[Sync] Add the Model API document.

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docs/sync/model_api.md

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+# Chrome Sync's Model API
+
+Chrome Sync operates on discrete, explicitly defined model types (bookmarks,
+preferences, tabs, etc). These model types are individually responsible for
+implementing their own local storage and responding to remote changes. This
+guide is for developers interested in syncing data for their model type to the
+cloud using Chrome Sync. It describes the newest version of the API, known as
+Unified Sync and Storage (USS). There is also the deprecated [SyncableService
+API] (aka Directory), which as of early 2016 is still used by most model types.
+
+[SyncableService API]: https://www.chromium.org/developers/design-documents/sync/syncable-service-api
+
+[TOC]
+
+## Overview
+
+To correctly sync data, USS requires that sync metadata be stored alongside your
+model data in a way such that they are written together atomically. **This is
+very important!** Sync must be able to update the metadata for any local data
+changes as part of the same write to disk. If you attempt to write data to disk
+and only notify sync afterwards, a crash in between the two writes can result in
+changes being dropped and never synced to the server, or data being duplicated
+due to being committed more than once.
+
+[`ModelTypeSyncBridge`][Bridge] is the interface the model code must implement.
+The bridge tends to be either a [`KeyedService`][KeyedService] or owned by one.
+The correct place for the bridge generally lies as close to where your model
+data is stored as possible, as the bridge needs to be able to inject metadata
+updates into any local data changes that occur.
+
+The bridge has access to a [`ModelTypeChangeProcessor`][MTCP] object, which it
+uses to communicate local changes to sync using the `Put` and `Delete` methods.
+The processor will communicate remote changes from sync to the bridge using the
+`ApplySyncChanges` method. [`MetadataChangeList`][MCL] is the way sync will
+communicate metadata changes to the storage mechanism. Note that it is typically
+implemented on a per-storage basis, not a per-type basis.
+
+[Bridge]: https://cs.chromium.org/chromium/src/components/sync/model/model_type_sync_bridge.h
+[KeyedService]: https://cs.chromium.org/chromium/src/components/keyed_service/core/keyed_service.h
+[MTCP]: https://cs.chromium.org/chromium/src/components/sync/model/model_type_change_processor.h
+[MCL]: https://cs.chromium.org/chromium/src/components/sync/model/metadata_change_list.h
+
+## Data
+
+### Specifics
+
+Model types will define a proto that contains the necessary fields of the
+corresponding native type (e.g. [`TypedUrlSpecifics`][TypedUrlSpecifics]
+contains a URL and a list of visit timestamps) and include it as a field in the
+generic [`EntitySpecifics`][EntitySpecifics] proto. This is the form that all
+communications with sync will use. This proto form of the model data is referred
+to as the specifics.
+
+[TypedUrlSpecifics]: https://cs.chromium.org/chromium/src/components/sync/protocol/typed_url_specifics.proto
+[EntitySpecifics]: https://cs.chromium.org/search/?q="message+EntitySpecifics"+file:sync.proto
+
+### Identifiers
+
+There are two primary identifiers for entities: **storage key** and **client
+tag**. The bridge will need to take an [`EntityData`][EntityData] object (which
+contains the specifics) and be able generate both of these from it. For
+non-legacy types without significant performance concerns, these will generally
+be the same.
+
+The storage key is used to uniquely identify entities locally within a client.
+It’s what’s used to refer to entities most of the time and, as its name implies,
+the bridge needs to be able to look up local data and metadata entries in the
+store using it. Because it is a local identifier, it can change as part of
+database migrations, etc. This may be desirable for efficiency reasons.
+
+The client tag is used to generate the **client tag hash**, which will identify
+entities **across clients**. This means that its implementation can **never
+change** once entities have begun to sync, without risking massive duplication
+of entities. This means it must be generated using only immutable data in the
+specifics. If your type does not have any immutable fields to use, you will need
+to add one (e.g. a GUID, though be wary as they have the potential to conflict).
+While the hash gets written to disk as part of the metadata, the tag itself is
+never persisted locally.
+
+[EntityData]: https://cs.chromium.org/chromium/src/components/sync/model/entity_data.h
+
+## Storage
+
+A crucial requirement of USS is that the model must add support for keeping
+sync’s metadata in the same storage as its normal data. The metadata consists of
+one [`EntityMetadata`][EntityMetadata] proto for each data entity, and one
+[`ModelTypeState`][ModelTypeState] proto containing metadata pertaining to the
+state of the entire type (the progress marker, for example). This typically
+requires two extra tables in a database to do (one for each type of proto).
+
+Since the processor doesn’t know anything about the store, the bridge provides
+it with an implementation of the [`MetadataChangeList`][MCL] interface. The
+change processor writes metadata through this interface when changes occur, and
+the bridge simply has to ensure it gets passed along to the store and written
+along with the data changes.
+
+[EntityMetadata]: https://cs.chromium.org/chromium/src/components/sync/protocol/entity_metadata.proto
+[ModelTypeState]: https://cs.chromium.org/chromium/src/components/sync/protocol/model_type_state.proto
+
+### ModelTypeStore
+
+While the model type may store its data however it chooses, many types use
+[`ModelTypeStore`][Store], which was created specifically to provide a
+convenient persistence solution. It’s backed by a [LevelDB] to store serialized
+protos to disk. `ModelTypeStore` provides two `MetadataChangeList`
+implementations for convenience; both accessed via
+[`ModelTypeStore::WriteBatch`][WriteBatch]. One passes metadata changes directly
+into an existing `WriteBatch` and another caches them in memory until a
+`WriteBatch` exists to consume them.
+
+The store interface abstracts away the type and will handle setting up tables
+for the type’s data, so multiple `ModelTypeStore` objects for different types
+can share the same LevelDB backend just by specifying the same path and task
+runner. Sync already has a backend it uses for DeviceInfo that can be shared by
+other types via the
+[`ProfileSyncService::GetModelTypeStoreFactory`][StoreFactory] method.
+
+[Store]: https://cs.chromium.org/chromium/src/components/sync/model/model_type_store.h
+[LevelDB]: http://leveldb.org/
+[WriteBatch]: https://cs.chromium.org/search/?q="class+WriteBatch"+file:model_type_store.h
+[StoreFactory]: https://cs.chromium.org/search/?q=GetModelTypeStoreFactory+file:profile_sync_service.h
+
+## Implementing ModelTypeSyncBridge
+
+### Initialization
+
+The bridge is required to load all of the metadata for its type from storage and
+provide it to the processor via the [`ModelReadyToSync`][ModelReadyToSync]
+method **before any local changes occur**. This can be tricky if the thread the
+bridge runs on is different from the storage mechanism. No data will be synced
+with the server if the processor is never informed that the model is ready.
+
+Since the tracking of changes and updating of metadata is completely
+independent, there is no need to wait for the sync engine to start before
+changes can be made. This prevents the need for an expensive association step in
+the initialization.
+
+[ModelReadyToSync]: https://cs.chromium.org/search/?q=ModelReadyToSync+file:/model_type_change_processor.h
+
+### MergeSyncData
+
+This method is called only once, when a type is first enabled. Sync will
+download all the data it has for the type from the server and provide it to the
+bridge using this method. Sync filters out any tombstones for this call, so
+`EntityData::is_deleted()` will never be true for the provided entities. The
+bridge must then examine the sync data and the local data and merge them
+together:
+
+*   Any remote entities that don’t exist locally must be be written to local
+    storage.
+*   Any local entities that don’t exist remotely must be provided to sync via
+    [`ModelTypeChangeProcessor::Put`][Put].
+*   Any entities that appear in both sets must be merged and the model and sync
+    informed accordingly. Decide which copy of the data to use (or a merged
+    version or neither) and update the local store and sync as necessary to
+    reflect the decision. How the decision is made can vary by model type.
+
+The [`MetadataChangeList`][MCL] passed into the function is already populated
+with metadata for all the data passed in (note that neither the data nor the
+metadata have been committed to storage yet at this point). It must be given to
+the processor for any `Put` or `Delete` calls so the relevant metadata can be
+added/updated/deleted, and then passed to the store for persisting along with
+the data.
+
+Note that if sync gets disabled and the metadata cleared, entities that
+originated from other clients will exist as “local” entities the next time sync
+starts and merge is called. Since tombstones are not provided for merge, this
+can result in reviving the entity if it had been deleted on another client in
+the meantime.
+
+[Put]: https://cs.chromium.org/search/?q=Put+file:/model_type_change_processor.h
+
+### ApplySyncChanges
+
+While `MergeSyncData` provides the state of sync data using `EntityData`
+objects, `ApplySyncChanges` provides changes to the state using
+[`EntityChange`][EntityChange] objects. These changes must be applied to the
+local state.
+
+Here’s an example implementation of a type using `ModelTypeStore`:
+
+```cpp
+base::Optional<ModelError> DeviceInfoSyncBridge::ApplySyncChanges(
+    std::unique_ptr<MetadataChangeList> metadata_change_list,
+    EntityChangeList entity_changes) {
+  std::unique_ptr<WriteBatch> batch = store_->CreateWriteBatch();
+  for (const EntityChange& change : entity_changes) {
+    if (change.type() == EntityChange::ACTION_DELETE) {
+      batch->DeleteData(change.storage_key());
+    } else {
+      batch->WriteData(change.storage_key(),
+                       change.data().specifics.your_type().SerializeAsString());
+    }
+  }
+
+  batch->TransferMetadataChanges(std::move(metadata_change_list));
+  store_->CommitWriteBatch(std::move(batch), base::Bind(...));
+  NotifyModelOfChanges();
+  return {};
+}
+```
+
+A conflict can occur when an entity has a pending local commit when an update
+for the same entity comes from another client. In this case, the bridge’s
+[`ResolveConflict`][ResolveConflict] method will have been called prior to the
+`ApplySyncChanges` call in order to determine what should happen. This method
+defaults to having the remote version overwrite the local version unless the
+remote version is a tombstone, in which case the local version wins.
+
+[EntityChange]: https://cs.chromium.org/chromium/src/components/sync/model/entity_change.h
+[ResolveConflict]: https://cs.chromium.org/search/?q=ResolveConflict+file:/model_type_sync_bridge.h
+
+### Local changes
+
+The [`ModelTypeChangeProcessor`][MTCP] must be informed of any local changes via
+its `Put` and `Delete` methods. Since the processor cannot do any useful
+metadata tracking until `MergeSyncData` is called, the `IsTrackingMetadata`
+method is provided. It can be checked as an optimization to prevent unnecessary
+processing preparing the parameters to a `Put` or `Delete` call.
+
+Here’s an example of handling a local write using `ModelTypeStore`:
+
+```cpp
+void WriteLocalChange(std::string key, ModelData data) {
+  std::unique_ptr<WriteBatch> batch = store_->CreateWriteBatch();
+  if (change_processor()->IsTrackingMetadata()) {
+    change_processor()->Put(key, ModelToEntityData(data),
+                            batch->GetMetadataChangeList());
+  }
+  batch->WriteData(key, specifics->SerializeAsString());
+  store_->CommitWriteBatch(std::move(batch), base::Bind(...));
+}
+```
+
+## Error handling
+
+If any errors occur during store operations that could compromise the
+consistency of the data and metadata, the processor’s
+[`ReportError`][ReportError] method should be called. The only exception to this
+is errors during `MergeSyncData` or `ApplySyncChanges`, which should just return
+a [`ModelError`][ModelError].
+
+This will inform sync of the error, which will stop all communications with the
+server so bad data doesn’t get synced. Since the metadata might no longer be
+valid, the bridge will asynchronously receive a `DisableSync` call (this is
+implemented by the abstract base class; subclasses don’t need to do anything).
+All the metadata will be cleared from the store (if possible), and the type will
+be started again from scratch on the next client restart.
+
+[ReportError]: https://cs.chromium.org/search/?q=ReportError+file:/model_type_change_processor.h
+[ModelError]: https://cs.chromium.org/chromium/src/components/sync/model/model_error.h
+
+## Sync Integration Checklist
+
+*   Define your specifics proto in [`//components/sync/protocol/`][protocol].
+*   Add a field for it to [`EntitySpecifics`][EntitySpecifics].
+*   Add it to the [`ModelType`][ModelType] enum and
+    [`kModelTypeInfoMap`][info_map].
+*   Add it to the [proto value conversions][conversions] files.
+*   Register a [`ModelTypeController`][ModelTypeController] for your type in
+    [`ProfileSyncComponentsFactoryImpl::RegisterDataTypes`][RegisterDataTypes].
+*   Tell sync how to access your `ModelTypeSyncBridge` in
+    [`ChromeSyncClient::GetSyncBridgeForModelType`][GetSyncBridge].
+*   Add to the [start order list][kStartOrder].
+*   Add an field for encrypted data to [`NigoriSpecifics`][NigoriSpecifics].
+*   Add to two encrypted types translation functions in
+    [`nigori_util.cc`][nigori_util].
+*   Add a [preference][pref_names] for tracking whether your type is enabled.
+*   Map your type to the pref in [`GetPrefNameForDataType`][GetPrefName].
+*   Check whether you should be part of a [pref group][RegisterPrefGroup].
+*   Add to the `SyncModelTypes` enum and `SyncModelType` suffix in
+    [`histograms.xml`][histograms].
+*   Add to the [`SYNC_DATA_TYPE_HISTOGRAM`][DataTypeHistogram] macro.
+
+[protocol]: https://cs.chromium.org/chromium/src/components/sync/protocol/
+[ModelType]: https://cs.chromium.org/chromium/src/components/sync/base/model_type.h
+[info_map]: https://cs.chromium.org/search/?q="kModelTypeInfoMap%5B%5D"+file:model_type.cc
+[conversions]: https://cs.chromium.org/chromium/src/components/sync/protocol/proto_value_conversions.h
+[ModelTypeController]: https://cs.chromium.org/chromium/src/components/sync/driver/model_type_controller.h
+[RegisterDataTypes]: https://cs.chromium.org/search/?q="ProfileSyncComponentsFactoryImpl::RegisterDataTypes"
+[GetSyncBridge]: https://cs.chromium.org/search/?q=GetSyncBridgeForModelType+file:chrome_sync_client.cc
+[kStartOrder]: https://cs.chromium.org/search/?q="kStartOrder[]"
+[NigoriSpecifics]: https://cs.chromium.org/chromium/src/components/sync/protocol/nigori_specifics.proto
+[nigori_util]: https://cs.chromium.org/chromium/src/components/sync/syncable/nigori_util.cc
+[pref_names]: https://cs.chromium.org/chromium/src/components/sync/base/pref_names.h
+[GetPrefName]: https://cs.chromium.org/search/?q=::GetPrefNameForDataType+file:sync_prefs.cc
+[RegisterPrefGroup]: https://cs.chromium.org/search/?q=::RegisterPrefGroups+file:sync_prefs.cc
+[histograms]: https://cs.chromium.org/chromium/src/tools/metrics/histograms/histograms.xml
+[DataTypeHistogram]: https://cs.chromium.org/chromium/src/components/sync/base/data_type_histogram.h
+
+## Testing
+
+The [`TwoClientUssSyncTest`][UssTest] suite is probably a good place to start
+for integration testing. Especially note the use of a `StatusChangeChecker` to
+wait for events to happen.
+
+[UssTest]: https://cs.chromium.org/chromium/src/chrome/browser/sync/test/integration/two_client_uss_sync_test.cc