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From Mathias Söderberg <math...@burtcorp.com>
Subject LeaderLatch recipe and error handling
Date Tue, 27 May 2014 19:32:33 GMT
Good evening,

I’m currently working on a project where we’re utilising Curator and more
specifically (quite heavily) the LeaderLatch recipe.

The documentation for error handling in “general” states the following for
a LOST notification:

The connection is confirmed to be lost. Close any locks, leaders, etc. and
attempt to re-create them. NOTE: it is possible to get a RECONNECTED state
after this but you should still consider any locks, etc. as dirty/unstable.

And the documentation for the LeaderLatch recipe states the following:

LeaderLatch instances add a ConnectionStateListener to watch for connection
problems. If SUSPENDED or LOST is reported, the LeaderLatch that is the
leader will report that it is no longer the leader (i.e. there will not be
a leader until the connection is re-established). If a LOST connection is
RECONNECTED, the LeaderLatch will delete its previous ZNode and create a
new one.

Users of LeaderLatch must take account that connection issues can cause
leadership to be lost. i.e. hasLeadership() returns true but some time
later the connection is SUSPENDED or LOST. At that point hasLeadership()
will return false. It is highly recommended that LeaderLatch users register
a ConnectionStateListener.

My conclusion from reading these two sections is that we’re supposed to add
a ConnectionStateListener and when we’re notified of a LOST event followed
by a RECONNECTED event, we’re supposed to close the current LeaderLatches
that we’re holding and re-create them?

However, looking through the actual code for the LeaderLatch, it appears
that this is actually already handled, i.e. it appears to create a new
znode when it encounters a RECONNECTED event, or am I reading this wrong?
(The documentation also states this as a fact).

My question is really: do we have to take any particular precaution
regarding the LeaderLatch recipe and connection loss scenarios? i.e. do we
have to close and re-create the LeaderLatches? Or can we be calm and just
carry on with our business as Curator handles this?

If anything is unclear, let me know.

Best regards,

Mathias Söderberg
Software Developer, Burt

www.burtcorp.com
Cell: + 46 762 79 57 55 | Skype: mthssdrbrg
http://twitter.com/mthssdrbrg | http://twitter.com/burtcorp
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