the Chromium logo

The Chromium Projects

Protection against data override by old Sync clients



This document outlines the necessary steps to prevent data loss scenarios with Sync's Model API for multi-client Sync users. A typical problematic scenario is as follows:

  1. New proto field F is introduced in data specifics (e.g. PasswordSpecifics).
  2. Client N (a newer client) submits a proto containing the introduced field F.
  3. Client O (an older client) receives the proto, but doesn’t know the field F, discarding it before storing in the local model.
  4. Client O submits a change to the same proto, which results in discarding field’s F data from client N.


To prevent the described data loss scenario, it is necessary for the old client (client O above) to keep a copy of a server-provided proto, including unknown fields (i.e. fields not even defined in the .proto file at the time the binary was built) and partially-supported fields (e.g. functionality guarded behind a feature toggle). The logic for caching these protos was implemented in ModelTypeChangeProcessor.

To have this protection for a specific datatype, its ModelTypeSyncBridge needs to be updated to include the cached data during commits to the server (more details in the Implementation).


To implement this solution, a Sync datatype owner should follow these steps:

  1. Override TrimAllSupportedFieldsFromRemoteSpecifics function (see this section).
  2. [Optional] Add DCHECK to local updates flow (see this section).
  3. Include unsupported fields in local changes (see this section).
  4. Redownload the data on browser upgrade (see this section).
  5. [Optional] Add sync integration test (see this section).

The result of these steps is that:



Storing a full copy of a proto may have performance impact (memory, disk). The Sync infrastructure allows and encourages to trim proto fields that do not need an additional copy (if the field is already well supported by the client).

Trimming is a functionality that allows each data type to specify which proto fields are supported in the current browser version. Any field that is not supported will be cached by the ModelTypeChangeProcessor and can be used during commits to the server to prevent the data loss.

Fields that should not be marked as supported:

TrimAllSupportedFieldsFromRemoteSpecifics is a function of ModelTypeSyncBridge that:

To add a data-specific unsupported fields caching, override the trimming function in the data-specific ModelTypeSyncBridge to clear all its supported fields (i.e. fields that are actively used by the implementation and fully launched):

sync_pb::EntitySpecifics DataSpecificBridge::TrimAllSupportedFieldsFromRemoteSpecifics(
   const sync_pb::EntitySpecifics& entity_specifics) const {
 sync_pb::EntitySpecifics trimmed_entity_specifics = entity_specifics;
 return trimmed_entity_specifics;

Safety check

Forgetting to trim fields that are supported might result in:

To prevent this scenario, add a check that:

This should be done before every commit to the Sync server:

DCHECK_EQ(TrimAllSupportedFieldsFromRemoteSpecifics(datatype_specifics.ByteSizeLong()), 0u);

Local update flow

To use the cached unsupported fields data during commits to the server, add the code that does the following steps:

  1. Query cached sync_pb::EntitySpecifics from the ModelTypeChangeProcessor (Passwords example).
  2. Use the cached proto as a base for a commit and fill it with the supported fields from the local proto representation (Passwords example).
  3. Commit the merged proto to the server (Passwords example).

Browser upgrade flow

To handle the scenario when unsupported fields become supported due to a browser upgrade, add the following code to your data-specific ModelTypeSyncBridge:

  1. On startup, check whether the unsupported fields cache contains any field that is supported in the current browser version. This can be done by using the trimming function on cached protos and checking if it trims any fields (Passwords example).
  2. If the cache contains any fields that are already supported, simply force the initial sync flow to deal with any inconsistencies between local and server states (Passwords example).

It’s important to implement the trimming function correctly, otherwise the client can run into unnecessary sync data redownloads if a supported field gets cached unnecessarily.

If the trimming function relies on having data-specific field present in the sync_pb::EntitySpecifics proto (example), make sure to skip entries without these fields present in the startup check (as e.g. cache can be empty for entities that were created before this solution landed). This can be tested with the following Sync integration test.

Integration test

Add a Sync integration test for the caching / trimming flow.


Sync support horizon

The proposed solution is intended to be a long-term one, but it will take some time until it can be used reliably. This is due to the facts that:

Deprecating a field

Deprecated fields should still be treated as supported to prevent their unnecessary caching.

Migrating a field

This requires client-side handling as the newer clients will have both fields present and the legacy clients will have access to the deprecated field only. Newer clients should:

Repeated fields

No client-side logic is required - the solution will work by default.

Nested fields

Protecting nested fields is possible, but requires adding client-side logic to trim single child fields or the top level field if none of the child fields are populated (Passwords notes example).