Bernd: Don't feel bad about missing it, I wrote the silly stuff and it took me some time to remember.....
Those are the rules. It's always humbling to look back at my own code and say "that idiot should have put some comments in here..." ;) yeah, I agree there are a lot of moving parts here. I have a note to myself to provide better feedback in the response. You're absolutely right that we fire all these commands and hope they all work. Just returning "success" status doesn't guarantee leadership change. I'll be on another task the rest of this week, but I should be able to dress things up over the weekend. That'll give you a patch to test if you're willing. The actual code changes are pretty minimal, the bulk of the patch will be the reworked test. Best, Erick On Wed, Jan 9, 2019 at 11:59 PM Bernd Fehling <bernd.fehl...@uni-bielefeld.de> wrote: > > Hi Erik, > > that is very valuable info I missed. > Shouldn't that belong into an issue about rework at REBALANCELEADERS? > > With your explanation the use of a queue makes sense and now I see some of > the logic behind. > - there is the leader and the firstWatcher > - if firstWatcher goes down or is inactive that one should positioned > at end of queue which automatically gives the baton to the next in queue > - preferredLeader has precedence if state=active > - there can be only one preferredLeader per shard > > Any more rules? > > From debugging I know there are a lot of actions taken behind the scene > and many things can go wrong. Parts included are ZkStateReader, Overseer, > ShardLeaderElectionContextBase, ShardLeaderElectionContext, ElectionContext, > LeaderElector, some Requests and a HttpSolrCall to REJOINLEADERELECTION. > > An info I see when rebalance goes wrong is from > ShardLeaderElectionContextBase: > "No version found for ephemeral leader parent node, won't remove previous > leader registration." > > Regards, Bernd > > > Am 09.01.19 um 17:22 schrieb Erick Erickson: > > Executive summary: > > > > The central problem is "how can I insert an ephemeral node > > in a specific place in a ZK queue". The code could be much, > > much simpler if there were a reliable way to do just that. I haven't > > looked at more recent ZKs to see if it's possible, I'd love it if > > there were a better way. > > > > On to details: > > > > bq. wonder if we can discover the "inventor" of all this and ask him > > how it should work > > > > Yeah, I can contact that clown. That would be me ;) > > > > The way leader election works is a ZK recipe where each > > ephemeral node only watches the one in front of it. When a > > node is deleted, the one watching it is notified. > > > > So let's say we have nodes in this order: 1 watched by 2 > > watched by 3... In this case 1 is the leader. If 2 should > > disappear, 3 gets notified and now watches 1. Nothing > > else really happens. > > > > But if the nodes are 1, 2, 3 and 1 disappears 2 gets notified > > and says, in effect "I'm first in line so I will be leader". 3 > > doesn't get any notification. > > > > bq. I wonder why the queue is not rotated until the new and preferred > > leader is at front (position 0) > > > > Because then you'd have N leader changes where N is the number > > of nodes between the preferred leader and the actual leader in > > the leader election queue at the start. That could result in 100s of > > changes when only 1 is desired. The original case for this was > > exactly that, there could be 100s of shards and several tens > > to 100s of replicas. > > > > Hmmmm, I suppose you wouldn't have that many leader changes if > > you sent the ephemeral node in the _second_ position to the end > > of the queue until the preferredLeader became the one in the second > > position. You'd still have a watch fired for every requeueing though. I > > haven't measured the cost there. That would also be an added > > burden for the Overseer, which has been overwhelmed in the past. > > > > I'm not against that solution, I don't have any real data to > > evaluate. > > > > bq. ....is possible to have more than one electionNode with the > > same sequence number. > > > > Yeah, and it causes complexity, but I don't have a good way around it. This > > is > > related to your sorting question. ZK itself has a simple ordering, > > sequential > > sequence numbers. Having two the same is the only way I could see (actually > > I > > patterned it off some other code) to insert an ephemeral node second. What > > you > > have then is two nodes "tied" for second by having the same sequence > > numbers. > > > > Which one ZK thinks is second (and > > thus which one becomes the leader if the zero'th ephemeral node disappears) > > is based on sorting which includes the session ID, so there's code in there > > that > > has to deal with sending the non-preferred node that's tied for second to > > the > > end of the queue. That's the code that got changed during various > > refactorings that I didn't take part in, and the code that's messed up. > > > > bq. Wherever I see any java code to get the content from the queue it > > is sorted. Where is the sense of this? > > > > This is implied by the above, but explicitly so the Java code can see the > > queue > > in the same order that ZK does and "do the right thing". In this case > > assign the preferred leader's ephemeral node with the same sequence number > > that the current second-in-line has and move the current second-in-line to > > the > > end of the queue. > > > > All that said, this code was written several years ago and I haven't looked > > at > > whether there are better methods available now. The actions that are > > necessary > > are: > > > > 1> ensure that the preferredLeader is the only node watching the leader in > > the > > leader election queue > > > > 2> re-queue the leader at the end of the leader election queue. Since we'd > > be > > sure the preferredLeader is watching the leader, this action would elect > > the proper node as the leader. > > > > Hmmm, note to self. It would help folks in the future if I, you know, > > documented > > those two points in the code. Siiggghhh. > > > > Last night I found what I _think_ is the problem I was having. Note that > > the > > current code has three problems. I think I have fixes for all of them: > > > > 1> assigning the preferredLeader (or any SHARDUNIQUE property) does not > > properly remove that property from other replicas in the shard if > > present. So you may have multiple preferredLeaders in a shard. > > > > 2> the code to resolve tied sequence numbers had been changed > > during some refactoring so the wrong node could be elected. > > > > 3> the response from the rebalanceleaders command isn't very useful, it's > > on my plate to fix that. Partly it was not reporting useful > > info, and partly your comment from the other day that it returns > > without verifying the leadership has actually changed is well taken. > > At > > present, it just changes the election queue and assumes that the > > right thing happens. The test code was supposed to point out when > > that assumption was incorrect, but you know the story there. > > > > Currently, the code is pretty ugly in terms of all the junk I put in trying > > to > > track this down, but when I clean it up I'll put up a patch. I added some > > code to restart some of the jettys in the test (it's now "@Slow:") that > > catches the restart case. Additionally, I changed the test to force > > unique properties to be concentrated on a particular node then issue the > > BALANCESHARDUNIQUE command to make sure that <1> above > > doesn't happen. > > > > Meanwhile, if there's an alternative approach that's simpler I'd be all > > for it. > > > > Best, > > Erick > > > > On Wed, Jan 9, 2019 at 1:32 AM Bernd Fehling > > <bernd.fehl...@uni-bielefeld.de> wrote: > >> > >> Yes, your findings are also very strange. > >> I wonder if we can discover the "inventor" of all this and ask him > >> how it should work or better how he originally wanted it to work. > >> > >> Comments in the code (RebalanceLeaders.java) state that it is possible > >> to have more than one electionNode with the same sequence number. > >> Absolutely strange. > >> > >> I wonder why the queue is not rotated until the new and preferred > >> leader is at front (position 0)? > >> But why is it a queue anyway? > >> Wherever I see any java code to get the content from the queue it > >> is sorted. Where is the sense of this? > >> > >> Also, the elctionNodes have another attribute with name "ephemeral". > >> Where is that for and why is it not tested in TestRebalanceLeaders.java? > >> > >> Regards, Bernd > >> > >> > >> Am 09.01.19 um 02:31 schrieb Erick Erickson: > >>> It's weirder than that. In the current test on master, the > >>> assumption is that the node recorded as leader in ZK > >>> is actually the leader, see > >>> TestRebalanceLeaders.checkZkLeadersAgree(). The theory > >>> is that the identified leader node in ZK is actually the leader > >>> after the rebalance command. But you're right, I don't see > >>> an actual check that the collection's status agrees. > >>> > >>> That aside, though, there are several problems I'm uncovering > >>> > >>> 1> BALANCESHARDUNIQUE can wind up with multiple > >>> "preferredLeader" properties defined. Some time between > >>> the original code and now someone refactored a bunch of > >>> code and missed removing a unique property if it was > >>> already assigned and being assigned to another replica > >>> in the same slice. > >>> > >>> 2> to make it much worse, I've rewritten the tests > >>> extensively and I can beast the rewritten tests 1,000 > >>> times and no failures. If I test manually by just issuing > >>> the commands, everything works fine. By "testing manually" > >>> I mean (working with 4 Vms, 10 shards 4 replicas) > >>>> create the collection > >>>> issue the BALANCESHARDUNIQUE command > >>>> issue the REBALANCELEADERS command > >>> > >>> > >>> However, if instead I > >>>> create the collection > >>>> issue the BALANCESHARDUNIQUE command > >>>> shut down 3 of 4 Solr instances so all the leaders > >>> are on the same host. > >>>> restart the 3 instances > >>>> issue the REBALANCELEADERS command then > >>> it doesn't work. > >>> > >>> At least that's what I think I'm seeing, but it makes no > >>> real sense yet. > >>> > >>> So I'm first trying to understand why my manual test > >>> fails so regularly, then I can incorporate that setup > >>> into the unit test (I'm thinking of just shutting down > >>> and restarting some of the Jetty instances). > >>> > >>> But it's a total mystery to me why restarting Solr instances > >>> should have any effect. But that's certainly not > >>> something that happens in the current test so I have > >>> hopes that tracking that down will lead to understanding > >>> what the invalid assumption I'm making is and we can > >>> test for that too., > >>> > >>> On Tue, Jan 8, 2019 at 1:42 AM Bernd Fehling > >>> <bernd.fehl...@uni-bielefeld.de> wrote: > >>>> > >>>> Hi Erick, > >>>> > >>>> after some more hours of debugging the rough result is, who ever invented > >>>> this leader election did not check if an action returns the estimated > >>>> result. There are only checks for exceptions, true/false, new sequence > >>>> numbers and so on, but never if a leader election to the preferredleader > >>>> really took place. > >>>> > >>>> If doing a rebalanceleaders to preferredleader I also have to check if: > >>>> - a rebalance took place > >>>> - the preferredleader has really become leader (and not anyone else) > >>>> > >>>> Currently this is not checked and the call rebalanceleaders to > >>>> preferredleader > >>>> is like a shot into the dark with hope of success. And thats why any > >>>> problems have never been discovered or reported. > >>>> > >>>> Bernd > >>>> > >>>> > >>>> Am 21.12.18 um 18:00 schrieb Erick Erickson: > >>>>> I looked at the test last night and it's...disturbing. It succeeds > >>>>> 100% of the time. Manual testing seems to fail very often. > >>>>> Of course it was late and I was a bit cross-eyed, so maybe > >>>>> I wasn't looking at the manual tests correctly. Or maybe the > >>>>> test is buggy. > >>>>> > >>>>> I beasted the test 100x last night and all of them succeeded. > >>>>> > >>>>> This was with all NRT replicas. > >>>>> > >>>>> Today I'm going to modify the test into a stand-alone program > >>>>> to see if it's something in the test environment that causes > >>>>> it to succeed. I've got to get this to fail as a unit test before I > >>>>> have confidence in any fixes, and also confidence that things > >>>>> like this will be caught going forward. > >>>>> > >>>>> Erick > >>>>> > >>>>> On Fri, Dec 21, 2018 at 3:59 AM Bernd Fehling > >>>>> <bernd.fehl...@uni-bielefeld.de> wrote: > >>>>>> > >>>>>> As far as I could see with debugger there is still a problem in > >>>>>> requeing. > >>>>>> > >>>>>> There is a watcher and it is recognized that the watcher is not a > >>>>>> preferredleader. > >>>>>> So it tries to locate a preferredleader with success. > >>>>>> It then calls makeReplicaFirstWatcher and gets a new sequence number > >>>>>> for > >>>>>> the preferredleader replica. But now we have two replicas with the same > >>>>>> sequence number. One replica which already owns that sequence number > >>>>>> and > >>>>>> the replica which got the new (and the same) number as new sequence > >>>>>> number. > >>>>>> It now tries to solve this with queueNodesWithSameSequence. > >>>>>> Might be something in rejoinElection. > >>>>>> At least the call to rejoinElection seems right. For preferredleader it > >>>>>> is true for rejoinAtHead and for the other replica with same sequence > >>>>>> number > >>>>>> it is false for rejoinAtHead. > >>>>>> > >>>>>> A test case should have 3 shards with 3 cores per shard and should try > >>>>>> to > >>>>>> set preferredleader to different replicas at random. And then try to > >>>>>> rebalance and check the results. > >>>>>> > >>>>>> So far, regards, Bernd > >>>>>> > >>>>>> > >>>>>> Am 21.12.18 um 07:11 schrieb Erick Erickson: > >>>>>>> I'm reworking the test case, so hold off on doing that. If you want to > >>>>>>> raise a JIRA, though. please do and attach your patch... > >>>>>>> > >>>>>>> On Thu, Dec 20, 2018 at 10:53 AM Erick Erickson > >>>>>>> <erickerick...@gmail.com> wrote: > >>>>>>>> > >>>>>>>> Nothing that I know of was _intentionally_ changed with this between > >>>>>>>> 6x and 7x. That said, nothing that I know of was done to verify that > >>>>>>>> TLOG and PULL replicas (added in 7x) were handled correctly. There's > >>>>>>>> a > >>>>>>>> test "TestRebalanceLeaders" for this functionality that has run since > >>>>>>>> the feature was put in, but it has _not_ been modified to create TLOG > >>>>>>>> and PULL replicas and test with those. > >>>>>>>> > >>>>>>>> For this patch to be complete, we should either extend that test or > >>>>>>>> make another that fails without this patch and succeeds with it. > >>>>>>>> > >>>>>>>> I'd probably recommend modifying TestRebalanceLeaders to randomly > >>>>>>>> create TLOG and (maybe) PULL replicas so we'd keep covering the > >>>>>>>> various cases. > >>>>>>>> > >>>>>>>> Best, > >>>>>>>> Erick > >>>>>>>> > >>>>>>>> > >>>>>>>> On Thu, Dec 20, 2018 at 8:06 AM Bernd Fehling > >>>>>>>> <bernd.fehl...@uni-bielefeld.de> wrote: > >>>>>>>>> > >>>>>>>>> Hi Vadim, > >>>>>>>>> I just tried it with 6.6.5. > >>>>>>>>> In my test cloud with 5 shards, 5 nodes, 3 cores per node it missed > >>>>>>>>> one shard to become leader. But noticed that one shard already was > >>>>>>>>> leader. No errors or exceptions in logs. > >>>>>>>>> May be I should enable debug logging and try again to see all > >>>>>>>>> logging > >>>>>>>>> messages from the patch. > >>>>>>>>> > >>>>>>>>> Might be they also changed other parts between 6.6.5 and 7.6.0 so > >>>>>>>>> that > >>>>>>>>> it works for you. > >>>>>>>>> > >>>>>>>>> I also just changed from zookeeper 3.4.10 to 3.4.13 which works > >>>>>>>>> fine, > >>>>>>>>> even with 3.4.10 dataDir. No errors no complains. Seems to be > >>>>>>>>> compatible. > >>>>>>>>> > >>>>>>>>> Regards, Bernd > >>>>>>>>> > >>>>>>>>> > >>>>>>>>> Am 20.12.18 um 12:31 schrieb Vadim Ivanov: > >>>>>>>>>> Yes! It works! > >>>>>>>>>> I have tested RebalanceLeaders today with the patch provided by > >>>>>>>>>> Endika Posadas. > >>>>>>>>>> (http://lucene.472066.n3.nabble.com/Rebalance-Leaders-Leader-node-deleted-when-rebalancing-leaders-td4417040.html) > >>>>>>>>>> And at last it works as expected on my collection with 5 nodes and > >>>>>>>>>> about 400 shards. > >>>>>>>>>> Original patch was slightly incompatible with 7.6.0 > >>>>>>>>>> I hope this patch will help to try this feature with 7.6 > >>>>>>>>>> https://drive.google.com/file/d/19z_MPjxItGyghTjXr6zTCVsiSJg1tN20 > >>>>>>>>>> > >>>>>>>>>> RebalanceLeaders was not very useful feature before 7.0 (as all > >>>>>>>>>> replicas were NRT) > >>>>>>>>>> But new replica types made it very helpful to keep big clusters in > >>>>>>>>>> order... > >>>>>>>>>> > >>>>>>>>>> I wonder, why there is no any jira about this case (or maybe I > >>>>>>>>>> missed it)? > >>>>>>>>>> Anyone who cares, please, help to create jira and improve this > >>>>>>>>>> feature in the nearest releaase > >>>>>>>>>> >
