[Dovecot] Dovecot v2.2 plans

[Timo Sirainen]

Here's a list of things I've been thinking about implementing for Dovecot v2.2. Probably not all of them will make it, but I'm at least interested in working on these if I have time.

Previously I've mostly been working on things that different companies were paying me to work on. This is the first time I have my own company, but the prioritization still works pretty much the same way:

 - 1. priority: If your company is highly interested in getting something implemented, we can do it as a project via my company. This guarantees that you'll get the feature implemented in a way that integrates well into your system.
 - 2. priority: Companies who have bought Dovecot support contract can let me know what they're interested in getting implemented. It's not a guarantee that it gets implemented, but it does affect my priorities. :)
 - 3. priority: Things other people want to get implemented.

There are also a lot of other things I have to spend my time on, which are before the 2. priority above. I guess we'll see how things work out.

dsync-based replication
-----------------------

I'll write a separate post about this later. Besides, it's coming for Dovecot v2.1 so it's a bit off topic, but I thought I'd mention it anyway.

Shared mailbox improvements
---------------------------

Support for private flags for all mailbox formats:

namespace {
  type = public
  prefix = Public/
  mail_location = mdbox:/var/vmail/public:PVTINDEX=~/mdbox/indexes-public
}

 - dsync needs to be able to replicate the private flags as well as shared flags.
 - might as well add a common way for all mailbox formats to specify which flags are shared and which aren't. $controldir/dovecot-flags would say which is the default (private or shared) and what flags/keywords are the opposite.
 - easy way to configure shared mailboxes to be accessed via imapc backend, which would allow easy shared mailbox accesses across servers or simply between two system users in same server. (this may be tricky to dsync.)
 - global ACLs read from a single file supporting wildcards, instead of multiple different files
 - default ACLs for each namespace/storage root (maybe implemented using the above..)

Metadata / annotations
----------------------

Add support for server, mailbox and mail annotations. These need to be dsyncable, so their changes need to be stored in various .log files:

1. Per-server metadata. This is similar to subscriptions: Add changes to dovecot.mailbox.log file, with each entry name a hash of the metadata key that was changed.

2. Per-mailbox metadata. Changes to this belong inside mailbox_transaction_context, which write the changes to mailbox's dovecot.index.log files. Each log record contains a list of changed annotation keys. This gives each change a modseq, and also allows easily finding out what changes other clients have done, so if a client has done ENABLE METADATA Dovecot can easily push metadata changes to client by only reading the dovecot.index.log file.

3. Per-mail metadata. This is pretty much equivalent to per-mailbox metadata, except changes are associated to specific message UIDs.

The permanent storage is in dict. The dict keys have components:
 - priv/ vs. shared/ for specifying private vs. shared metadata
 - server/ vs mailbox/<mailbox guid>/ vs. mail/<mailbox guid>/<uid>
 - the metadata key name

This would be a good time to improve the dict configuration to allow things like:
 - mixed backends for different hierarchies (e.g. priv/mailbox/* goes to a file, while the rest goes to sql)
 - allow sql dict to be used in more relational way, so that mail annotations could be stored with tables: mailbox (id, guid) and mail_annotation (mailbox_id, key, value), i.e. avoid duplicating the guid everywhere.

Things to think through:
 - How to handle quota? Probably needs to be different from regular mail quota. Probably some per-user "metadata quota bytes" counter/limit.
 - Dict lookups should be done asynchronously and prefetched as much as possible. For per-mail annotation lookups mail_alloc() needs to include a list of annotations that are wanted.

Configuration
-------------

Copy all mail settings to namespaces, so it'll be possible to use per-namespace mailbox settings. Especially important for imapc_* settings, but can be useful for others as well. Those settings that aren't explicitly defined in the namespace will use the global defaults. (Should doveconf -a show all of these values, or simply the explicitly set values?)

Get rid of *.conf.ext files. Make everything part of dovecot.conf, so doveconf -n outputs ALL of the configuration. There are mainly 3 config files I'm thinking about: dict-sql, passdb/userdb sql, passdb/userdb ldap. The dict-sql is something I think needs a bigger redesign (mentioned above in "Metadata" section), but the sql/ldap auth configs could be merged. One way could be:

sql_db sqlmails {
  # most settings from dovecot-sql.conf.ext, except for queries
  driver = mysql
  connect = ...
}

ldap_db ldapmails {
  # most settings from dovecot-ldap.conf.ext, except attributes/filters
}

passdb {
  driver = sql
  db = sqlmails
  sql_query = select password from users where username = '%u'
}
passdb {
  driver = ldap
  db = ldapmails
  ldap_attributes {
    password = %{ldap:userPassword}
  }
  ldap_filter = ...
}

The sql_db {} and ldap_db {} would be generic enough to be used everywhere (e.g. dict-sql), not just for passdb/userdb.

Some problems:
 - Similar to the per-namespace mail settings, doveconf -a would output all sql_query, ldap_attributes, ldap_filter, etc. settings for all passdbs/userdbs. Perhaps a similar solution?
 - The database configs contain passwords, so they should be readable only by root. This makes running dovecot-lda and maybe doveadm difficult, since they fail at "permission denied" when trying to open the config. There are probably only two solutions: a) The db configs need to be !include_try'd or b) the configs can be world-readable, but only passwords are placed to only-root-readable files by using "password = <db.password"

IMAP state saving/restoring
---------------------------

IMAP connections are often long running. Problems with this:

1. Currently each connection requires a separate process (at least to work reliably), which means each connection also uses quite a lot of memory even when they aren't doing anything for a long time.
2. Some clients don't handle lost connections very nicely. So Dovecot can't be upgraded without causing some user annoyance. Also in a cluster if you want to bring down one server, the connections have to be disconnected before they can be moved to another server.

If IMAP session state could be reliably saved and later restored to another process, both of the above problems could be avoided entirely. Typically when a connection is IDLEing there are really just 4 things that need to be remembered: username, selected mailbox name, its UIDVALIDITY and HIGHESTMODSEQ. With this information the IMAP session can be fully restored in another process without losing any state. So, what we could do is:

1. When an IMAP connection has bee IDLEing for a while (configurable initial time, could be dynamically adjusted):
 - move the IMAP state and the connection fd to imap-idle process
 - the old imap process is destroyed
 - imap-idle process can handle lots of IMAP connections
 - imap-idle process also uses inotify/etc. to watch for changes in the specified mailbox
 - if any mailbox changes happen or IMAP client sends a command, start up a new imap process, restore the state and continue from where we left off
 - This could save quite a lot of memory at the expense of some CPU usage

2. Dovecot proxy <-> backend protocol could be improved to support moving connection to another backend. Possibly using a separate control connection to avoid making the proxying less efficient in normal operation.

3. When restarting Dovecot, move all the connections to a process that keeps the connections open for a while. When Dovecot starts up, create imap processes back to the connections. This allows changing configuration for existing client connections (which sometimes may be bad! need to add checks against client-visible config conflicts), upgrading Dovecot, etc. without being visible to clients. The only problem is SSL connections: OpenSSL doens't provide a way to save/restore state, so either you need to set shutdown_clients=no (and possibly keep some imap-login processes doing SSL proxying for a long time), or SSL connections need to be killed. Of course the SSL handling could be outsourced to some other software/hardware outside Dovecot.

The IMAP state saving isn't always easy. Initially it could be implemented only for the simple cases (which are a majority) and later extended to cover more.

IMAP extensions
---------------

 - CATENATE is already implemented by Stephan
 - URLAUTH is also planned to be implemented, somewhat differently than in Apple's patch. The idea is to create a separate imap-urlauth service that provides extra security.
 - NOTIFY extension could be implemented efficiently using mailbox list indexes, which already exists in v2.1.
 - FILTERS extension can be easily implemented once METADATA is implemented
 - There are also other missing extensions, but they're probably less important: BINARY & URLAUTH=BINARY, CONVERT, CONTEXT=SORT, CREATE-SPECIAL-USE, MULTISEARCH, UTF8=* and some i18n stuff.

Backups
-------

Filesystem based backups have worked well enough with Dovecot in the past. But with new features like single instance storage it's becoming more difficult. There's no 100% consistent way to even get filesystem level backups with SIS enabled, because deleting both the message file and its attachment files can't be done atomically (although usually this isn't a real problem). Restoring SIS mails is more difficult though, first you need to restore the dbox mail files and then you need to figure out what attachment files from SIS need to be restored, and finally you'll need to do doveadm import to put them into their final destination.

I don't have much experience with backup software, but other people in my company do. The initial idea is to implement a Dovecot backup agent to one (commercial) backup software, which allows doing online backups and restoring mails one user/mailbox/mail at a time. I don't know the details yet how exactly this is going to be implemented, but the basic plan is probably to implement a "backup" mail storage backend, which is a PostgreSQL pg_dump-like flat file containing mails from all mailboxes. doveadm backup/import can then export/import this format via stdout/stdin. Incremental backups could possibly be done by giving a timestamp of previous backup run (I'm not sure about this yet).

Once I've managed to implement the first fully functional backup agent, it should become clearer how to implement it to other backup solutions.

Random things
-------------

 - dovecot.index.cache file writing is too complex, should be simplified
 - Enable auth_debug[_passwords]=yes on-the-fly for some specific users/IPs via doveadm
 - Optimize virtual mailboxes using mailbox list indexes. It wouldn't anymore need to keep all the backend mailboxes' index files open.
 - Would be nice to go forward with supporting key-value databases as mail storage backends.