[HDFS-8221] HDFS have two Standby NNs because ActiveStandbyElectorLock ephemeralOwner in ZK is different with the sessionId stored in ZKFC

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Duplicate
• Affects Version/s: 2.4.1
• Component/s: auto-failover
• Assignee: Unassigned
• Reporter: XingFeng Shen
• Due: 22/May/15
• Created: 22/Apr/15 09:41
• Updated: 23/Apr/15 10:50
• Resolved: 23/Apr/15 05:46

Description

Firstly, NN1 is active, NN2 is standby. When NN1 become standby due to some reasons, NN2 then take over the active state imediately. But after NN2 becoming active, It changed to standby again. And, HDFS got two standby NN forever.
After check the log, I found that NN2 become standby beacuse It have wrong sessionID with ActiveStandbyElectorLock ephemeralOwner stored in Znode.
And the rootcause is when NN1 go to standby, NN2 create one session A with zk, and become active. Ideally, NN2 should have the same sessionID with ActiveStandbyElectorLock ephemeralOwner stored in Znode, but some network reason can result in NN2'ZKFC sessionID changed.
So, I think when NN2 become standby due to different sessionid, NN2 should unlock the state in Znode in order to failover again.

ActiveStandyElector.processResult
==================
Code code = Code.get(rc);
if (isSuccess(code)) {
// the following owner check completes verification in case the lock znode
// creation was retried
if (stat.getEphemeralOwner() == zkClient.getSessionId()) {
// we own the lock znode. so we are the leader
if (!becomeActive())

{ reJoinElectionAfterFailureToBecomeActive(); }

} else

{ // we dont own the lock znode. so we are a standby. becomeStandby(); }

// the watch set by us will notify about changes
return;
}

ActiveStandbyElectorLock content
==================
[zk: 160.149.0.114:24002(CONNECTED) 1] get /hadoop-ha/hacluster/ActiveStandbyElectorLock

160-149-0-117 锟斤拷(锟斤拷
cZxid = 0x2000a38d9
ctime = Thu Apr 16 11:32:54 CST 2015
mZxid = 0x2000a38d9
mtime = Thu Apr 16 11:32:54 CST 2015
pZxid = 0x2000a38d9
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x164cb2b3e4b36ae4
dataLength = 38
numChildren = 0

Issue Links

• is duplicated by: HDFS-8161 Both Namenodes are in standby State