Thank Erick,
I'm not fully understand it, but it kinda makes sense somehow.
Thank you!
On Thu, Aug 6, 2020 at 6:40 PM Erick Erickson <erickerick...@gmail.com>
wrote:
> Dat:
>
> That’s a confusingly-named variable, surely needs a comment. Remembering
> from a long time ago trying to work with PreferredLeader, the problem is
> that you can’t _replace_ the head because that’s the one that actually is
> the leader. You have to spoof that by inserting a node that points to the
> leader, then making the leader go away so election takes place. IIRC (and
> lots may have changed since then), you wind up with a situation where
>
> znode0 is the current leader
> znode1 points to znode0
> znode2 _also_ points to znode0
> znode3 points to znode1
> znode 4 points to znode3
>
> znode1 here was the original znode put in the queue to become the next
> leader. znode2 is the one added by the code you’re looking at. When leader
> election actually happens, this situation is detected and znode2 becomes
> the leader and znode3 is changed to point to znode2 WARNING: This is what I
> remember from code I worked on a long time ago, it may have been changed a
> lot since then. But I’m fairly sure that’s what’s behind that code.
>
> Best,
> Erick
>
>
>
> > On Aug 6, 2020, at 6:18 AM, Cao Mạnh Đạt <da...@apache.org> wrote:
> >
> > Hi guys,
> >
> > When I look at these lines,
> > if(joinAtHead){
> > log.debug("Node {} trying to join election at the head", id);
> > List<String> nodes =
> OverseerTaskProcessor.getSortedElectionNodes(zkClient, shardsElectZkPath);
> > if(nodes.size() <2){
> > leaderSeqPath = zkClient.create(shardsElectZkPath + "/" + id +
> "-n_", null,
> > CreateMode.EPHEMERAL_SEQUENTIAL, false);
> > } else {
> > String firstInLine = nodes.get(1);
> > log.debug("The current head: {}", firstInLine);
> > Matcher m = LEADER_SEQ.matcher(firstInLine);
> > if (!m.matches()) {
> > throw new IllegalStateException("Could not find regex match in:"
> > + firstInLine);
> > }
> > leaderSeqPath = shardsElectZkPath + "/" + id + "-n_"+ m.group(1);
> > zkClient.create(leaderSeqPath, null, CreateMode.EPHEMERAL, false);
> > }
> > } else {
> > the line firstInLine = nodes.get(1) seems problematic to me. Why do we
> get the middle of a sorted list and say that it is the first thing in line?
> Should I open an issue for this?
> >
> > Thanks
>
>
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--
*Best regards,*
*Cao Mạnh Đạt*
*E-mail: caomanhdat...@gmail.com <caomanhdat...@gmail.com>*
