Dovecot Director: Preferred backend server

[Steven Varco]

Hi Aki

Thanks for pointing  out the tag feature which sound really interesting in the first place.

However, if I understand the documentation correctly:
> With tags you can use a single director ring to serve multiple backend clusters. Each backend cluster is assigned a tag name, which can be anything 
> you want. By default everything has an empty tag. A passdb lookup can return "director_tag" field containing the wanted tag name. If there aren't any 
> backend servers with the wanted tag, it's treated the same as if there aren't any backend servers available (= wait for 30 secs for a backend and
> then return temporary failure).

As of my understanding, this only helps if there are multiple IMAP _clusters_ in the doveadm ring.
In my case I have only one cluster (with two IMAP _servers_) and would want to go to a specific server, failing over to another if that is unavailable.
Now if I have the following scenario:

# Director Server
(DEV) root at lb01 [~] # doveadm director status
mail server ip tag  vhosts state state changed users
mx01.example.com   mx01 100    up    -             0
mx02.example.com   mx02 100    up    -             1

# IMAP Server
(DEV) root at mx01 [~] # doveadm user 'test at example.com'
field	value
uid	1025
gid	12
home	/srv/mail/example.com/test
mail	maildir:~/Maildir
maildir	example.com/test/
mail_home	/srv/mail/example.com/test
quota_rule	*:storage=20480
sieve_dir	/srv/mail/example.com/test/sieve
director_tag	mx01

Than user 'test at example.com‘ would go to the backend host mx01.example.com, BUT, if mx01.example.com goes down, it would probably fail, because user 'test at example.com‘ wants tag „mx01“, which is now down and the only server with that tag?


By the way, I did a quick live test and it does not even seem to work, when both hosts are up, failing with the log entry on the dovecot server:
Aug 31 11:11:11 lb01 dovecot: director: Error: director: User test at example.com host lookup failed: Timeout because no hosts - queued for 30 secs (Ring synced for 385 secs, hash=1561836376)

Do see what I’m missing out here?
Using dovecot 2.2.36 (1f10bfa63) on both the directror and IMAP backend.

thanks,
Steven

-- 
https://steven.varco.ch/ 

> Am 30.08.2021 um 19:20 schrieb Aki Tuomi <aki.tuomi at open-xchange.com>:
> 
> 
>> On 30/08/2021 19:09 Steven Varco <dovecot.org at bbs.varco.ch> wrote:
>> 
>> 
>> Hi All
>> 
>> I have a dovecot cluster with directror and two IMAP Servers behind.
>> 
>> Since they are in geographical different locations I would like to have users to go to a specific IMAP backend server (if both are up) and only switch to the other if one goes down (failover).
>> 
>> As to my current knowledge the PassDB extra field „host=„ is not suitable in this case as it would never route the client to a different backend, even if the „user specific backend“ would be down.
>> 
>> Is their a way in dovecot to achive this? As far as I searched the documentation I could not find any information on this so far.
>> 
>> If not, it may also help if I could get certain users to „initially" go to a specific backend (since director usually routes a client/user to the same backend server it initially connects) and therefore it would be interesting to know how dovecot director chooses wether a user goes to server1 or server2?
>> And if a client already gets to server2, how to bring it „back“ to server1?
>> 
>> thanks in advance,
>> Steven
>> 
>> -- 
>> https://steven.varco.ch/ 
>> https://www.tech-island.com/
> 
> 
> Hi!
> 
> Use dovecot director tag feature. You can match users with tag= to a specific backend at tag.
> 
> Aki