lrhkobe opened a new issue #457:
URL: https://github.com/apache/incubator-eventmesh/issues/457
**The scenes of message downstream action:**
1. normal downstream
```
public void subscribe(SubscriptionItem subscriptionItem) throws Exception {
....
//msg put in eventmesh,waiting client ack
session.getPusher().unAckMsg(downStreamMsgContext.seq,
downStreamMsgContext);
session.downstreamMsg(downStreamMsgContext);
...
}
```
2. when downstream fail, push retry queue, execute downstream
```
private void retryHandle(DownStreamMsgContext downStreamMsgContext) {
...
downStreamMsgContext.session = rechoosen;
rechoosen.downstreamMsg(downStreamMsgContext);
...
}
```
3. when closeSession, the session has downstreamed msg which is unAcked, and
there is other session which is belonged to same consumer group, the unAcked
msg will repush to other client with same group
```
private void handleUnackMsgsInSession(Session session) {
...
downStreamMsgContext.session = reChooseSession;
reChooseSession.downstreamMsg(downStreamMsgContext);
...
}
```
The scene2 and scene3 have ack problem, **because the msg does not put in
unAckMap of the rechoosed session**. When the rechoosed session repley ack,
this leads EventMesh can not find the msg to ack .
**Message downstream process in EventMesh:**
1. select a session(client)
2. put the msg in a map of selected session
3. execute downstream action
**Message ack process in EventMesh:**
1. session(client) reply ack msg to EventMesh
2. EventMesh execute ack action in `MessageAckTask.java`
```
DownStreamMsgContext downStreamMsgContext =
session.getPusher().getUnAckMsg().get(seq);
if (downStreamMsgContext != null) {
downStreamMsgContext.ackMsg();
session.getPusher().getUnAckMsg().remove(seq);
}else {
logger.warn("MessageAckTask, seq:{}, downStreamMsgContext not in
downStreamMap,client:{}", seq, session.getClient());
}
```
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