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Mailbox Synchronization
=======================

The idea behind synchronization is to find out what changes other sessions have
done to the mailbox and to finalize our own changes to the mailbox.

For example if you expunge a message in a transaction and commit it, the commit
will only write a "please expunge UID n" record to Dovecot's transaction log
file. The message still exists on the disk. The next time Dovecot syncs the
mailbox (either the session that wrote the record or another one), it goes
through all the non-synchronized records in transaction log and applies the
requested changes to the backend mailbox. Syncing can be a bit heavyweight
operation, so it's possible to commit multiple transactions and perform a
single sync for all of them. Dovecot attempts to do this with IMAP protocol
when pipelining commands.

The other important job of syncing is to refresh mailbox's state:

 * Finding out about external modifications to mailbox (e.g. a new mail
   delivered to Maildir/new/).
 * Updating in-memory view of what messages exist, what their flags are, etc.

When a mailbox is opened, its state starts with what index files contain at the
time. Since the backend mailbox may have already changed, and syncing an
up-to-date mailbox is usually really cheap, there isn't much point in not
syncing mailbox immediately after opening. The mailbox state stays the same
until you synchronize the mailbox again, before that no new messages show up
and no messages get expunged.

Typically you would sync the mailbox

 * after committing a transaction that modifies backend mailbox in any way
   (instead of just internal index data), such as after changing message flags
   or expunging a message.
 * whenever you want to find out if there are any changes. With IMAP protocol
   this is done every time after running a command.

Initializing
------------

'mailbox_sync_init()' initializes syncing.

There are some flags that control how much effort is spent on syncing:

 * 'MAILBOX_SYNC_FLAG_FAST' can be given when you're ready for mailbox to be
   refreshed, but don't care much if it actually is or not. When this flag is
   set, Dovecot still notices all internal changes, but external changes are
   checked only once every few seconds or so.
 * 'MAILBOX_SYNC_FLAG_FULL_READ' is mainly useful with mboxes. If
   'mbox_dirty_syncs=yes' and a new mail gets appended to mbox by an external
   program, Dovecot assumes that the only change was the added mail, even
   though the program may have also modified existing messages' flags by
   rewriting Status: headers. If 'mbox_very_dirty_syncs=no', these changes are
   noticed after the next time mailbox is opened. So when this flag is enabled,
   it means Dovecot should try harder to find out if there were any external
   unexpected changes. It's currently used only with IMAP SELECT and CHECK
   commands and POP3 startup. Probably unnecessary elsewhere.
 * 'MAILBOX_SYNC_FLAG_FULL_WRITE' is again mainly useful with mboxes. If
   'mbox_lazy_writes=no', Dovecot delays writing flag changes to mbox file
   until mailbox is closed or IMAP CHECK command is issued. Using this
   elsewhere is probably unnecessary, except as an optimization if mailbox is
   in any case synced just before closing it, you might as well give this flag
   to it to avoid double-syncing with mbox.
 * 'MAILBOX_SYNC_FLAG_FORCE_RESYNC' is used to force resyncing indexes. The
   only time this should be done is when manually triggered by administrator.

Then there are also other syncing flags:

 * 'MAILBOX_SYNC_FLAG_NO_EXPUNGES': No expunged messages are removed from the
   in-memory mailbox view. Their removal is delayed until syncing is done
   without this flag. Attempting to access the expunged messages may or may not
   work, depending on what information is accessed and what storage backend is
   used.
 * 'MAILBOX_SYNC_FLAG_FIX_INCONSISTENT': Normally when the internal mailbox
   state can't be consistently updated (typically due to index file
   corruption), the syncing fails. When this flag is set, it means that the
   caller doesn't care about mailbox's previous state and just wants to get it
   accessible again. Typically this is used when the mailbox is being opened,
   but not afterwards.
 * 'MAILBOX_SYNC_FLAG_EXPUNGE' is mainly intended for virtual plugin with IMAP
   protocol. You probably shouldn't use it.

Reading changes
---------------

While 'mailbox_sync_next()' returns TRUE, it fills out sync record:

 * seq1, seq2: Message sequence numbers that were affected
 * type: expunge, flag change or modseq change.

Expunge records don't immediately change the view's sequence numbers. After
seeing an expunge record you can still fetch the expunged messages' flags and
possibly other information. Only after syncing is deinitialized, the sequences
change.

Message flag change records don't actually show what the changes were. You can
find the new flags just by fetching them ('mail_get_flags()', etc.), they're
available immediately. You'll need to create a <transaction>
[Design.Storage.Mailbox.Transaction.txt] and a <mail> [Design.Storage.Mail.txt]
for that. For example:

---%<-------------------------------------------------------------------------
sync_ctx = mailbox_sync_init(box, flags);
trans = mailbox_transaction_begin(box, 0);
mail = mail_alloc(trans, MAIL_FETCH_FLAGS, 0);
---%<-------------------------------------------------------------------------

If you don't actually care about sync records, you don't necessarily have to
even call 'mailbox_sync_next()'. In that case it's actually easiest to perform
the whole sync using a one-step 'mailbox_sync()' function. This function also
sets 'MAILBOX_SYNC_FLAG_FIX_INCONSISTENT' flag automatically.

Deinitializing
--------------

'mailbox_sync_deinit()' finalizes the syncing. If any errors occurred during
sync, it'll return -1.

If 'MAILBOX_SYNC_FLAG_NO_EXPUNGES' was used and some expunges were actually
delayed,'status_r->sync_delayed_expunges' is set to TRUE.

Implementing sync for a storage backend
---------------------------------------

FIXME: talk about mail_index_sync_*() and how to change stuff and how to update
internal state.

(This file was created from the wiki on 2019-06-19 12:42)