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From "Robert Newson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (COUCHDB-1259) Replication ID is not stable if local server has a dynamic port number
Date Mon, 05 Nov 2012 13:06:12 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-1259?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13490609#comment-13490609
] 

Robert Newson commented on COUCHDB-1259:
----------------------------------------

I'll summarize my reasons for not finding a technical justification in your veto as follows:
"We do not trust the host:port pair today, we will not trust the uuid substitute with this
patch, no security aspect has changed". Your argument implies that seeing foo:5984 now, and
seeing foo:5984 five minutes from now means that you are talking to the same machine. This
is obviously false. if we are talking to https://foo:5984, and we trust the certificate now,
and five minutes from now, then we are sure we are talking to the same machine.

All this patch does is give a stable identifier to a couchdb server. To date, we have assumed
that full-qualified hostname and port number is a stable identifier. It is, up to a point.

So, I would like comments on whether the patch does what it claims, and on the separate point
about clarifying the meaning of the patch by using UUID in place of Host and Port in the replication_id
calculations.

I will also note that, at your step 3, replication could not resume, since the node could
not be contacted on the old port. A new replication, with the new port, would start up, and
I see no reason why it should redo from start.

All this said, I think we need other voices on this ticket now.
                
> 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
>            Assignee: Robert Newson
>            Priority: Blocker
>             Fix For: 1.3
>
>         Attachments: couchdb-1259.patch, couchdb-1259.patch
>
>
> 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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