Oh, I did not articulate myself well. I mean the sync when a follower starts up
("syncWithLeader" as it were), which doesn't seem to use the actual sync
feature. Or does it and I'm just not seeing where it is in the code?
It seems like we rely on the LearnerHandler thread startup to capture all of
the missing committed transactions in the SNAP or DIFF, but I don't see
anything (especially in the DIFF case) that is preventing us for committing
more transactions before we actually start forwarding updates to the new
follower.
Let me explain using my example from ZOOKEEPER-919. Assume we have quorum
already, so the leader can be processing transactions while my follower is
starting up.
I'm a follower at zxid N-5, the leader is at N. I send my FOLLOWERINFO packet
to the leader with that information. The leader gets the proposals from its
committed log (time T1), then syncs on the proposal list (LearnerHandler line
267. Why? It's a copy of the underlying proposal list... this might be part of
our problem). I check to see if the peerLastZxid is within my max and min
committed log and it is, so I'm going to send a diff. I set the zxidToSend to
be the maxCommittedLog at time T3 (we already know this is sketchy), and
forward the proposals from my copied proposal list starting at the
peerLastZxid+1 up to the last proposal transaction (as seen at time T1).
After I have queued up all those diffs to send, I tell the leader to
startFowarding updates to this follower (line 308).
So, let's say that at time T2 I actually swap out the leader to the thread that
is handling the various request processors, and see that I got enough votes to
commit zxid N+1. I commit N+1 and so my maxCommittedLog at T3 is N+1, but this
proposal is not in the list of proposals that I got back at time T1, so I don't
forward this diff to the client. Additionally, I processed the commit and
removed it from my leader's toBeApplied list. So when I call startForwarding
for this new follower, I don't see this transaction as a transaction to be
forwarded.
There's one problem. Let's also imagine, however, that I commit N+1 at time T4.
The maxCommittedLog value is consistent with the max of the diff packets I am
going to send the follower. But, I still committed N+1 and removed it from the
toBeApplied list before calling startFowarding with this follower. How does the
follower get this transaction? Does it?
To put it another way, here is the thread interaction, hopefully formatted so
you can read it...
LearnerHandlerThread
RequestProcessorThread
T1(LH): get list of proposals (COPY)
T2(RPT): commit
N+1, remove from toBeApplied
T3(LH): get maxCommittedLog
T4(LH): send diffs from view at T1
T5(LH): startForwarding
Or
T1(LH): get list of proposals (COPY)
T2(LH): get maxCommittedLog
T3(RPT): commit
N+1, remove from toBeApplied
T4(LH): send diffs from view at T1
T5(LH): startFowarding
I'm trying to figure out what, if anything, keeps the requests from being
committed, removed, and never seen by the follower before it fully starts up.
Thanks,
C
-----Original Message-----
From: Benjamin Reed [mailto:br...@yahoo-inc.com]
Sent: Monday, December 20, 2010 4:06 PM
To: dev@zookeeper.apache.org
Subject: Re: Question about leader/follower coherence
it turns out that there is a simple answer.
first the sync guarantee: the client will see the effect of all
operations that happened before the sync started.
to make that guarantee we just need to make sure that the follower that
the client is connected to has all transactions that were in flight when
the sync was received.
as a side note let me point out that if you do a write, even if it
fails, you will get the same guarantee as the sync, but it will be
heavier weight because the write result will get pushed through the
atomic broadcast.
to implement sync, the follower forwards the sync to the leader. the
processing pipeline at the follower will delay any requests after the
sync until the leader replies to the sync. when the leader get sync
there are two things that can happen:
1) there aren't any outstanding transactions: the leader queues a sync
reply to the follower. it will get queued behind any pending operations
that were previously sent to the follower.
2) there are outstanding transactions: leader notes the zxid of the last
outstanding transaction and installs a trigger to queue the sync reply
when that zxid gets committed.
because everything is processed in order once a follower processes a
sync reply that follower will have processed all operations started
before the sync. note that the implementation has a stronger guarantee
than needed because it covers all operations started at the leader
before the sync. however, it is hard to reason about "started before"
since the leader determines the ordering.
ok, that was a rather long simple answer :)
ben
On 12/20/2010 11:22 AM, Fournier, Camille F. [Tech] wrote:
> Hi everyone,
>
> A simple question with a possibly not simple answer:
> For transactions that happen and are committed on the leader/in the cluster
> (given a cluster with quorum already) during the time in which a new follower
> is being synced (sending diffs, sync, etc), what mechanism is it that ensure
> that those transactions also make it to the follower that was syncing at that
> time?
>
> Thanks,
> Camille
>
