The ClientTagBasedModelTypeProcessor is a crucial piece of the USS codepath. It lives on the model thread and performs the tracking of sync metadata for the ModelTypeSyncBridge that owns it by implementing the ModelTypeChangeProcessor interface, as well as sending commit requests to the ModelTypeWorker on the sync thread via the CommitQueue interface and receiving updates from the same worker via the ModelTypeProcessor interface.

This processor supports types that use a client tag, which is currently includes all except bookmarks. This means all changes in flight (either incoming remote changes provided via the ModelTypeWorker, or local changes reported by the ModelTypeSyncBridge) must specify a client tag, which is considered (after being hashed) the main global identifier of a sync entity.


The bridge owns a processor object at all times and operates on the same thread as it. If sync is disabled, the processor is destroyed but a new one is immediately created to replace it.

Processor State Machines

The processor sits between the model bridge and the sync engine. It has knowledge of what state each is in based on the calls it has receieved and performed. The states are not stored explicitly, but are implicit based on state stored in the processor. Here are the states of each, with notes on their transitions and how to determine them.

Model States

    • Waiting for ModelReadyToStart to be called.
    • Determined by: waiting_for_metadata_ && !model_error_
    • Waiting for data for pending commits to be loaded.
    • This state is skipped if there are no pending commits.
    • Determined by: waiting_for_pending_data_ && !model_error_
    • The model is completely ready to sync.
    • Determined by: !waiting_for_metadata_ && !waiting_for_pending_data && !model_error
    • Something in the model or storage broke.
    • This state is permanent until DisableSync destroys the object.
    • Determined by: !!model_error_

Sync States

    • Sync for this type has not started.
    • This state can be re-entered from any other state if Disconnect is called.
    • Determined by: !error_handler_.
    • Sync has started but the model is not yet READY (or ERROR).
    • This state is skipped if the model is ready before sync is.
    • Determined by: error_handler_ && start_callback_
    • Both the model and sync are ready. The start callback has been called and we're waiting to connect to the sync thread.
    • If the model was ERROR, the error is passed along and the callback is cleared; we're really waiting for DisableSync instead of connect.
    • Determined by: error_handler_ && !start_callback_

Processor States

Based on the interplay of the model and sync states, the processor effectively progresses through 3 states worth noting:

    • Metadata isn't loaded so we have no knowledge of entities.
    • Put and Delete calls are not allowed in this state (will DCHECK).
    • Indicates that not metadata is being tracked and that Put and Delete calls will be ignored.
    • This state is entered if the loaded metadata shows an initial merge hasn't happened (ModelTypeState::initial_sync_done is false).
    • Exposed via IsTrackingMetadata for optimization, not correctness.
    • Indicates that metadata is being tracked and Put and Delete calls must happen for entity changes.
    • This state is entered if the loaded metadata shows an initial merge has happened (ModelTypeState::initial_sync_done is true).
    • Indicates that commits can be sent and updates can be received from the sync server. This is a superstate of TRACKING.
    • If the processor was in TRACKING, it progresses to this state as soon as it gets connected to the worker.
    • If the processor was in NOT_TRACKING, it progresses to this state after MergeSyncData is called and the metadata is initialized.

Entity Tracker

The ProcessorEntity tracks the state of individual entities for the processor. It keeps the EntityMetadata proto in memory, as well as any pending commit data until it gets acked by the server. It also stores the special commit_requested_sequence_number_, which tracks the sequence number of the last version that's been sent to the server.

The tracker holds the metadata in memory forever, which is needed so we know what to update the on-disk memory with when we get a new local or remote change. Changing this would require being able to handle updates asynchronously.