cyrus replication over a WAN

[Jon .]

I've done and am doing this. For questions 1 and 2 I don't really have an
answer because bandwidth wasn't an issue for me / the people I set this up
for.

In the past I found that if Cyrus is restarted on the replica, the
sync_client on the master server fails. I also saw instances of sync_client
failing on the master if the replica isn't available. The lower levels
pertaining to why I don't know (or remember), but I can recreate this and
post the exact error that is logged if any one on this list is interested.

Here's a bash script executed by the cron daemon on a pair of Cyrus servers
every minute for watch-dogging sync_client.

#!/bin/bash

if [ "$(pgrep cyrus-master)" ] ; then
   if [ "$(cat /etc/cyrus-role)" == "master" ] ; then
      if [ ! "$(pgrep sync_client)" ] ; then
         su cyrus -c "/usr/lib/cyrus-imapd/sync_client -r" ;
      fi
   fi
fi

if [ "$(cat /etc/cyrus-role)" == "replica" ] ; then
   if [ "$(pgrep sync_client)" ] ; then
      kill $(pgrep sync_client)
   fi
fi

The script checks for the conditions that the server is set as the master
(which it is set to if Cyrus is started) and that the sync_client is not
running. If so it runs the sync_client again.

The script does not check if the Cyrus daemon is running because it was
written taking into consideration a scenario where replication hasn't
occurred for some arbitrary amount of time while the Cyrus daemon was
running but replication once again becomes possible while the Cyrus daemon
local to the master is not running.

You can also use this script to enable bi-directional IMAP replication. Why
you would want to do such a thing (with a two-server master/replica
configuration) is if you have SMTP daemons running on both of your Cyrus
servers and want the "replica"/"passive" (IMAP-side at least) server to be
able to push mail accepted by it from the outside world onto the master
Cyrus server. This is probably better (and faster for the user) than simply
dropping packets from outside SMTP servers (which is what I've seen on
two-server master/replica Cyrus servers out there, fencing was done through
setting ports as "filtered'). Because a user's IMAP activity on the master
server shouldn't conflict with incoming mail, in theory bi-directional Cyrus
replication for incoming messages from the SMTP daemon is not only optimal
but also possible without creating a split-brain scenario. I plan to test
this myself when I have time.

This was all done with Cyrus 2.3.7 on CentOS 5.3 and 5.4 in each case. Your
mileage may vary and if any of the behavior described above is expected to
be different with new versions of Cyrus please share.

- Jon


On Wed, Dec 9, 2009 at 8:08 AM, Robert Mueller (web) <robm at fastmail.fm>wrote:

> I just wanted to check if any people have any experience or reports
> about doing cyrus replication over a WAN?
>
> A few potential issues spring to mind:
>
> 1. I know the replication protocol is pretty "chatty", so does the
> higher latency over a WAN (ms vs us) cause it to get behind more
> regularly?
> 2. The protocol uses more bandwidth than really needed, has that caused
> unexpected bandwidth usage problems?
> 3. A WAN isn't nearly as reliable as a local connection. If you're over
> a VPN, and the connection goes down for a minute and then comes back,
> does that confuse the sync_client, or does it tend to just continue and
> work?
>
> Any insights would be appreciated.
>
> Rob
> Rob Mueller
> robm at fastmail.fm
>
> ----
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>
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