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Syncable Service API

http://go/syncapi (also, see http://go/syncapipreso [Googlers only, but a PDF of the presentation is attached to this document])

Background

As sync continues to expand to handle more data types, the need for an easier and more scalable way for Chrome services to interact with sync is becoming more apparent. This document proposes a new API with the following goals:
  • A Chrome service should be able to use this API to sync its data without having to know the details of the sync code base and protocol.
  • This API should encourage new Chrome services to define its interactions with sync up front.
  • The "business logic" for syncing a service's data should live in the service itself.

SyncableService Interface and related classes

First, the message types that Chrome services will use to talk to sync (and vice versa):

enum SyncType { BOOKMARKS, PREFERENCES, ... };

class SyncData {
 public:
  SyncType sync_type();

  // The service-specific data is in a protobuf extension of EntitySpecifics
  // which corresponds to sync_type().
  EntitySpecifics specifics();
};

class SyncChange {
 public:
  enum ChangeType { ADD, UPDATE, DELETE };

  ChangeType change_type();

  // If change_type is ADD or UPDATE, this contains the new data; if
  // change_type is DELETE, this contains the data right before deletion.
  SyncData sync_data();
};

SyncData CreateSyncData(SyncType sync_type, EntitySpecifics specifics);

// |tag| should be a unique data-type specific ID that can be used to
// prevent duplicate entries for the same object, e.g. for extensions,
// it would be the extension ID.
SyncChange CreateSyncChange(SyncChange::ChangeType type, SyncType sync_type, EntitySpecifics specifics, string tag);

Both SyncData and SyncChange are immutable, thread-safe, and cheaply copyable/assignable.

Then, the SyncChangeProcessor interface, which is implemented by sync classes (and also by local servicessee below):

interface SyncChangeProcessor {
  void ProcessSyncChanges(SyncType type, vector<SyncChange> changes);
};

When ProcessSyncChanges is called on a sync class, it processes the given list of sync changes for the given type, which eventually gets propagated to the sync server and then to other clients.

Finally, the SyncableService interface, which Chrome services should implement:

interface SyncableService inherits SyncChangeProcessor {
  // Inherited from SyncChangeProcessor.
  void ProcessSyncChanges(SyncType type, vector<SyncChange> changes);

  vector<SyncData> GetAllSyncData(SyncType type);

  bool MergeDataAndStartSyncing(SyncType type, vector<SyncData> sync_data_to_merge, SyncChangeProcessor sync_change_processor);

  void StopSyncing(SyncType type);
};

interface ProfileKeyedService inherits SyncableService {
  ...
};

Some Chrome services map to multiple sync data types, hence the need for a SyncType parameter to all SyncableService methods (e.g., ExtensionService handles both extensions and apps, and WebDataService handles both autofill and autofill profiles).

A SyncableService is also a SyncChangeProcessor.  When ProcessSyncChanges is called on a Chrome service, it must process the given list of sync changes for the given type (which came from other clients via the sync server).

When GetAllSyncData is called, it must return a list of SyncDatas which represents the currently known local data for the given type.  GetAllSyncData may be called at any time, even before syncing starts; it should always return an accurate representation of the local state, as it may be used for diagnostics and verification, e.g. sync may call GetAllSyncData after it calls StartSync and verify that the local state is equivalent to the remote state.

When MergeDataAndStartSyncing is called on a Chrome service, it must do an "initial sync": it must merge sync_data_to_merge with its local data and/or push changes to sync_change_processor such that, after the function returns, the local sync state (i.e., the return value of GetAllSyncData) matches the state represented by sync_data_to_merge plus any changes pushed to sync_change_processor.  It should also store the given sync_change_processor reference and use it to send changes for type until StopSyncing is called for type.

A Chrome service can assume that all SyncableService methods are called on the same thread, usually its "native" thread.

SyncChanges can be assumed to be processed atomically by sync, i.e. a Chrome service will never receive a "partial" SyncChange.

SyncableService should be a super-interface of ProfileKeyedService.  Most Chrome services should implement SyncableService, either directly or via ProfileKeyedService.

How to write a new sync-aware Chrome service

  1. Important: Loop in the sync team early!  Send an e-mail to chrome-sync-dev@google.com explaining what you want to sync.  We're happy to help.
  2. You will need to add your type to the SyncType enum as well define a protobuf to use for your data and make it an extension of EntitySpecifics.
  3. Except for some special cases, you will probably need to make your service inherit from ProfileKeyedService.  Make sure your service has one thread (not necessarily the UI thread) that it can use to communicate with sync.
  4. Write unit tests to:
    1. Make sure GetAllSyncData accurately reflects your service's state.
    2. Make sure that ProcessSyncChanges handles SyncChanges intelligently (even erroneous ones).
    3. Make sure that calling MergeDataAndStartSyncing on an instance of your service with another instance's sync state and a reference to that instance as a SyncChangeProcessor brings both instances to the same state.  In other words, the following should hold:

      SyncType my_sync_type = ...;
      SyncableService my_service = ...;
      SyncableService my_other_service = ...;
      my_service.MergeDataAndStartSyncing(my_sync_type, my_other_service.GetAllSyncData(), my_other_service);
      EXPECT_EQ(my_service.GetAllSyncData(), my_other_service.GetAllSyncData());

  5. Write sync integration tests for your service.  See http://go/syncautomation for details.

Suggested migration for existing synced Chrome services

  1. Make the service "consume" SyncChanges, i.e. implement ProcessSyncChanges.  This most likely involves moving some logic from the change processor to the service itself.
  2. The change processor is most likely listening to notifications from the service and using the notification's details or querying the service directly to find out what changed.  Make the service "produce" SyncChanges: i.e., when the change processor receives a notification from the service, it should only have to query the service to get one or more SyncChanges to process.  This most likely involves moving the rest of the logic from the change processor to the service itself.
  3. Make the service inherit from SyncableService; this most likely involves moving the logic from the model associator to MergeDataAndStartSyncing.  The model associator itself can stick around and just call MergeDataAndStartSyncing/StopSyncing from Associate/DisassociateModels.

Implementation Notes

See the PDF attached for an overview of the Syncable Service API.

SyncType should just be syncable::ModelType.

Internally, a SyncData will contain a SyncEntity protobuf.  Or rather, a thread-safe ref-counted pointer to an immutable one, so SyncData/SyncChange can be cheaply copyable and passable across threads.  TODO: We should try to make sync_data.h/sync_change.h not include our protobuf header files.

Some special handling is needed for bookmarks; currently, it is the only data type which uses sync's support for hierarchies.  The most practical solution for this is probably to just add bookmark-specific accessors to SyncChange for id, first_child_id, and successor_id.

Extensions and apps may need to use sync's support for ordering, i.e. it may also need to use successor_id.

There will most likely be one instance per thread of SyncChangeProcessor which handles all the data types which live on that thread.

An earlier API proposal by Nicolas Zea is here (available only to Googlers).

Life of a Sync Item Update is useful for understanding how exactly sync works behind the scenes.

Future plans

Support may be added to enable SyncableServices to talk to chrome://sync-internals.  This will most likely involve passing some sort of Javascript bridge object in MergeDataAndStartSyncing that a SyncableService can use to raise events, and/or an additional method in SyncableService to enable it to respond to Javascript queries.
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Nicolas Zea,
May 24, 2011, 3:22 PM
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