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Date: Sun, 7 Sep 2014 21:53:28 +0000 (UTC)
From: "Nolan Lawson (JIRA)" <jira@apache.org>
To: dev@couchdb.apache.org
Message-ID: <JIRA.12737963.1409362863000.14040.1410126808751@Atlassian.JIRA>
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Subject: [jira] [Commented] (COUCHDB-2310) Add a bulk API for revs &
 open_revs
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    [ https://issues.apache.org/jira/browse/COUCHDB-2310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14125068#comment-14125068 ] 

Nolan Lawson commented on COUCHDB-2310:
---------------------------------------

For the short term, I'm on Calvin's side. I think we can solve this by just turning the CouchDB replication algorithm into a simple stream. I've started an initial version here: [pouchdb-replication-stream|https://github.com/nolanlawson/pouchdb-replication-stream]. If it works, it could be very exciting.

For the long term, I'd still like to see a new API endpoint that simply fixes the immediate problem of {{GET}} with {{open_revs}}/{{revs}} being too slow. Ideally it should be a solution that can be easily integrated into existing replication algorithms &ndash; not just PouchDB's, but also TouchDB, Cloudant Mobile, CSG, Cloudant, etc.

> Add a bulk API for revs & open_revs
> -----------------------------------
>
>                 Key: COUCHDB-2310
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-2310
>             Project: CouchDB
>          Issue Type: Bug
>      Security Level: public(Regular issues) 
>          Components: HTTP Interface
>            Reporter: Nolan Lawson
>
> CouchDB replication is too slow.
> And what makes it so slow is that it's just so unnecessarily chatty. During replication, you have to do a separate GET for each individual document, in order to get the full {{_revisions}} object for that document (using the {{revs}} and {{open_revs}} parameters &ndash; refer to [the TouchDB writeup|https://github.com/couchbaselabs/TouchDB-iOS/wiki/Replication-Algorithm] or [Benoit's writeup|http://dataprotocols.org/couchdb-replication/] if you need a refresher).
> So for example, let's say you've got a database full of 10,000 documents, and you replicate using a batch size of 500 (batch sizes are configurable in PouchDB). The conversation for a single batch basically looks like this:
> {code}
> - REPLICATOR: gimme 500 changes since seq X (1 GET request)
>   - SOURCE: okay
> - REPLICATOR: gimme the _revs_diff for these 500 docs/_revs (1 POST request)
>   - SOURCE: okay
> - repeat 500 times:
>   - REPLICATOR: gimme the _revisions for doc n with _revs [...] (1 GET request)
>     - SOURCE: okay
> - REPLICATOR: here's a _bulk_docs with 500 documents (1 POST request)
>     - TARGET: okay
> {code}
> See the problem here? That 500-loop, where we have to do a GET for each one of 500 documents, is a lot of unnecessary back-and-forth, considering that the replicator already knows what it needs before the loop starts. You can parallelize, but if you assume a browser (e.g. for PouchDB), most browsers only let you do ~8 simultaneous requests at once. Plus, there's latency and HTTP headers to consider. So overall, it's not cool.
> So why do we even need to do the separate requests? Shouldn't {{_all_docs}} be good enough? Turns out it's not, because we need this special {{_revisions}} object.
> For example, consider a document {{'foo'}} with 10 revisions. You may compact the database, in which case revisions {{1-x}} through {{9-x}} are no longer retrievable. However, if you query using {{revs}} and {{open_revs}}, those rev IDs are still available:
> {code}
> $ curl 'http://nolan.iriscouch.com/test/foo?revs=true&open_revs=all'
> {
>   "_id": "foo",
>   "_rev": "10-c78e199ad5e996b240c9d6482907088e",
>   "_revisions": {
>     "start": 10,
>     "ids": [
>       "c78e199ad5e996b240c9d6482907088e",
>       "f560283f1968a05046f0c38e468006bb",
>       "0091198554171c632c27c8342ddec5af",
>       "e0a023e2ea59db73f812ad773ea08b17",
>       "65d7f8b8206a244035edd9f252f206ad",
>       "069d1432a003c58bdd23f01ff80b718f",
>       "d21f26bb604b7fe9eba03ce4562cf37b",
>       "31d380f99a6e54875855e1c24469622d",
>       "3b4791360024426eadafe31542a2c34b",
>       "967a00dff5e02add41819138abb3284d"
>     ]
>   }
> }
> {code}
> And in the replication algorithm, _this full \_revisions object is required_ at the point when you copy the document from one database to another, which is accomplished with a POST to {{_bulk_docs}} using {{new_edits=false}}. If you don't have the full {{_revisions}} object, CouchDB accepts the new revision, but considers it to be a conflict. (The exception is with generation-1 documents, since they have no history, so as it says in the TouchDB writeup, you can safely just use {{_all_docs}} as an optimization for such documents.)
> And unfortunately, this {{_revision}} object is only available from the {{GET /:dbid/:docid}} endpoint. Trust me; I've tried the other APIs. You can't get it anywhere else.
> This is a huge problem, especially in PouchDB where we often have to deal with CORS, meaning the number of HTTP requests is doubled. So for those 500 GETs, it's an extra 500 OPTIONs, which is just unacceptable.
> Replication does not have to be slow. While we were experimenting with ways of fetching documents in bulk, we tried a technique that just relied on using {{_changes}} with {{include_docs=true}} ([|\#2472|https://github.com/pouchdb/pouchdb/pull/2472]). This pushed conflicts into the target database, but on the upside, you can sync ~95k documents from npm's skimdb repository to the browser in less than 20 minutes! (See [npm-browser.com|http://npm-browser.com] for a demo.)
> What an amazing story we could tell about the beauty of CouchDB replication, if only this trick actually worked!
> My proposal is a simple one: just add the {{revs}} and {{open_revs}} options to {{_all_docs}}. Presumably this would be aligned with {{keys}}, so similar to how {{keys}} takes an array of docIds, {{open_revs}} would take an array of array of revisions. {{revs}} would just be a boolean.
> This only gets hairy in the case of deleted documents. In this example, {{bar}} is deleted but {{foo}} is not:
> {code}
> curl -g 'http://nolan.iriscouch.com/test/_all_docs?keys=["bar","foo"]&include_docs=true'
> {"total_rows":1,"offset":0,"rows":[
> {"id":"bar","key":"bar","value":{"rev":"2-eec205a9d413992850a6e32678485900","deleted":true},"doc":null},
> {"id":"foo","key":"foo","value":{"rev":"10-c78e199ad5e996b240c9d6482907088e"},"doc":{"_id":"foo","_rev":"10-c78e199ad5e996b240c9d6482907088e"}}
> ]}
> {code}
> The cleanest would be to attach the {{_revisions}} object to the {{doc}}, but if you use {{keys}}, then the deleted documents are returned with {{doc: null}}, even if you specify {{include_docs=true}}. One workaround would be to simply add a {{revisions}} object to the {{value}}.
> If all of this would be too difficult to implement under the hood in CouchDB, I'd also be happy to get the {{_revisions}} back in {{_changes}}, {{_revs_diff}}, or even in a separate endpoint. I don't care, as long as there is some bulk API where I can get multiple {{_revisions}} for multiple documents at once.
> On the PouchDB end of things, we would really like to push forward on this. I'm happy to implement a Node.js proxy to stand in front of CouchDB/Cloudant/CSG and add this new API, plus adding it directly to PouchDB Server. I can invent whatever API I want, but the main thing is that I would like this API to be something that all the major players can agree upon (Apache, Cloudant, Couchbase) so that eventually the proxy is no longer necessary.
> Thanks for reading the WoT. Looking forward to a faster CouchDB replication protocol, since it's the thing that ties us all together and makes this crazy experiment worthwhile.
> Background: [this|https://github.com/pouchdb/pouchdb/issues/2686] and [this|https://gist.github.com/nolanlawson/340cb898f8ed9f3db8a0].


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