(I swear I'll stop soon...) Using /_db_updates as a cheap mechanism to detect activity at the source for any database we're interested in is an important optimization. We didn't discuss it this past week as we felt that /_db_updates wasn't sufficiently reliable. We can save a lot of churn in the scheduler by simply not resuming any job unless we have seen an update to the source database.
B. > On 20 Mar 2016, at 14:36, Robert Samuel Newson <[email protected]> wrote: > > I missed a point in Adam's earlier post. > > The current scheme uses couch_event for runtime changes to _replicator docs > but has to read all updates of all _replicator databases at startup. In the > steady state it is just receiving couch_event notifications. The /_db_updates > option would change that only slightly (we'd read /_db_updates from 0 to find > all _replicator databases, rather than reading the changes feed for the > node-local 'dbs' database). > > CouchDB itself has a single /_replicator database, of course, but the code > will consider any database to be a /_replicator database if the name ends > that way. i.e, today, if you made a database called foo/_replicator it would > be considered a /_replicator database by the system (and we'd inject the > ddoc, etc). > > B. > >> On 20 Mar 2016, at 14:31, Robert Samuel Newson <[email protected]> wrote: >> >> Since I'm typing anyway, and haven't yet been dinged for top-posting, I >> wanted to mention one other optimization we had in mind. >> >> Currently each replicator job has its own connection pool. When we introduce >> the notion that we can stop and restart jobs, those become approximately >> useless. So we will obvious hoist that 'up' to a higher level and manage >> connection pools at the manager level. >> >> One optimization that seems obvious from the Cloudant perspective is to >> allow reuse of connections to the same destinations even though they are >> ostensibly for different domains. That is, a connection to >> rnewson.cloudant.com is ultimately a connection to lbX.jenever.cloudant.com. >> This connection could just as easily be used for any other user in the >> jenever cluster. Thus, if it's idle, we could borrow that connection rather >> than create a new one. >> >>> host rnewson.cloudant.com >> rnewson.cloudant.com is an alias for jenever.cloudant.com. >> jenever.cloudant.com is an alias for lb2.jenever.cloudant.com. >> lb2.jenever.cloudant.com has address 5.153.0.207 >> >> Rather than add rnewson.cloudant.com > 5.153.0.207 to the pool, we would add >> lb2.jenever.cloudant.com -> 5.153.0.207 and resolve rnewson.cloudant.com to >> its ultimate CNAME before consulting the pool. >> >> Does this optimization help elsewhere than Cloudant? >> >>> On 20 Mar 2016, at 14:22, Robert Samuel Newson <[email protected]> wrote: >>> >>> My point is that we can (and currently do) trigger the replication manager >>> on receipt of the database updated event, so it avoids all of the other >>> parts of the sequence you describe which could fail. >>> >>> The obvious difference, and I suspect this is what motivates Adam's >>> position, is that _db_updates can be called remotely. A solution using >>> /_db_updates as its feed can run somewhere else, it wouldn't even need to >>> be a couchdb cluster. With the current 2.0 scheme, the _replicator db has >>> to live on the nodes performing replication management (and therefore it >>> depends on couch_{btree,file} etc). That's a huge incentive to go the >>> /_db_updates route and it would serve as a model for others like pouchdb >>> that cannot choose to co-locate. >>> >>> One side-benefit we get from using database updated events from the >>> _replicator shards, though, is that it helps us determine which node will >>> run any particular job. We allocate a job to the lowest live erlang node >>> that hosts the document. If we go with /_db_updates, we'll need some other >>> scheme. That's not a bad thing (indeed, it could be a very good thing), but >>> it would need more thought. While in Seattle we did discuss both directions >>> at some length and believe we'd need some form of leader election system, >>> the leader would then assign (and rebalance) replication jobs across the >>> erlang cluster. I pointed at a proof-of-concept implementation of an >>> algorithm I trust that I wrote a while back at >>> https://github.com/cloudant/sobhuza as a possible starting point. >>> >>> B. >>> >>> P.S. I'm using Mail.app and simply replying where it sticks the cursor (at >>> the top), but in other forums I've been berated for top-posting. Should I >>> modify my reply style here? >>> >>> On 19 Mar 2016, at 21:42, Benjamin Bastian <[email protected]> wrote: >>>> >>>> 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 >>> >> >
