As NFS wiki page describes, the main problem with NFS has always been caching problems. One NFS client changes two files, but another NFS client sees only one of the changes, which Dovecot then assumes is caused by corruption.
The recommended solution has always been to redirect the same user to only a single server at the same time. User doesn't have to be permanently assigned there, but as long as a server has some of user's files cached, it should be the only server accessing the user's mailbox. Recently I was thinking about a way to make this possible with an SQL database.
The company here in Italy didn't really like such idea, so I thought about making it more transparent and simpler to manage. The result is a new "director" service, which does basically the same thing, except without SQL database. The idea is that your load balancer can redirect connections to one or more Dovecot proxies, which internally then figure out where the user should go. So the proxies act kind of like a secondary load balancer layer.
When a connection from a newly seen user arrives, it gets assigned to a mail server according to a function:
host = vhosts[ md5(username) mod vhosts_count ]
This way all of the proxies assign the same user to the same host without having to talk to each others. The vhosts is basically an array of hosts, except each host is initially listed there 100 times (vhost count=100). This vhost count can then be increased or decreased as necessary to change the host's load, probably automatically in future.
The problem is then of course that if (v)hosts are added or removed, the above function will return a different host than was previously used for the same user. That's why there is also an in-memory database that keeps track of username → (hostname, timestamp) mappings. Every new connection from user refreshes the timestamp. Also existing connections refresh the timestamp every n minutes. Once all connections are gone, the timestamp expires and the user is removed from database.
The final problem then is how multiple proxies synchronize their state. The proxies connect to each others forming a connection ring. For example with 4 proxies the connections would go like A → B → C → A. Each time a user is added/refreshed, a notification is sent to both directions in the ring (e.g. B sends to A and C), which in turn forward it until it reaches a server that has already seen it. This way if a proxy dies (or just hangs for a few seconds), the other proxies still get the changes without waiting for it to timeout. Host changes are replicated in the same way.
It's possible that two connections from a user arrive to different proxies while (v)hosts are being added/removed. It's also possible that only one of the proxies has seen the host change. So the proxies could redirect users to different servers during that time. This can be prevented by doing a ring-wide sync, during which all proxies delay assigning hostnames to new users. This delay shouldn't be too bad because a) they should happen rarely, b) it should be over quickly, c) users already in database can still be redirected during the sync.
The main complexity here comes from how to handle proxy server failures in different situations. Those are less interesting to describe and I haven't yet implemented all of it, so let's just assume that in future it all works perfectly. :) I was also thinking about writing a test program to simulate the director failures to make sure it all works.
Finally, there are the doveadm commands that can be used to:
- List the director status:
# doveadm director status
mail server ip vhosts users
184.108.40.206 100 1312
220.127.116.11 50 1424
- Add a new mail server (defaults are in dovecot.conf):
# doveadm director add 18.104.22.168
- Change a mail server's vhost count to alter its connection count (also works during adding):
# doveadm director add 22.214.171.124 50
- Remove a mail server completely (because it's down):
# doveadm director remove 126.96.36.199
If you want to slowly get users away from a specific server, you can assign its vhost count to 0 and wait for its user count to drop to zero. If the server is still working while "doveadm director remove" is called, new connections from the users in that server are going to other servers while the old ones are still being handled.