsntw:SubscribeCreationFailedFault"/>
            <wsdl:fault 
                name="TopicPathDialectUnknownFault" 
                message="wsntw:TopicPathDialectUnknownFault"/>
        </wsdl:operation>

        
        <wsdl:operation name="GetCurrentMessage">
            <wsdl:input message="wsntw:GetCurrentMessageRequest"
               wsa:Action="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification/GetCurrentMessage"/>
            <wsdl:output message="wsntw:GetCurrentMessageResponse"
          wsa:Action="http://docs.oasis-open.org/wsn/2004/06/wsn-WS-BaseNotification/GetCurrentMessageResponse"/>
            <wsdl:fault  name="ResourceUnknownFault" 
                message="wsntw:ResourceUnknownFault" />
            <wsdl:fault  
                name="InvalidTopicExpressionFault" 
                message="wsntw:InvalidTopicExpressionFault" />
            <wsdl:fault  name="TopicNotSupportedFault" 
                message="wsntw:TopicNotSupportedFault" />

            <wsdl:fault  
                name="NoCurrentMessageOnTopicFault" 
                message="wsntw:NoCurrentMessageOnTopicFault" />
        </wsdl:operation>
        
    </portType>
    
    <binding name="DatabaseSoapBinding" type="tns:DatabasePortType">
        <soap:binding style="document" transport="http://schemas.xmlsoap.org/soap/http"/>
...
        <operation name="renewDatabase">

            <input>
                <soap:body use="literal"/>
            </input>
            <output>
                <soap:body use="literal"/>
            </output>

        </operation>
        <operation name="Subscribe">
            <input>
                <soap:body use="literal"/>
            </input>
            <output>

                <soap:body use="literal"/>
            </output>
        </operation>
        <operation name="GetCurrentMessage">
            <input>
                <soap:body use="literal"/>

            </input>
            <output>
                <soap:body use="literal"/>
            </output>
        </operation>
    </binding>

...

By adding these two operations, the guts of which are defined in the imported specifications, you can turn any Web service into a NotificationProducer.

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