I’ll never berate anyone for top-posting (or bottom-posting for that matter). I just follow suit with whatever the current thread is doing — in this, very very clearly top-posting ;)
Thank you for making this distinction clear. Personally I was only ever interested in the first case. Scoping the replicator manager to only learn about _replicator docs on the local cluster through internal APIs is a smart move — I wasn’t suggesting anything different there. I do think we should have an efficient way for a client to learn about the existence of new updates to an arbitrary number of databases on a remote cluster using a single socket. Adam > On Mar 20, 2016, at 10:45 AM, Robert Samuel Newson <[email protected]> wrote: > > Final note, we've conflated two uses of /_db_updates that I want to be very > clear on; > > 1) using /_db_updates to detect active source databases of a replication job. > 2) using /_db_updates to hear about new/updated/deleted _replicator documents. > > It was the 2nd case where the unreliability was a concern, since the update > frequency is very low, one expects, for _replicator databases. > >> On 20 Mar 2016, at 14:44, Robert Samuel Newson <[email protected]> wrote: >> >> (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 >>>>> >>>> >>> >> >
