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From Jens Alfke <>
Subject Re: A million databases
Date Tue, 25 Mar 2014 18:52:44 GMT

On Mar 25, 2014, at 11:36 AM, Stanley Iriele <> wrote:

> What is the difference between filtered changes feed and sync gateway?. Are
> they that comparable?

The Sync Gateway has a notion called “channels”, which are sort of like tags that get
applied to a document revision when it’s added to the database, by an app-supplied JS “sync

Clients can then filter their pull replications by channel, by using the magic filter name
“sync_gateway/bychannel” and a query parameter “channels” whose value is a comma-separated
list of channel names. This works with CouchDB, Cloudant, PouchDB, etc. as well as Couchbase

The big difference from traditional filtered replication is that the _changes feed can filter
by channels very efficiently. It doesn’t have to load (or call a JS function on) every revision;
it’s got per-channel change indexes it can consult.

Channels are also used for access control. Every user account has a set of channels that it’s
allowed to access, and it can’t read documents that aren’t tagged with one or more of
its channels. This is the thing that lets you avoid having to create a database per user.
Also, an unfiltered pull replication will automatically be limited to the set of the user’s
channels, so if you ignore filters you get all the docs you’re allowed to see and nothing

(Hopefully this isn’t off-topic. I believe the Sync Gateway is a valid member of the CouchDB
family of databases, albeit perhaps that red-headed cousin off in the corner…)

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