Overview
The Gmail API provides server push notifications that let you watch for changes to Gmail mailboxes. You can use this feature to improve the performance of your application. It allows you to eliminate the extra network and compute costs involved with polling resources to determine if they have changed. Whenever a mailbox changes, the Gmail API notifies your backend server application.
Initial Cloud Pub/Sub Setup
The Gmail API uses the Cloud Pub/Sub API to deliver push notifications. This allows notification via a variety of methods including webhooks and polling on a single subscription endpoint.
Prerequisites
In order to complete the rest of this setup, make sure you fulfill the Cloud Pub/Sub Prerequisites and then set up a Cloud Pub/Sub client.
Create a topic
Using your Cloud Pub/Sub client, create the topic that the Gmail API should send notifications to. The topic name can be any name you choose under your project (i.e. matching projects/myproject/topics/*, where myproject is the Project ID listed for your project in the Google Developers Console).
Create a subscription
Follow the Cloud Pub/Sub Subscriber Guide to set up a subscription to the topic that you created. Configure the subscription type to be either a webhook push (i.e. HTTP POST callback) or pull (i.e. initiated by your app). This is how your application will receive notifications for updates.
Grant publish rights on your topic
Cloud Pub/Sub requires that you grant Gmail privileges to publish notifications to your topic.
To do this, you need to grant publish privileges to [email protected]. You can do this using the Cloud Pub/Sub Developer Console permissions interface following the resource-level access control instructions.
Getting Gmail mailbox updates
Once the initial Cloud Pub/Sub setup is finished, configure Gmail accounts to send notifications for mailbox updates.
Watch request
To configure Gmail accounts to send notifications to your Cloud Pub/Sub topic, simply use your Gmail API client to call watch on the Gmail user mailbox similar to any other Gmail API call. To do so, provide the topic name created above and any other options in your watch request, such as labels to filter on. For example, to be notified any time a change is made to the Inbox:
Protocol
POST "https://www.googleapis.com/gmail/v1/users/me/watch" Content-type: application/json { topicName: "projects/myproject/topics/mytopic", labelIds: ["INBOX"], labelFilterBehavior: "INCLUDE", } Python
request = { 'labelIds': ['INBOX'], 'topicName': 'projects/myproject/topics/mytopic', 'labelFilterBehavior': 'INCLUDE' } gmail.users().watch(userId='me', body=request).execute() Watch response
If the watch request is successful you will receive a response like:
{ historyId: 1234567890 expiration: 1431990098200 } with the current mailbox historyId for the user. All changes after that historyId will be notified to your client. If you need to process changes prior to this historyId, refer to the sync guide.
Additionally, a successful watch call should cause a notification to immediately be sent to your Cloud Pub/Sub topic.
If you receive an error from the watch call, the details should explain the source of the problem, which is typically with the setup of the Cloud Pub/Sub topic and subscription. Refer to the Cloud Pub/Sub documentation to confirm that the setup is correct and for help with debugging topic and subscription issues.
Renewing mailbox watch
You must re-call watch at least every 7 days or else you will stop receiving updates for the user. We recommend calling watch once per day. The watch response also has an expiration field with the timestamp for the watch expiration.
Receiving notifications
Whenever a mailbox update occurs that matches your watch, your application will receive a notification message describing the change.
If you have configured a push subscription, a webhook notification to your server will conform to a PubsubMessage:
POST https://yourserver.example.com/yourUrl Content-type: application/json { message: { // This is the actual notification data, as base64url-encoded JSON. data: "eyJlbWFpbEFkZHJlc3MiOiAidXNlckBleGFtcGxlLmNvbSIsICJoaXN0b3J5SWQiOiAiMTIzNDU2Nzg5MCJ9", // This is a Cloud Pub/Sub message id, unrelated to Gmail messages. "messageId": "2070443601311540", // This is the publish time of the message. "publishTime": "2021-02-26T19:13:55.749Z", } subscription: "projects/myproject/subscriptions/mysubscription" } The HTTP POST body is JSON and the actual Gmail notification payload is in the message.data field. That message.data field is a base64url-encoded string that decodes to a JSON object containing the email address and the new mailbox history ID for the user:
{"emailAddress": "[email protected]", "historyId": "9876543210"} You can then use history.list to get the change details for the user since their last known historyId, as per the sync guide.
For example, to use history.list to identify changes that occurred between your initial watch call and the receipt of the notification message shared in the previous example, pass 1234567890 as the startHistoryId to history.list. Afterwards,9876543210 can be persisted as the last known historyId for future use cases.
If you have configured a pull subscription instead, refer to the code samples in the Cloud Pub/Sub Subscriber Pull Guide for more details on receiving messages.
Responding to notifications
All notifications need to be acknowledged. If you use webhook push delivery, then responding successfully (e.g. HTTP 200) will acknowledge the notification.
If using pull delivery (REST Pull, RPC Pull , or RPC StreamingPull) then you must follow up with an acknowledge call (REST or RPC). Refer to the code samples in the Cloud Pub/Sub Subscriber Pull Guide for more details on acknowledging messages either asynchronously or synchronously using the official RPC-based client libraries.
If the notifications are not acknowledged (e.g. your webhook callback returns an error or times out), Cloud Pub/Sub will retry the notification at a later time.
Stopping mailbox updates
To stop receiving updates on a mailbox, call stop and all new notifications should stop within a few minutes.
Limitations
Max notification rate
Each Gmail user being watched has a maximum notification rate of 1 event/sec. Any user notifications above that rate will be dropped. Be careful when handling notifications to be sure not to trigger another notification, and thereby start a notification loop.
Reliability
Typically all notifications should be delivered reliably within a few seconds; however in some extreme situations notifications may be delayed or dropped. Make sure to handle this possibility gracefully, so that the application still syncs even if no push messages are received. For example, fall back to periodically calling history.list after a period with no notifications for a user.
Cloud Pub/Sub Limitations
The Cloud Pub/Sub API also has its own limitations, specifically detailed in its pricing and quotas documentation.