Starting from Curator 3, after losing the connection to zookeeper, curator emits a SUSPENDED event and then starts an internal timer, if the time of the negotiatied session timeout get hit and curator didn't reconnect to zookeeper it emits a LOST event.
For example :
Given negotiated session timeout = 40
|0||SUSPENDED (curator has been disconnected from zookeeper)|
|40||LOST (curator is still disconnected, it must have been lost the sesion as it is disconnected for 40sec)|
Given this scenario we could ,theoretically, ignore the SUSPENDED event and consider the leadership as lost just if the ConnectionStateListener receives the LOST event.
But this feature seems to have introduced a bug (from my point of view)
Case of failure
|0||Something went wrong with the connected zookeeper (in my case, the network inteface of zookeeper's server has gone down). Curator stops hearing heart beats from zookeeper, but doesn't lose the connection (from some reason that I don't know, if the network interface of the server goes down, Curator doesn't lose the connection)|
|~26.666||SUSPENDED (after 26 seconds without hearing heartbeats, curator emits a SUSPENDED event) 26 is from "readTimeout = sessionTimeout * 2 / 3" from the class ClientCnxn.java from zookeeper client. At this point, curator starts counting 40 sec.|
|26.666 to 40||During this period, Curator is trying to connect to other zookeeper instances but the other instances, in my example case, are also uncheachable.|
|40||Session has expired and the other instance has taken leadership ( in my example, the other instance can connect to zookeeper )|
|66.666||LOST ( after 40sec from SUSPENDED, curator finally sends LOST)|
As you can see, if we are ignoring the SUSPENDED event, the second application instance acquires the leadership ~26 seconds before the first instance notice that it lost the leadership.
I understand it seems to be a very rare case, but either way I think it should be addressed.
I have came up with a way to reproduce this easily. I know this is not a recommended production setup, but it reproduces the problem anyway.
1) On a virtual machine, with 2 network interfaces (eth0[192.168.0.101], eth1[192.168.0.102]) , I installed one zookeeper instance.
2) I setup application1 with the leadership receipe, with 40sec of negotiated timeout, pointing just to 192.168.0.101 .Now it is the leader
3) I setup application2 with the leadership receipe, with 40sec of negotiated timeout, pointing just to 192.168.0.102 .Now it it is NOT the leader
4) On the server I turn the eth0[192.168.0.101] interface down [ ifconfig eth0 down ]
5) After 26 seconds, application1 says :
:ClientCnxn$SendThread@1111] - Client session timed out, have not heard from server in 26679ms for sessionid
ConnectionStateManager@237] - State change: SUSPENDED
NOTE: I'm ignoring the SUSPENDED event
6) After 40 seconds, application2 takes leadership
7) After 66 seconds, application1 says :
ConnectionStateManager@237] - State change: LOST
NOTE: Just at this point, I consider that the application1 has lost leadership
Then, for 26 seconds, we had 2 leaders.
If you confirm it as a bug, I think I could help.
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