openstack/oslo.messaging
Code Issues Proposed changes

Clarify the documentation for pooled Notification Listeners

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Change-Id: I184b045edaa8e61f075df576992426e94bf7386a
Closes-Bug: #1814544
This commit is contained in:
Kenneth Giusti 2019-03-11 09:31:38 -04:00
parent 472d483942
commit 855a85fa2d
1 changed files with 22 additions and 16 deletions
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38
oslo_messaging/notify/listener.py
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@ -20,22 +20,6 @@ A notification listener subscribes to the topic - and optionally exchange - in
the supplied target. Notification messages sent by notifier clients to the
target's topic/exchange are received by the listener.
If multiple listeners subscribe to the same target, the notification will be
received by only one of the listeners. The receiving listener is selected from
the group using a best-effort round-robin algorithm.
This delivery pattern can be altered somewhat by specifying a pool name for the
listener. Listeners with the same pool name behave like a subgroup within the
group of listeners subscribed to the same topic/exchange. Each subgroup of
listeners will receive a copy of the notification to be consumed by one member
of the subgroup. Therefore, multiple copies of the notification will be
delivered - one to the group of listeners that have no pool name (if they
exist), and one to each subgroup of listeners that share the same pool name.
Note that not all transport drivers have implemented support for listener
pools. Those drivers that do not support pools will raise a NotImplementedError
if a pool name is specified to get_notification_listener().
A notification listener exposes a number of endpoints, each of which contain a
set of methods. Each method's name corresponds to a notification's priority.
When a notification is received it is dispatched to the method named like the
@ -62,6 +46,28 @@ support requeueing, it will raise NotImplementedError at this point.
The message is acknowledged only if all endpoints either return
oslo_messaging.NotificationResult.HANDLED or None.
*NOTE*: If multiple listeners subscribe to the same target, the notification
will be received by only *one* of the listeners. The receiving listener is
selected from the group using a best-effort round-robin algorithm.
This delivery pattern can be altered somewhat by specifying a pool name for the
listener. Listeners with the same pool name behave like a subgroup within the
group of listeners subscribed to the same topic/exchange. Each subgroup of
listeners will receive a copy of the notification to be consumed by one member
of the subgroup. Therefore, multiple copies of the notification will be
delivered - one to the group of listeners that have no pool name (if they
exist), and one to each subgroup of listeners that share the same pool name.
**NOTE WELL:** This means that the Notifier always publishes notifications to a
non-pooled Listener as well as the pooled Listeners. Therefore any application
that uses listener pools **must have at least one listener that consumes from
the non-pooled queue** (i.e. one or more listeners that do not set the *pool*
parameter.
Note that not all transport drivers have implemented support for listener
pools. Those drivers that do not support pools will raise a NotImplementedError
if a pool name is specified to get_notification_listener().
Each notification listener is associated with an executor which controls how
incoming notification messages will be received and dispatched. Refer to the
Executor documentation for descriptions of the other types of executors.