CODEWITHSUNDEEP
spring-integration
adeelmahmood
adeelmahmood

Reputation: 2431

How does group timeout work?

Aggregator is a passive component and the release logic is only triggered when a new message arrives. How then does the group timeout work?

Is it a scheduled task similar to reaper that constantly monitors the state of the aggregator. Does that also mean it repeatedly evaluates the group-timeout-expression to determine the value of group-timeout, or is it evaluated once at the start? I am assuming, since there are some examples based on size of payload, that means it must evaluate the group-timeout-expression repeatedly but if that's the case how often does that happen? Can that frequency of evaluation be controlled/modified. Along the same lines if aggregator is a POJO, has this group-timeout functionality already flexible enough to be able to specify the timout from a POJO method.

Another interesting thing I noticed is that for my group-timeout-expression I was trying a spell expression and was passing payload or headers but those apparently aren't available in the context. Seems like the context within this group-timeout-expression points to SimpleMessageGroup which doesn't have payload or headers properties. So, how can I access payload or headers within the spel expression of group-timeout-expression?

In fact in my case I want the actual message (the wrapper around the payload) because my method signature expects an actual SI message passed to it not the payload.

Upvotes: 1

Views: 1691

Answers (1)

Gary Russell
Gary Russell

Reputation: 174484

  1. Prior to Spring Integration 4.0, the aggregator was a passive component and you had to configure a MessageGroupStoreReaper to expire groups.

  2. The group-timeout* attributes were added in 4.0; when a new message arrives, a task is scheduled to time out the group. If the next message arrives before the timeout, the task is cancelled, the new message is added to the group and, if the release doesn't happen, a new task is scheduled. The expression is re-evaluated each time (the example in the documentation looks at the group size).

Yes, the root object for expression evaluation is the message group.

Which "payload and headers" to you need? There are likely multiple messages in the group. You can easily access the first message in the group using one.payload or one.headers['foo'] (these expressions use group.getOne() which peeks at the first message in the group).

If you need to access a different message, you would need to write some code to iterate over the messages in the group. We don't currently, but it would be possible to make the newly arrived message available as a variable #newMessage or similar; feel free to open an 'Improvement' JIRA issue for that.

Upvotes: 2

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