Documentation > Programming Guide > Registration Meta Events and Procedures

Registration Meta-Events and Procedures

A registration is created within Crossbar.io when a first client issues a registration request for a topic, and it is deleted when the last client unregistrers or its session is disconnected. In between, depending on the invocation rule used during the first registration, other client sessions may be attached to the registration or removed from it.

Registration meta-events give information about these events.

Additionally, there is a set of meta-procedures which can be called to receive information about currently existing registrations.

Meta-events are created by the router itself. This means that the events as well as the data received when calling a meta-procedure can be accorded the same trust level as the router.

Use cases for registration meta-events and procedures

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Events

The set of meta events covers the lifecycle of a registration. A client can subscribe to the following registration meta events:

  • wamp.registration.on_create: Is fired when a registration is created through a registration request for an URI which was previously without a registration.
  • wamp.registration.on_register: Is fired when a session is added to a registration.
  • wamp.registration.on_unregister: Is fired when a session is removed from a registration.
  • wamp.registration.on_delete: Is fired when a registration is deleted after the last session attached to it has been removed.

Note: A wamp.registration.on_register event is always fired subsequent to a wamp.registration.on_create event, since the first registration results in both the creation of the registration and the addition of a session. Similarly, the wamp.registration.on_delete event is always preceded by a wamp.registration.on_unregister event.

The WAMP registration meta events are dispatched by the router to the same realm as the WAMP session which triggered the event.

Important: To receive and process these events, your component will need to have subscribe permission on the respective topic.

Procedures

It is possible to actively retrieve information about registrations via the following procedures.

Retrieving registration IDs

You can retrieve registration IDs via the following three procedures:

  • wamp.registration.list: Returns an object with three lists of the registration IDs for all current registrations for exact matching, prefix matching and wildcard matching.
  • wamp.registration.lookup: Returns the registration ID for an existing registration to the provided URI, or null if no such registration exists. The matching policy to apply is set as an option, with exact matching applied if this is omitted.
  • wamp.registration.match: Returns a list of IDs of registrations which currently match the URI using any matching strategy.

Example code for retrieving the lists of current registrations:

session.call("wamp.registration.list").then(session.log, session.log)

Example code for looking up a registration:

session.call("wamp.registration.lookup", ["com.myapp.procedure1"]).then(session.log, session.log)
session.call("wamp.registration.lookup", ["com.myapp", { match: "prefix" }]).then(session.log, session.log)
session.call("wamp.registration.lookup", ["com.myapp..create", { match: "wildcard" }]).then(session.log, session.log)

Example code for matching registrations:

session.call("wamp.registration.match", ["com.myapp.procedure1"]).then(session.log, session.log)

Retrieving information about a specific registration

Using a registration ID, information about a specific registration can be retrieved using:

  • wamp.registration.get: Returns data about the registration itself: the registration URI, ID, matching policy, invocation rule and creation date.
  • wamp.registration.list_callees: Returns a list of session IDs for sessions currently attached to the registration.
  • wamp.registration.count_callees: Returns the number of sessions currently attached to the registration.

Example code for getting data about a registration:

session.call("wamp.registration.get", [23560753]).then(session.log, session.log)

Example code for getting the callees for a registration:

session.call("wamp.registration.list_callees", [23560753]).then(session.log, session.log)

Example code for getting the callee count:

session.call("wamp.registration.count_callees", [23560753]).then(session.log, session.log)

Forcefully removing a callee

It is possible to forcefully remove an individual callee from a registration by using

  • wamp.subscription.remove_callee: Removes a single calle from a registration based on a provided registration ID and callee ID.

Example code for removing a subscriber:

session.call("wamp.registration.remove_callee", [23560753, 483984922713478]).then(session.log, session.log)

Note: Access the to the meta-API should, of course, be limited in your configuration to avoid rogue clients wreaking havoc on your application.

Note: the above examples are for Autobahn|JS since we also maintain and use this WAMP client library, and JavaScript is the closest there is to a lingua franca in programming. Users of other WAMP client libraries should feel free to add code examples for these!

Working Example

For a full working example in JavaScript, see Crossbar Examples.