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From Zili Chen <wander4...@gmail.com>
Subject Re: Leader election and leader operation based on zookeeper
Date Sat, 21 Sep 2019 03:21:22 GMT
>>I am assuming the "write operation" here is write to ZooKeeper

Yes.

>>Looks like contender-1 was not reusing same ZooKeeper client object, so
this explains how the previous supposed to be fail operation succeeds?

Yes. Our communication to ZK is based on Curator, which will re-instance a
client and retry the operation. Due to asynchronously schedule the error
execute order is possible.

>>record the session ID and don't commit any write operations if session ID
changes.

Sounds reasonable. Currently in our ongoing design we treat the latch path
as "session id" so we use multi-op to atomically verify it.
It seems Curator does not expose session id. And in my option 2 above even
I think of falling back to zookeeper so that we just fail on
session expired and re-instance another contender, contending for
leadership. This will save us from maintaining mutable state during
leadership epoch(to be clear, Flink scope leadership, not ZK).

Best,
tison.


Michael Han <hanm@apache.org> 于2019年9月21日周六 上午4:03写道:

> >> thus contender-1 commit a write operation even if it is no longer the
> leader
>
> I am assuming the "write operation" here is write to ZooKeeper (as opposed
> to write to an external storage system)? If so:
>
> >> contender-1 recovers from full gc, before it reacts to revoke
> leadership event, txn-1 retried and sent to ZK.
>
> contender-2 becomes the leader implies that the ephemeral node appertains
> to contender-1 has been removed, which further implies that the session
> appertains to contender-1 is either explicitly closed (by client), or
> expired. So if contender-1 was still using same client ZooKeeper object,
> then it's not possible for txn-1 succeeded as session expire was an event
> ordered prior to txn-1, which wouldn't commit after an expired session.
>
> >> Curator always creates a new client on session expire and retry the
> operation.
> Looks like contender-1 was not reusing same ZooKeeper client object, so
> this explains how the previous supposed to be fail operation succeeds?
>
> If my reasoning make sense, one idea might be on Flink side, once you
> finish leader election with ZK, record the session ID and don't commit any
> write operations if session ID changes.
>
> The fencing token + multi might also work, but that sounds a little bit
> heavier.
>
> On Fri, Sep 20, 2019 at 1:31 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
>> [2]
>> https://zookeeper.apache.org/doc/current/recipes.html#sc_leaderElection
>> [3] 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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