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Subject: Re: [amqp] [management] Proposal for event notifications (initial outline)
I have a couple of thoughts for extending the event notification proposal, the original is below for reference. I'll put them all together if after any feedback. *Event meta-data*: Add a GET-EVENTS operation to the "self" entity (like existing GET-ATTRIBUTES) to get a map of event-type name to attribute name list. *Compact event representation*: The current proposal is very fat on the wire - attribute names are repeated in every notification and there's no way to select a subset of attributes. I suggest the following addition inspired by the existing QUERY response: When subscribing for events the link attach properties MAY contain the key `attributeNames` with a list of attribute names. If so, event messages are tested against the filter as before, but are sent in a compact form. Attribute names/values are not sent in the application properties, instead the body contains a list of attribute values in the same order as `attributeNames`. Missing attributes are represented by NULL values. Without `attributeNames` you get the names and values. This is less efficient but may be easier to work with for dynamic or exploratory tools that can interact with many different implementations. With `attributeNames` you get just what you ask for with no overhead. This is better for tools that know what they want, and in heavy traffic systems where the overhead of attribute names is excessive. *Standard event types*: We could probably generate a reasonable set of standard event types directly from the performatives in the AMQP spec. Every performative (link-open, connection-open, disposition etc.) does signify and event that might be of interest to a management console. Is it too early to think about standardizing this? Are there any pitfalls? Original proposal ----------------- # AMQP Management Event Notification Management clients often need to know when the state of a managed entity has changed. This can be for monitoring or logging purposes, or to react when something becomes ready for use. Polling the management agent for changes with READ or QUERY operations is inefficient. Clients either poll too often, which overloads the agent, or too seldom which causes needless delays in the system. Event notification allows management clients to subscribe for continuous notification of selected events as soon as the agent becomes aware of them. ## Event Types An Event Type is a named set of attributes. The name is a case-sensitive string. Implementers MAY define their own Event Types which MUST be named using a reverse domain name prefix owned by the implementer, e.g. "com.example.broker.queueDeleted". Every Event Type MUST have an attribute "eventType" which carries the Event Type name as a string. Implementers may define additional required or optional attributes for their Event Types. Event Types do not have operations, annotations or extensions. No standard event types are defined by this specification. ## Notification Messages A notification message contains a single event. The message subject MUST be the Event Type name. The application-properties MUST include all required attributes for that Event Type, in particular "eventType" with the Event Type name. They MAY include any optional attributes for the Event Type. ## Subscribing for Event Notification To subscribe for notification messages, a client creates a link from the address "$management.events" with a *filter* (TODO xref) describing the events that it is interested in. Closing the link ends the subscription. The filtering capabilities depend on the filters provided by the agent. An agent SHOULD provide the APACHE.ORG:SELECTOR described in <http://www.amqp.org/specification/1.0/filters>. This allows clients to filter on arbitrary SQL-like expressions over all the attributes of the event including "eventType". Since the event type MUST also be included in the message subject, even simple filters (for example the legacy filters described in <http://www.amqp.org/specification/1.0/filters>) MAY be used to filter by event type. Any filter that can operate on the subject or application-properties of a message will be able to work with event notification messages, but for flexibility and interoperability implementers SHOULD provide APACHE.ORG:SELECTOR. ---- # Notes, not part of the spec: Needs cross references to relevant sections on filters etc. This design does not allow multiple events to be batched in a single message. IMO an implementation should batch messages at the transport (into TCP packets) NOT batch events into a message. Batching in messages complicates the client, defeats transport level batching (by forcing "fat" messages on the transport) and rules out (or badly complicates) the use of AMQP filters to select events. I considered an alternative design where the client sends a subscription request with a reply-to address and the agent then sends notification messages to the reply-to address. I rejected that design for the following reasons: 1. It is impossible in general to tell when such a subscriber goes away if it crashes or fails to send an explicit unsubscribe message. Implementations must already deal with closing links when clients disconnect or heartbeat out so using links solves this problem. Even in an indirect topology (like link-routed Qpid dispatch networks) the routers must proxy link closure to be AMQP-correct. 2. We should use AMQP standard features to solve AMQP problems, not re-invent the wheel in a form only useful for management. Filters/selectors were designed for exactly this purpose we should make them work for us.
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