WS-Notification: Overview
WS-Notification enables web services to use the publish and subscribe messaging pattern.
WS-Notification became a stabilized feature in WebSphere® Application Server Version 8.5.5.
You use publish and subscribe messaging to publish one message to many subscribers. In this pattern a producing application inserts (publishes) a message (event notification) into the messaging system having marked it with a topic that indicates the subject area of the message. Consuming applications that have subscribed to the topic in question, and have appropriate authority, all receive an independent copy of the message that was published by the producing application. Any consuming application can further filter messages for a given topic, by using a message content filter that is evaluated over the XML message content of the message body.
The WS-Notification implementation in WebSphere Application Server supports the WS-Notification standards, complies with the WS-I Basic Profile 1.0 requirements, and composes with other related standards such as WS-Addressing for High Availability and Workload Management, and WS-ReliableMessaging for reliable communication between components. At an application level, this enables a standardized approach for web service applications to participate in the publish and subscribe messaging pattern, whether this be listening for notification of a particular event occurrence, or inserting event notifications into the system for consumption by other applications or system management tooling. The open-standards nature of this web services specification mean that applications can communicate with each other irrespective of the underlying hardware platforms, software languages or vendor environments.
The WS-Notification standards
WebSphere Application Server implements the WS-Notification Version 1.3 family of standards that are developed under the supervision of the Organization for the Advancement of Structured Information Standards (OASIS). These standards define web service message exchanges that enable web service applications to use the publish and subscribe messaging pattern.
- WS-BaseNotification Version 1.3 OASIS Standard, which defines basic producer/consumer application roles, and the filtering of message content through a selector expression.
- WS-BrokeredNotification Version 1.3 OASIS Standard, which extends Base Notification to define a broker role.
- WS-Topics Version 1.3 OASIS Standard, which defines topic syntaxes that can be used by implementers of either base notification or brokered notification.
- WS-ReliableMessaging allows web service endpoints to be configured to ensure that web service operations are invoked reliably across inherently unreliable transports such as HTTP. The WS-Notification standard does not guarantee the reliability with which messages are published or received by applications, so you must compose WS-Notification with WS-ReliableMessaging to provide reliability.
- WS-Distributed Management (WS-DM) defines specialized applications that are WS-Notification NotificationProducers, and a topic namespace document that describes the topics on which these applications should emit event notifications to provide management of a resource (such as a printer) by a web services client.
The WS-Notification implementation in WebSphere Application Server
The key component of this implementation is the notification broker. This is a point of separation between producing applications that want to insert event notifications into the system, and consuming applications that want to receive the event notifications. WebSphere Application Server provides this broker ready for use, so that applications can concentrate on the business level functional requirements of sending and receiving events without needing to implement the more complex infrastructure aspects of the WS-Notification specifications, such as maintaining lists of active subscribers; parsing and matching topics and wildcards; distributing event notifications to subscribers; handling subscription lifecycles. This separation between producing and consuming business application means that the producer and consumer applications do not have to be available at the same time in order for them to communicate. The broker retains a publication until the consumer becomes available.
- A web service application contacts the server by using the Web service endpoints exposed by the WS-Notification service point.
- The endpoint passes this invocation request through to the notification broker, which is responsible for parsing the request information and taking the appropriate action depending upon the type of request received.
The following figure shows an application server that contains a notification broker and a messaging engine. Within the messaging engine, there is a durable subscription and a bus topic space. Between the application server and the outside world, there is a Web service endpoint. In the outside world, there is a publisher, a subscriber and a notification consumer. The publisher sends a notification message on a given topic, and the subscriber sends a subscribe request on behalf of the notification consumer to subscribe to the same topic. Both these messages are received by the web service endpoint, then routed into the associated broker and on to the topic space. Details of the subscription are filed as a durable subscription. The received notification message is forwarded by the broker to the notification consumer that has subscribed to the topic.
- Version 7.0: Use this type of service if you want to compose a JAX-WS WS-Notification service with web service qualities of service (QoS) via policy sets, or if you want to apply JAX-WS handlers to your WS-Notification service. This is the recommended type of service for new deployments. This WS-Notification option has been available in WebSphere Application Server from Version 7.0.
- Version 6.1: Use this type of service if you want to expose a JAX-RPC WS-Notification service that uses the same technology provided in WebSphere Application Server Version 6.1, including the ability to apply JAX-RPC handlers to the service. This WS-Notification option has been available in WebSphere Application Server from Version 6.1.
- Notification broker
- Subscription manager
- Publisher registration manager
- Notification broker
- Subscription manager
- Publisher registration manager