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From "Jens Alfke (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (COUCHDB-1259) Replication ID is not stable if local server has a dynamic port number
Date Wed, 24 Aug 2011 03:17:29 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-1259?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13089976#comment-13089976
] 

Jens Alfke commented on COUCHDB-1259:
-------------------------------------

@Jason: The reason I filed this bug report is that the URL of a database in Couchbase Mobile
*isn't* unique. It's barely meaningful at all; it's of the form "http://127.0.0.1:nnnnn" where
"nnnnn" is an upredictable port number assigned by the TCP stack at launch time. That URL
isn't even exposed to the outside world because the CouchDB server is only listening on the
loopback interface.

The state that's being represented by a replication ID is the contents of the database (at
least as it was when it last synced.) So it seems the ID should be something that sticks to
the database itself, not to any ephemeral manifestation like a URL.

> Replication ID is not stable if local server has a dynamic port number
> ----------------------------------------------------------------------
>
>                 Key: COUCHDB-1259
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-1259
>             Project: CouchDB
>          Issue Type: Bug
>          Components: Replication
>    Affects Versions: 1.1
>            Reporter: Jens Alfke
>
> I noticed that when Couchbase Mobile running on iOS replicates to/from a remote server
(on iriscouch in this case), the replication has to fetch the full _changes feed every time
it starts. Filipe helped me track down the problem -- the replication ID is coming out different
every time. The reason for this is that the local port number, which is one of the inputs
to the hash that generates the replication ID, is randomly assigned by the OS. (I.e. it uses
a port number of 0 when opening its listener socket.) This is because there could be multiple
apps using Couchbase Mobile running on the same device and we can't have their ports colliding.
> The underlying problem is that CouchDB is attempting to generate a unique ID for a particular
pair of {source, destination} databases, but it's basing it on attributes that aren't fundamental
to the database and can change, like the hostname or port number.
> One solution, proposed by Filipe and me, is to assign each database (or each server?)
a random UUID when it's created, and use that to generate replication IDs.
> Another solution, proposed by Damien, is to have CouchDB let the client work out the
replication ID on its own, and set it as a property in the replication document (or the JSON
body of a _replicate request.) This is even more flexible and will handle tricky scenarios
like full P2P replication where there may be no low-level way to uniquely identify the remote
database being synced with.

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