Hi Team,
Good day.
Recently came across “Double Locking Issue (i.e. two clients acquiring lock)”
using Curator code ( InterProcessMutex lock APIs ) in our application
Our use case:
* Two clients attempts to acquire the zookeeper lock using Curator
InterProcessMutex and whoever owns it would release it once sees the connection
disconnect ( on receiving Connection.SUSPENDED / Connection.LOST Curator
Connection Events from Connection listener)
Issue we noticed:
* After session expired & reconnected with new session, both client seems
to have acquired the lock. Interesting thing that we found is that one of the
clients still holds the lock while its lock node (ephemeral) was gone
Things we found:
* Based on our initial analysis and few test runs, we saw that Curator
acquire() method acquires the lock based on “about to be deleted lock node of
previous session”. Explanation : Ephemeral node created by previous session was
still seen by client that reconnected with new session id until server cleans
that up. If this happens, Curator acquire() would hold the lock.
* Clearly we could see the race condition (in zookeeper code) between 1).
Client reconnecting to server with new session id and 2). server deleting the
ephemeral nodes of client’s previous session. We were able to reproduce this
issue using the following approach,
* Artificially break the socket connection between client and server for
30s
* Artificially pausing the set of server codes for a min and unpause
* On the above mentioned race condition, if client manage to reconnect to
server with new session id before server cleans up the ephemeral nodes of
client’s previous session, Curator lock acquire() who is trying to acquire the
lock will hold the lock as it still sees the lock node in zookeeper directory.
Eventually server would be cleaning up the ephemeral nodes leaving the Curator
local lock thread data stale giving the illusion that it still hold the lock
while its ephemeral node is gone
* Timeline events described below for better understanding,
* At t1, Client A and Client B establishes zookeeper session with
session id A1 and B1 respectively
* At t2, Client A creates the lock node N1 & acquires the lock
* At t3, Client B creates the lock node N2 & blocked on acquire() to
acquire the lock
* At t4, session timed out for both clients & server is about to clean
up the old session • Client A trying to release the lock
* At t5, Client A and Client B reconnects to server with new session id
A2 and B2 respectively before server deletes the ephemeral node N1 & N2 of
previous client session. Client A releases the lock, deleting N1 and trying to
acquire it again by creating N3 node and Client B who is blocked on acquire()
acquires the lock based on N2 (about to be deleted node created by previous
session)
* At t6, server cleans up the ephemeral node N2 created by Client B’s
previous session. Client A acquires the lock with node N3 as its previous
sequence N2 gets deleted, whereas Client B who incorrectly acquired the lock at
t5 timeline still holds the lock
Note:
* We are not certain that this race condition that we noticed in zookeeper
code is intentional design.
Questions:
* Given this race condition seen in zookeeper code, we would like to hear
your recommendations / suggestions to avoid this issue while using Curator lock
code?
* We also see that Interprocess Mutex has the makeRevocable() API that
enables application to revoke lock, but that handles Node change event only but
not for Node deleted event. I understand that this makes sense to handle Node
change event alone, as to enable application user to revoke lock externally
from application side. But would it be also to okay to have one for Node delete
event, so as application can register the listener for Node delete event. I
would like to hear your thoughts.
Many Thanks,
Viswa
