Mail sync

Viauna keeps a local index of each mailbox's folders: which messages are in them, their senders, subjects, flags, labels and a line of preview text. Lists, counts and paging come from the index, so they're instant. The index is a cache: the mail itself stays on your mail server, and if the index is lost Viauna builds it again.

The connections to your mail server

Each signed-in mailbox has one runtime, shared by every browser and tab signed in to it. It starts when someone uses the mailbox, and holds three IMAP connections:

• one for what you do (opening a message, moving, starring, searching), so a click never waits behind sync;
• one for background sync;
• one idling on the Inbox (IMAP IDLE), to hear of new mail.

The runtime stops when no page has the mailbox's live-update stream open and nothing has asked for it for 10 minutes (checked every minute). While it's stopped nothing is synced; the next visit fetches only what changed.

Besides those three, Viauna logs in briefly to check a password: at sign-in, and at most every 10 minutes after (see Sessions). Loading or saving filters opens a ManageSieve connection each time, and contacts sync uses CardDAV; see Filters and Contacts.

Which folders show

Viauna syncs, shows and searches the folders you're subscribed to, plus the special ones (Inbox, Sent, Drafts, Archive, Junk, Trash), subscribed or not. A folder made in another app and not subscribed there doesn't appear; subscribe to it in that app. Folders made in Viauna are subscribed, and renaming one keeps its subfolders' subscriptions.

How fresh the list is

• The Inbox is watched with IDLE. When the server reports new, deleted or re-flagged messages, Viauna waits 300 ms to gather a burst, then syncs the Inbox. An open page shows it within about a second.
• Every other folder, Archive and the folders filters file mail into included, is checked once a minute: Viauna asks the server for every folder's status (message count, next UID, highest modification sequence, unread count) in one LIST-STATUS command, and syncs the folders that moved.
• Changes made in other apps (reading, starring, moving) appear the same way: within about a second in the Inbox, within a minute elsewhere.
• What you do in Viauna updates the index at once.

All of this happens only while the mailbox's runtime runs.

The order things are done in

Background work runs one task at a time, in this order: the folder list, the Inbox, other folders (Sent, Drafts, Archive, the rest, then Junk and Trash), preview text, then older mail. A sync waits for the task already running to finish, which is at most one batch.

• First sync of a folder: the newest 300 messages, so the list appears at once. The rest are fetched 500 at a time, newest first. Until a folder is fully indexed, its unread and total counts are the server's.
• Preview text: the first 3 KB (3,072 bytes) of each message's text, 60 messages at a time, Inbox first.
• After that: new messages from the folder's next UID on; flag changes with CONDSTORE (CHANGEDSINCE) where the server has it, or by reading every flag again where it doesn't; and deleted messages by comparing the UIDs the index holds with the server's.
• If a folder's UIDVALIDITY changes, every UID Viauna held for it is meaningless, so the folder is indexed again from scratch.

When something fails

Each task retries on its own, after 4 s, then 8 s, 16 s and so on, doubling up to 10 minutes, so one that keeps failing neither spins nor holds the others back. A message the server refuses to return (a corrupt file, an unknown encoding) is fetched alone, then skipped. If the Inbox's IDLE connection drops, it's opened again after 5 s, or 30 s if connecting failed.

Search

A search runs on the mail server, so it covers message text, using the server's full-text index where it has one.

• Viauna sends an IMAP SEARCH to each folder in scope: every synced folder except Trash and Spam, or the one named with in:.
• It takes the newest 3,000 matches in each folder. Matches older than what the index has reached are indexed there and then.
• has:attachment is decided from the index; everything else is the server's search.
• The same search within 60 s gives the same results, without asking the server again.
• An operator Viauna doesn't know (a URL, re:) is searched as text. For older_than: and newer_than:, a month is 30 days and a year 365.