Migrating pull queues to Cloud Pub/Sub

This page explains how to migrate pull queue code from Task Queues to Cloud Pub/Sub. Cloud Pub/Sub is now the preferred way of performing pull queue work in App Engine.

If your app uses both pull queues and push queues, use this guide to migrate your pull queues to Cloud Pub/Sub before migrating your push queues to the new push queue service Cloud Tasks. Migrating your pull queues after migrating push queues to Cloud Tasks is not recommended because the required use of the queue.yaml file is likely to cause unexpected behavior with Cloud Tasks.

Features not currently available in Cloud Pub/Sub

The following Task Queues features are not currently available in Cloud Pub/Sub:

  • Batching by tag
  • Automatic deduplication

Pricing and quotas

Migrating your pull queues to Cloud Pub/Sub might affect the pricing and quotas for your app.

Pricing

Cloud Pub/Sub has its own pricing. As with Task Queues, sending requests to your App Engine app with Cloud Pub/Sub can cause your app to incur costs.

Quotas

The Cloud Pub/Sub quotas are different from the quotas for Task Queues. Like with Task Queues, sending requests to your App Engine app from Cloud Pub/Sub might impact your App Engine request quotas.

Before migrating

This section discusses topics you need to cover before migrating your pull queues to Cloud Pub/Sub.

Enabling the Cloud Pub/Sub API

To enable the Cloud Pub/Sub API, click Enable on the Cloud Pub/Sub API in the API Library. If you see a Manage button instead of an Enable button, you have previously enabled the Cloud Pub/Sub API for your project and do not need to do so again.

Authenticating your app to the Cloud Pub/Sub API

You must authenticate your app to the Cloud Pub/Sub API. This section discusses authentication for two different use cases.

To develop or test your app locally, we recommend using a service account. For instructions on setting up a service account and connecting it to your app, read Obtaining and providing service account credentials manually.

To deploy your app on App Engine, you do not need to provide any new authentication. The Application Default Credentials (ADC) infer authentication details for App Engine apps.

Downloading the Cloud SDK

Download and install the Cloud SDK to use the gcloud tool with the Cloud Pub/Sub API if you have not installed it previously. Run the following command from your terminal if you already have the Cloud SDK installed.

gcloud components update

Importing the client library for Java

Follow the Cloud Pub/Sub instructions for installing the client library to use the client library with your app.

Cloud Pub/Sub and pull queues

Feature comparison

Cloud Pub/Sub sends work to workers through a publisher/subscriber relationship. A pull subscription in Cloud Pub/Sub is like a pull queue in Task Queues because the subscriber pulls the message from the topic. The table below lists the core feature for pull queues in Task Queues and the associated feature for pull subscriptions in Cloud Pub/Sub.

Task Queues feature Cloud Pub/Sub feature
Queue Topic
Task Message
Worker Subscriber

To learn more about the Cloud Pub/Sub architecture, read Cloud Pub/Sub: A Google-Scale Messaging Service.

Workflow comparison

Below is a comparison of a typical workflow for a pull queue in Task Queues and a pull subscription in Cloud Pub/Sub.

Task Queues workflow Cloud Pub/Sub workflow
You create the pull queue You create the topic and subscribe your subscriber (i.e. worker) to the topic
You create and enqueue the task You create the message and publish it to the topic
The worker leases the task The subscriber pulls the message from the topic
The worker processes the task The subscriber processes the message
The worker deletes the task from the queue The subscriber acknowledges the message
The lease expires The topic deletes the message when all of its subscribers have acknowledged the message

Creating pull subscriptions in Cloud Pub/Sub

You can use a Cloud Pub/Sub pull subscription like a Task Queues pull queue. Subscriptions to a topic do not expire and can exist simultaneously for multiple workers. This means that a message can be processed by more than one worker, which is one of the primary use cases for Cloud Pub/Sub. To recreate your Task Queues pull queues as Cloud Pub/Sub pull subscriptions, create a topic for each worker and subscribe only the associated worker to the topic. This ensures that each message is processed by exactly one worker as in Task Queues. To learn about creating and managing pull subscriptions, read about managing topics and subscriptions.

Deleting pull queues

After you migrate your Task Queues pull queues to Cloud Pub/Sub pull subscriptions, delete them from Task Queues using your queue.yaml file. We recommend deleting each pull queue before migrating the next one. This prevents your app from duplicating the work it receives from the new Cloud Pub/Sub pull subscription while you migrate your other pull queues. Note that deleting your Task Queues pull queues one by one rather than in a single deployment might have a greater effect on your App Engine deployment quota.

After you delete all your pull queues from Task Queues, you can omit the queue.yaml file from future deployments of your app.

If your app only uses pull queues, remove any references to the Task Queues API in your code. If your app uses both pull queues and push queues, you can either remove the references to the Task Queues API that occur in files that only use pull queues, or wait until you have also migrated your push queues and then remove references to the Task Queues API from all files.

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