This patch fixes a slightly pathological state that occurs when the
statestore is under heavy load. The result of the bug is that
subscribers cannot successfully re-register because the statestore never
marks them as failed.
The exact sequence of events is as follows:
1. Subscriber registers with state-store.
2. Statestore does not send heartbeats in timely fashion to
subscriber. Subscriber times-out.
3. Subscriber is restarted quickly. Statestore does not detect
restart.
4. Subscriber's RegisterSubscriber() call fails, because statestore
detects duplicate registration.
5. Subscriber restarts again. Since state-store is slow to send
heartbeats, the state-store has not detected the restart and the
subscriber receives a heartbeat message from the statestore and
does not reject it.
6. Statestore continues to believe subscriber is alive, since the
heartbeats are not being rejected.
To fix this, we add a registration ID to each successfully registered
subscriber that is known to both subscriber and statestore. If the
subscriber should restart and re-register, it receives a new
registration ID. Whenever a heartbeat arrives, it compares its
registration ID to that sent by the statestore with the heartbeat, and
rejects the heartbeat if they do not match.
We also allow re-registration of existing subscribers (getting rid of
the dreaded "Duplicate subscription" message). A new registration
overwrites an old one.
Change-Id: Ie32df3a586ccb375375ebfbcbec1aaeb930b6bfe
Reviewed-on: http://gerrit.ent.cloudera.com:8080/778
Tested-by: jenkins
Reviewed-by: Henry Robinson <henry@cloudera.com>
