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From Nick Vatamaniuc <vatam...@gmail.com>
Subject Re: [DISCUSS] FDB and CouchDB replication
Date Fri, 12 Apr 2019 18:52:40 GMT
Hi Jan,

On Thu, Apr 11, 2019 at 4:16 AM Jan Lehnardt <jan@apache.org> wrote:

> An advanced implementation could maintain lists of [{node: n1, ratio:
> 0.2}, {node: n2, ratio: 0.4}, {node: n3, ratio: 0.4}] where nodes or admins
> can signify the ratio of jobs any given node should be handling. Of course
> this gets complicated quickly if the sum of ratio is <1 or >1, but I’m sure
> there are clever ways out of that. It might also be too complicated to get
> started with this, but it might be worth using a structured list, to allow
> extensions later.
>
>
I thought of adding an info or capacity to each node / executor and it
might be a good idea. Although also leaning to going with a single queue as
other contributor have commented. But with the current scheme, we have 500
max_jobs that will run at the same time. Any node can absorb a higher
number of course, and then they'd be scheduled on that node as needed. With
the assumption that all nodes are about the same capacity, it ends up
even-ing out. Say out of 10k jobs, randomly distributed amongst 10 nodes
chances are each node would get about 1000 jobs and would have about the
same load. Now if we only had 10 jobs, then yeah one node could end up with
3 heavy replications and most others will be idle.

* * *
>
> Nick, I like your proposal. I was briefly worrying about Adam’s suggestion
> to add a fast-path for the replicator, because I use a similar owner model
> for the db-per-user server, and thought I wanted to advocate for a more
> general framework that can be used for various types of jobs (something
> that the Roles variable in the model already allows), but I now agree that
> the importance of making replication as efficient as possible is a
> worthwhile optimisation vs. the other jobs we already have, or could
> imagine running in the future.
>

Good point. Yeah I remember couch-per-user was using a few similar pattern.
We are reading _changes from _users shard on each node and then maintaining
changes feed to all them. Then using the same "ownership" function
consistently pick one of the nodes for each db update to handle db
creation. Maybe the _replicator and _users both should participate in this
optimized changes notification. And then would it mean having an
"db_per_user" role and separate queue with node executing jobs? It might
makes sense, but we should think about it more in depth.


> Best
> Jan
> —
>
> > On 11. Apr 2019, at 01:51, Robert Newson <rnewson@apache.org> wrote:
> >
> > As long as any given replicator node can grab as much work as it can
> handle, It doesn't need to be 'fair' in the way we currently do it. The
> notion of an 'owner' node drops away imo. As nick mentions, the fun part is
> recognising when jobs become unowned due to a resource failure somewhere
> but this is a very standard thing, a pool of workers competing over jobs.
> >
> > --
> >  Robert Samuel Newson
> >  rnewson@apache.org
> >
> > On Thu, 11 Apr 2019, at 00:43, Adam Kocoloski wrote:
> >> Hi Nick,
> >>
> >> Good stuff. On the first topic, I wonder if it makes sense to use a
> >> dedicated code path for updates to _replicator DB docs, one that would
> >> automatically register these replication jobs in a queue in FDB as part
> >> of the update transaction. That’d save the overhead and latency of
> >> listening to _db_updates and then each _replicator DB’s _changes feed
> >> just to discover these updates (and then presumably create jobs in a
> >> job queue anyway).
> >>
> >> On the second topic — is it important for each node declaring the
> >> replicator role to receive an equal allotment of jobs and manage its
> >> own queue, or can the replication worker processes on each node just
> >> grab the next job off the global queue in FDB whenever they free up? I
> >> could see the latter approach decreasing the tail latency for job
> >> execution, and I think there are good patterns for managing high
> >> contention dequeue operations in the case where we’ve got more worker
> >> processes than jobs to run.
> >>
> >> Regardless, you make a good point about paying special attention to
> >> liveness checking now that we’re not relying on Erlang distribution for
> >> that purpose. I didn’t grok all the details of approach you have in
> >> mind for that yet because I wanted to bring up these two points above
> >> and get your perspective.
> >>
> >> Adam
> >>
> >>> On Apr 10, 2019, at 6:21 PM, Nick Vatamaniuc <vatamane@apache.org>
> wrote:
> >>>
> >>> I was thinking how replication would work with FDB and so far there
> are two
> >>> main issues I believe would need to be addressed. One deals with how we
> >>> monitor _replicator db docs for changes, and other one is how
> replication
> >>> jobs coordinate so we don't run multiple replication jobs for the same
> >>> replication document in a cluster.
> >>>
> >>> 1) Shard-level vs fabric-level notifications for _replicator db docs
> >>>
> >>> Currently replicator is monitoring and updating individual _replicator
> >>> shards. Change notifications are done via change feeds (normal,
> >>> non-continuous) and couch event server callbacks.
> >>>
> https://github.com/apache/couchdb/blob/master/src/couch/src/couch_multidb_changes.erl#L180
> ,
> >>>
> https://github.com/apache/couchdb/blob/master/src/couch/src/couch_multidb_changes.erl#L246
> .
> >>> With fdb we'd have to get these updates via a fabric changes feeds and
> rely
> >>> on the global _db_updates. That could result in a performance impact
> and
> >>> would be something to keep an eye on.
> >>>
> >>> 2) Replicator job coordination
> >>>
> >>> Replicator has a basic constraint that there should be only one
> replication
> >>> job running for each replicator doc per cluster.
> >>>
> >>> Each replication currently has a single "owner" node. The owner is
> picked
> >>> to be one of 3 nodes were the _replicator doc shards live. If nodes
> connect
> >>> or disconnect, replicator will reshuffle replication jobs and some
> nodes
> >>> will stop running jobs that they don't "own" anymore and then proceed
> to
> >>> "rescan" all the replicator docs to possibly start new ones. However,
> with
> >>> fdb, there are no connected erlang nodes and no shards. All
> coordination
> >>> happens via fdb, so we'd have to somehow coordinate replication job
> >>> ownership through there.
> >>>
> >>> For discussion, here is a proposal for a worker registration layer do
> that
> >>> job coordination:
> >>>
> >>> The basic idea is erlang fabric nodes would declare, by writing to fdb,
> >>> that they can take on certain "roles". "replicator" would be one such
> role.
> >>> And so, for each role there is a list of nodes. Each node picks a
> fraction
> >>> of jobs based on how many other nodes of the same role are in the list.
> >>> When membership changes, nodes which are alive might have to pick up
> new
> >>> jobs or stop running existing jobs since they'd be started by other
> nodes.
> >>>
> >>> For example, there are 3 nodes with "replicator" roles: [n1, n2, n3].
> n1 is
> >>> currently down so the membership list is [n2, n3]. If there are 60
> >>> replication jobs then n2 might run 30, and n3 another 30. n1 comes
> online
> >>> and adds itself to the roles list, which now looks like [n1, n2, n3].
> n1
> >>> then picks 20 replication jobs. At about the same time n2 and n3
> notice n1
> >>> is online and decide to stop running the jobs that n1 would pick up and
> >>> they each would end up running roughly 20 jobs.
> >>>
> >>> The difficulty here comes from maintaining liveliness. A node could
> stop at
> >>> any time without removing itself from the membership list of its roles.
> >>> That means all of the sudden a subset of jobs would stop running
> without
> >>> anyone picking them up. So, the idea is to have nodes periodically
> update
> >>> their health status in fdb to indicate they are alive. Once a node
> doesn't
> >>> update its status often enough it will be considered dead and others
> can
> >>> pick up its share of jobs.
> >>>
> >>> To start the discussion, I sketched this data layout and pseudocode:
> >>>
> >>> Data layout:
> >>>
> >>> ("couch_workers", Role, "members") = (MembersVersionStamp, [WId1, WId2,
> >>> ...])
> >>> ("couch_workers", Role, "health", WId) = (WorkerVersionStamp, Timeout,
> >>> Timestamp)
> >>>
> >>> Role : In our case it would be "replicator", but it could be any other
> role.
> >>>
> >>> WId : are workers IDs. These should unique identify workers. It would
> be
> >>> nice if it could be persisted, such that a worker doing a quick restart
> >>> will end up with the same id and the membership list won't change.
> However,
> >>> a random UUID would work as well.
> >>>
> >>> Timeout : This is the timeout declared by the nodes themselves. These
> need
> >>> not be the same for all node. Some nodes might decide they run slower
> so
> >>> their timeouts would be larger. But they essentially promise to update
> >>> their health status at least that often.
> >>>
> >>> Timestamp: The time of the last health report from that node.
> Timestamps
> >>> technically might not be needed as neighbor monitors could remember the
> >>> time delta between when it saw changes to the health values' version
> stamp.
> >>>
> >>> Pseudocode:
> >>>
> >>> init(Role) ->
> >>>   Members = tx(add_to_members(self(), Role)
> >>>   spawn health_ping(Members, Role)
> >>>   spawn neighbor_check(Members, Role)
> >>>   loop()
> >>>
> >>> terminate() ->
> >>>   tx(remove_self_from_members_and_health_list())
> >>>
> >>> loop() ->
> >>>   {Members, Watch} = tx(add_members(self(), Role), get_watch())
> >>>   receive
> >>>   {Watch, NewMembers} ->
> >>>       case diff(Members, NewMembers) of
> >>>       no_diff ->
> >>>           ok;
> >>>       {Added, Removed} ->
> >>>           update_neighbor_check(NewMembers)
> >>>           fire_callbacks(Added, Removed)
> >>>   end,
> >>>   loop()
> >>>
> >>> health_ping(Members, Role) ->
> >>>  tx(update_health(Role, self(), Timestamp))
> >>>  sleep(Timeout / 3)
> >>>  health_ping(Members, Role)
> >>>
> >>> neighbor_check(Members, Role) ->
> >>> Neighbor = next_in_list(self(), Members)
> >>> {Timeout, Timestamp} = tx(read_timestamp(Neighbor, Role))
> >>> case now() - Timestamp > Timeout of
> >>> true ->
> >>>     NewMembers = Tx(remove_neighbor(Neighbor, Role))
> >>>     neighbor_check(NewMembers, Role)
> >>> false ->
> >>>     sleep(Timeout)
> >>>     neighbor_check(Members, Role)
> >>> end
> >>>
> >>>
> >>> Description:
> >>>
> >>> Nodes add themselves to a membership list for each role they
> participate
> >>> in. The membership list has a version stamp. It's there to ensure that
> the
> >>> watch that is created during the update would find any change occurring
> >>> since their update.
> >>>
> >>> tx(...) is pseudocode for "runs in a transaction"
> >>>
> >>> neighbor_check() is how entries for dead workers are cleaned up. Each
> node
> >>> will monitor its neighbor's status. If it sees the neighbor has stopped
> >>> responding it will remove it from the list. That will update the
> membership
> >>> list and will fire the watch. Everyone will notice and rescan their
> >>> replicator docs.
> >>>
> >>> fire_callbacks() is just reporting to the replicator app that
> membership
> >>> has changed it and might need to rescan. On top of this code currently
> >>> there is a cluster stability logic that waits a bit before rescanning
> in
> >>> case there is a flurry of node membership changes. Like say on rolling
> node
> >>> reboots or cluster startup.
> >>>
> >>> I am not entirely sure on the semantics of watches and how lightweight
> or
> >>> heavyweight they are. Creating a watch and a version stamp will
> hopefully
> >>> not lose updates. That is, all updates after that transaction's watch
> will
> >>> fire the watch. Watches seem to have limits and then I think we'd need
> to
> >>> revert to polling
> >>>
> https://github.com/apple/foundationdb/blob/8472f1046957849a97538abb2f47b299e0ae2b2d/fdbserver/storageserver.actor.cpp#L789
> >>> which make sense but wondering if we should just start with polling
> first
> >>> and larger poll intervals. I guess it depends on how many other places
> we'd
> >>> have use watches and if we'd ever come close the even needing to handle
> >>> that error case.
> >>>
> >>>
> >>> What does everyone think? The idea would to be turn the proposal from
> 2)
> >>> into an RFC but wanted to open it for a general discussion and see what
> >>> everythone thought about it.
> >>
> >>
>
> --
> Professional Support for Apache CouchDB:
> https://neighbourhood.ie/couchdb-support/
>
>

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