multi sync (>2 servers) + selective sync + trigger

[Daniel van Ham Colchete]

B,

I really like the idea of N-way replication. Pairs are ugly, they cost
double! Even if you have 20 servers, when one goes down all that IO traffic
goes to just one.

So, what I did here was a (kind of) DHT-based n-way replication, where the
node for the second copy is independent of where the first copy is.

For that you will have to use the mail_replica inside your userdb. Here I'm
using MySQL. The catch is that the mail_replica is now always the same for
the same user. The first server need a mail_replica pointing to the second
while the second need a mail_replica pointing to the first. In order to do
that I have a table with two fields: hostA and hostB. Each dovecot will
have to figure out witch one to use. So, my dovecot-sql.conf-ext  looks
like:

user_query = select email, uid, gid, home, concat('tcp:', IF(hostA =
'10.0.3.11', hostB, hostA), ':12345') as mail_replica from users where
email='%Lu' and (hostA='10.0.3.11' or hostB='10.0.3.11')
password_query = SELECT email as user, password FROM users WHERE email =
'%Lu'
iterate_query = SELECT email AS user FROM users WHERE hostA = '10.0.3.11'
or hostB='10.0.3.11'

On each host you have to put the right IP on each dovecot-sql.conf.ext.

It works perfectly! And it is nice too! Imagine you have a 10 node cluster
and the first 2 nodes fail. Instead of having 10% of your users down, only
2.2% (2 x 1/10 x 1/9) of  your users will be offline! Also if each node can
handle 10k users and if you want to have N+1 redundancy you can have 90k
users in the same servers, instead of 50k if you had pairs. So,
availability is UP and costs are DOWN! Is there really a choice here? Hehe.

Caveats:

- Dovecot Proxy don't understand the concept of having 2 (or more) active
places for the same account. It seems like you would need an outside
monitor that would check the network every x seconds and change the proxy
field on the database when a node went down to hostA or hostB. I think this
is madness so I had to write my own POP3/IMAP/LMTP Proxy (it's not that
hard) to try the primary first and if that socket don't connect in 1 second
we go straight to the second without the user noticing it. It is really bad
that Dovecot Proxy can't talk replication.

- Your availability goes really UP (lots of nines) when you add a third
copy while still having lower costs than with pairs, but you can't use
Dovecot's notificator/aggregator/replication plugins for that to make
everything run smoothly and I don't think that an outside/cron-based dsync
is an option.

 - If you really use DHT-like algorithms, rebalancing your cluster will
require mininum data movement and you add or remove nodes. It will require
a small change on the SQL queries above so that the new node also
recognizes the new user, but it is easy to do.

Let me know what you think!

Best,
Daniel Colchete

On Fri, May 29, 2015 at 4:25 PM, <b-dovecot.org at grmbl.net> wrote:

> Hi all,
>
>
> I've been researching ways to replicate mail across multiple mailstores
> and have a few questions.
>
> Synching 2 mailstores (M1 & M2) via dsync works fine.
> I want to add a 3rd and 4th (M3 & M4) server to also be synced.
>
>
> Multi-sync (>2 servers):
>
> 1) How do I tell M1 and/or M2 to also sync to M3 and/or M4?
>
> Selective sync:
>
> 2) How do I sync specific domains to specific servers and not to others?
>
> Trigger:
>
> 3) Does a normal sync check only check the mailbox being changed or does
> it get triggered on mailbox changes to check ALL indexes and sync all
> changed mailboxes accordingly?
> 4) Can I use an email to trigger a sync for any other mailboxes which
> might be out of sync?
>
>
> TIA
>
> B
>