Spring Integration - ReleaseStrategy that looks at messages that weren't yet added

I have messages that are fetched to the system in groups (let's say 50), need to be grouped by AGGREGATION_ID into lists of messages and send further into the flow.

I can use correlationStrategy to aggregate with that id, but I need to know when to release the aggregated message. In ReleaseStrategy I can only look at the messages already added to the aggregate, but I need to know when there's no more messages in the fetched group of 50 with the same AGGREGATION_ID to know when to send the group. How can I do that?

A ReleaseStrategy could be any bean with a full access to the whole application context. If you have some place where you store those messages before aggregation, you definitely can take a look into that place from a custom ReleaseStrategy implementation.

On the other hand I would suggest to take a look into the groupTimeout option of an aggregator: https://docs.spring.io/spring-integration/docs/5.3.0.M4/reference/html/message-routing.html#agg-and-group-to. So, with the normal behavior your groups are going to be gathered according an expected size of 50, but when there is no new messages for the group during some time, a group is going to be released with whatever is there so far. You also can configure that groupTimeout as a SpEL expression, so there is an access to application context, too.