When a shard is updated, it'll trigger a "database updated" event. CouchDB will hold those updates in memory for a configurable amount of time in order to dedupe updates. It'll then cast lists of updated databases to nodes which host the relevant _db_updates shards for further deduplication. It's only at that point that the updates are persisted. Only a single update needs to reach the _db_updates DB. IIRC, _db_updates triggers up to n^3 (assuming the _db_updates DB and the updated DB have the same N), so it may be a bit tricky for all of them to fail. You'd need coordinated node failure. Perhaps something like datacenter power loss. Another possible issue is if all the nodes which host a shard range of the _db_updates DB are unreachable by the nodes which host a shard range of any other DB. Even if it was momentary, it'd cause messages to be dropped from the _db_updates feed.
For n=3 DBs, it seems like it'd be difficult for all of those things to go wrong (except perhaps in the case of power loss or catastrophic network failure). For n=1 DBs, you'd simply need to reboot a node soon after an update. On Sat, Mar 19, 2016 at 1:31 PM, Adam Kocoloski <[email protected]> wrote: > Hi Bob, comments inline: > > > On Mar 19, 2016, at 2:36 PM, Robert Samuel Newson <[email protected]> > wrote: > > > > Hi, > > > > The problem is that _db_updates is not guaranteed to see every update, > so I think it falls at the first hurdle. > > Do you mean to say that a listener of _db_updates is not guaranteed to see > every updated *database*? I think it would be helpful for the discussion to > describe the scenario in which an updated database permanently fails to > show up in the feed. My recollection is that it’s quite byzantine. > > > What couch_replicator_manager does in couchdb 2.0 (though not in the > version that Cloudant originally contributed) is to us ecouch_event, notice > which are to _replicator shards, and trigger management work from that. > > Did you mean to say “couch_event”? I assume so. You’re describing how the > replicator manager discovers new replication jobs, not how the jobs > discover new updates to source databases specified by replication jobs. > Seems orthogonal to me unless I missed something. > > > Some work I'm embarking on, with a few other devs here at Cloudant, is > to enhance the replicator manager to not run all jobs at once and it is > indeed the plan to have each of those jobs run for a while, kill them (they > checkpoint then close all resources) and reschedule them later. It's TBD > whether we'd always strip feed=continuous from those. We _could_ let each > job run to completion (i.e, caught up to the source db as of the start of > the replication job) but I think we have to be a bit smarter and allow > replication jobs that constantly have work to do (i.e, the source db is > always busy), to run as they run today, with feed=continuous, unless > forcibly ousted by a scheduler due to some configuration concurrency > setting. > > So I think this is really the crux of the issue. My contention is that > permanently occupying a socket for each continuous replication with the > same source and mediator is needlessly expensive, and that _db_updates > could be an elegant replacement. > > > I note for completeness that the work we're planning explicitly > includes "multi database" strategies, you'll hopefully be able to make a > single _replicator doc that represents your entire intention (e.g, > "replicate _all_ dbs from server1 to server2”). > > Nice! It’ll be good to hear more about that design as it evolves, > particularly in aspects like discovery of newly created source databases > and reporting of 403s and other fatal errors. > > Adam
