Re: [amqp] [management] Proposal for event notifications (initial outline)

[Matthew Arrott <marrott@novgp.com>]

As I read this, there are two parts to this proposal:

1) Attribute Name Representation Optimization to obtain small size messages

+1 with question: Is this attributeName list optimization something that
can be applied to all attributes across all subscriptions?

2) Generate a set of Standard EventTypes from the performatives in the
AMQP spec

Question: Are the associated  EventType attributes for each of the
performatives all well know and/or null?

-----Original Message-----
From: Alan Conway <aconway@redhat.com>
Date: Monday, February 16, 2015 at 8:24 AM
To: John O'Hara <john@rjohara.com>
Cc: <amqp@lists.oasis-open.org>, Matthew Arrott <marrott@ucsd.edu>,
"Godfrey, Robert X" <robert.godfrey@jpmorgan.com>
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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