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From Jordan Zimmerman <jor...@jordanzimmerman.com>
Subject Re: Leader election and leader operation based on zookeeper
Date Sat, 21 Sep 2019 15:05:29 GMT
I took a quick look at "Rethink High-Availability Stores" and I think using the leader path
may not work. I think the best solution will be something akin to combining a leader election
with a common ZNode versioning scheme. i.e.

Create a single ZNode to be used for coordination
Elect a leader in the normal manner
When an instance becomes leader it:
Gets the version of the coordination ZNode
Sets the data for that ZNode (the contents don't matter) using the retrieved version number
If the set succeeds you can be assured you are currently leader (otherwise release leadership
and re-contend)
Save the new version
Every time you need to perform a transaction, include a setData of the coordination ZNode
with the saved version number
As long as this succeeds you should be guaranteed of consistency
If it ever fails with an invalid version number, release leadership and re-contend

I haven't completely thought this through so others should try to poke holes in it. When I
get a chance, I'll try to write a test to prove this.


> On Sep 20, 2019, at 3:30 AM, Zili Chen <wander4096@gmail.com> wrote:
> Hi ZooKeepers,
> Recently there is an ongoing refactor[1] in Flink community aimed at
> overcoming several inconsistent state issues on ZK we have met. I come
> here to share our design of leader election and leader operation. For
> leader operation, it is operation that should be committed only if the
> contender is the leader. Also CC Curator mailing list because it also
> contains the reason why we cannot JUST use Curator.
> The rule we want to keep is
> **Writes on ZK must be committed only if the contender is the leader**
> We represent contender by an individual ZK client. At the moment we use
> Curator for leader election so the algorithm is the same as the
> optimized version in this page[2].
> The problem is that this algorithm only take care of leader election but
> is indifferent to subsequent operations. Consider the scenario below:
> 1. contender-1 becomes the leader
> 2. contender-1 proposes a create txn-1
> 3. sender thread suspended for full gc
> 4. contender-1 lost leadership and contender-2 becomes the leader
> 5. contender-1 recovers from full gc, before it reacts to revoke
> leadership event, txn-1 retried and sent to ZK.
> Without other guard txn will success on ZK and thus contender-1 commit
> a write operation even if it is no longer the leader. This issue is
> also documented in this note[3].
> To overcome this issue instead of just saying that we're unfortunate,
> we draft two possible solution.
> The first is document here[4]. Briefly, when the contender becomes the
> leader, we memorize the latch path at that moment. And for
> subsequent operations, we do in a transaction first checking the
> existence of the latch path. Leadership is only switched if the latch
> gone, and all operations will fail if the latch gone.
> The second is still rough. Basically it relies on session expire
> mechanism in ZK. We will adopt the unoptimized version in the
> recipe[2] given that in our scenario there are only few contenders
> at the same time. Thus we create /leader node as ephemeral znode with
> leader information and when session expired we think leadership is
> revoked and terminate the contender. Asynchronous write operations
> should not succeed because they will all fail on session expire.
> We cannot adopt 1 using Curator because it doesn't expose the latch
> path(which is added recently, but not in the version we use); we
> cannot adopt 2 using Curator because although we have to retry on
> connection loss but we don't want to retry on session expire. Curator
> always creates a new client on session expire and retry the operation.
> I'd like to learn from ZooKeeper community that 1. is there any
> potential risk if we eventually adopt option 1 or option 2? 2. is
> there any other solution we can adopt?
> Best,
> tison.
> [1] https://issues.apache.org/jira/browse/FLINK-10333 <https://issues.apache.org/jira/browse/FLINK-10333>
> [2] https://zookeeper.apache.org/doc/current/recipes.html#sc_leaderElection <https://zookeeper.apache.org/doc/current/recipes.html#sc_leaderElection>
> [3] https://cwiki.apache.org/confluence/display/CURATOR/TN10 <https://cwiki.apache.org/confluence/display/CURATOR/TN10>
> [4] https://docs.google.com/document/d/1cBY1t0k5g1xNqzyfZby3LcPu4t-wpx57G1xf-nmWrCo/edit?usp=sharing

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