The Sync engines in the tree

Unless otherwise specified, the engine implementations can be found here

Please read the Introduction to Sync.


The clients engine is a special engine in that it’s invisible to the user and can not be disabled - think of it as a “meta” engine. As such, it doesn’t really have a sensible concept of store or tracker.

The engine is mainly responsible for keeping its own record current in the clients collection. Some parts of Sync use this collection to know what other clients exist and when they last synced (although alot of this is moving to using the Firefox Accounts devices).

Clients also has the ability to handle commands - in short, some other client can write to this client’s commands, and when this client notices, it will execute the command. Commands aren’t arbitrary, so commands must be understood by both sides for them to work. There are commands to “wipe” collections etc. In practice, this is used only by bookmarks when a device restores bookmarks - in that case, the restoring device will send a wipe command to all other clients so that they take the new bookmarks instead of merging them.

If not for this somewhat limited commands functionality, this engine could be considered deprecated and subsumed by FxA devices - but because we can’t just remove support for commands and also do not have a plan for replacing them, the clients engine remains important.


The bookmarks engine has changed so that it’s tightly integrated with the places database. Instead of an external tracker, the tracking is integrated into Places. Each bookmark has a syncStatus and a syncChangeCounter and these are managed internally by places. Sync then just queries for changed bookmarks by looking for these fields.

Bookmarks is somewhat unique in that it needs to maintain a tree structure, which makes merging a challenge. The dogear component (written in Rust and also used by the application-services bookmarks component) performs this merging.

Bookmarks also pioneered the concept of a “mirror” - this is a database table which tracks exactly what is on the server. Because each sync only fetches changes from the server since the last sync, each sync does not supply every record on the server. However, the merging code does need to know what’s on the server - so the mirror tracks this.


History is similar to bookmarks described above - it’s closely integrated with places - but is less complex because there’s no tree structure involved.

One unique characteristic of history is that the engine takes steps to not upload everything - old profiles tend to have too much history to reasonably store and upload, so typically uploads are limited to the last 5000 visits.


Logins has also been upgraded to be closely integrated with Services.logins - the logins component itself manages the metadata.


Tabs is a special engine in that there’s no underlying storage at all - it both saves the currently open tabs from this device (which are enumerated every time it’s updated) and also lets other parts of Firefox know which tabs are open on other devices. There’s no database - if we haven’t synced yet we don’t know what other tabs are open, and when we do know, the list is just stored in memory.

The SyncedTabs module is the main interface the browser uses to get the list of tabs from other devices.


Addons is still an “old school” engine, with a tracker and store which aren’t closely integrated with the addon manager. As a result it’s fairly complex and error prone - eg, it persists the “last known” state so it can know what to sync, where a better model would be for the addon manager to track the changes on Sync’s behalf.

It also attempts to sync themes etc. The future of this engine isn’t clear given it doesn’t work on mobile platforms.

Addresses / Credit-Cards

Addresses and Credit-cards have Sync functionality tightly bound with the store. Unlike other engines above, this engine has always been tightly bound, because it was written after we realized this tight-binding was a feature and not a bug.

Technically these are 2 separate engines and collections. However, because the underlying storage uses a shared implementation, the syncing also uses a shared implementation - ie, the same logic is used for both - so we tend to treat them as a single engine in practice.

As a result, only a shim is in the services/sync/modules/engines/ directory, while the actual logic is next to the storage implementation.

This engine has a unique twist on the “mirror” concept described above - whenever a change is made to a fields, the original value of the field is stored directly in the storage. This means that on the next sync, the value of the record on the server can be deduced, meaning a “3-way” merge can be done, so it can better tell the difference between local only, remote only, or conflicting changes.


webext-storage is implemented in Rust and lives in application services and is vendored into the addons code - note that this includes the storage and Sync code. The Sync engine itself is a shim in the sync directory.

See the How Rust Engines are implemented document for more about how rust engines are integrated.