WHat I suggested is almost exactly what Jordan suggested. I should have read the rest of the thread before posting.
On Sat, Sep 21, 2019 at 9:54 AM Ted Dunning <ted.dunn...@gmail.com> wrote: > > I would suggest that using an epoch number stored in ZK might be helpful. > Every operation that the master takes could be made conditional on the > epoch number using a multi-transaction. > > Unfortunately, as you say, you have to have the update of the epoch be > atomic with becoming leader. > > The natural way to do this is to have an update of an epoch file be part > of the leader election, but that probably isn't possible using Curator. The > way I would tend to do it would be have a persistent file that is updated > atomically as part of leader election. The version of that persistent file > could then be used as the epoch number. All updates to files that are gated > on the epoch number would only proceed if no other master has been elected, > at least if you use the sync option. > > > > > > On Fri, Sep 20, 2019 at 1:31 AM Zili Chen <wander4...@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 >> >> >>