Synchronizing Clients with Gmail

Keeping your client synchronized with Gmail is important for most application scenarios. There are two overall synchronization scenarios: full synchronization and partial synchronization. Full synchronization is required the first time your client connects to Gmail and in some other rare scenarios. If your client has recently synchronized, partial synchronization is a lighter-weight alternative to a full sync. You can also use push notifications to trigger partial synchronization in real-time and only when necessary, thereby avoiding needless polling.

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Full synchronization

The first time your application connects to Gmail, or if partial synchronization is not available, you must perform a full sync. In a full sync operation, your application should retrieve and store as many of the most recent messages or threads as are necessary for your purpose. For example, if your application displays a list of recent messages, you may wish to retrieve and cache enough messages to allow for a responsive interface if the user scrolls beyond the first several messages displayed. The general procedure for performing a full sync operation is as follows:

1. Call messages.list to retrieve the first page of message IDs.
2. Create a batch request of messages.get requests for each of the messages returned by the list request. If your application displays message contents, you should use format=FULL or format=RAW the first time your application retrieves a message and cache the results to avoid additional retrieval operations. If you are retrieving a previously cached message, you should use format=MINIMAL to reduce the size of the response as only the labelIds may change.
3. Merge the updates into your cached results. Your application should store the historyId of the most recent message (the first message in the list response) for future partial synchronization.

Note: You can also perform synchronization using the equivalent Threads resource methods. This may be advantageous if your application primarily works with threads or only requires message metadata.

Partial synchronization

If your application has synchronized recently, you can perform a partial sync using the history.list method to return all history records newer than the startHistoryId you specify in your request. History records provide message IDs and type of change for each message, such as message added, deleted, or labels modified since the time of the startHistoryId. You can obtain and store the historyId of the most recent message from a full or partial sync to provide as a startHistoryId for future partial synchronization operations.

Limitations

History records are typically available for at least one week and often longer. However, the time period for which records are available may be significantly less and records may sometimes be unavailable in rare cases. If the startHistoryId supplied by your client is outside the available range of history records, the API returns an HTTP 404 error response. In this case, your client must perform a full sync as described in the previous section.