Sync Data Best Practices

Changing fields in specifics

The new sync API uses protobufs to communicate with Chrome services, which is nice because protobufs were written to be robust against protocol changes (see protobuf docs for details). However, once you start syncing data, changing your protobuf format isn't completely painless; not only can sync users upgrade from one Chrome version to another, but they may have different Chrome versions running at the same time! Fortunately, there are some best practices to help make it easy. There are listed in the Sync Protocol Style guide.

Conflict resolution

A conflict can occur when an entity has a pending local commit and an update for the same entity comes from another client during an incremental update. There is a default implementation of conflict resolution which is sufficient in most cases. However it's possible to provide in the bridge a custom ResolveConflict implementation. The result can be either keeping the local pending change, or fully apply the remote update. Note that currently merging is not supported during the conflict resolution.

It is recommended to avoid any additional logic to detect conflicts or merge local data with remote updates. This may lead to inconsistency between the client and the server, and hence between different clients. Re-uploading of such merged data should also be avoided because it may result in a ping-pong issue when two different clients would indefinitely reupload merged results of each other.

Avoiding ping-pong

tl;dr: Do not call change_processor()->Put(..) in your ApplyIncrementalSyncChanges implementation! In other words, an incoming sync update must never directly lead to an outgoing update.

Sometimes, incoming changes from the server can be "bad" in some way - for example, some older version of Chrome committed invalid or incomplete data. In such cases, it's tempting to fix the data, by committing a "fixed" version directly when receiving the "bad" data in ApplyIncrementalSyncChanges.

However, doing this can lead to "ping-pong" issues: If two clients try to fix bad data, but disagree on what the correct state is, they'll continuously re-update the same entity back and forth, as fast as their network connections allow. If enough clients get into this state, this amounts to a DDoS attack on the sync server.
Note that it's effectively impossible to ensure that no past or future Chrome version disagrees with your current code's behavior.

Instead, it's usually best to fix up the data locally without committing it, and relying on the next natural change to commit the fixed data to the server. If you absolutely must re-upload the fixed data, this must be rate-limited in some way, e.g. do it at most once per browser startup.