This page explains how to migrate pull queue code from Task Queues to Pub/Sub. 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 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 Pub/Sub
The following Task Queues features are not currently available in Pub/Sub:
- Batching by tag
- Automatic deduplication
Pricing and quotas
Migrating your pull queues to Pub/Sub might affect the pricing and quotas for your app.
Pricing
Pub/Sub has its own pricing. As with Task Queues, sending requests to your App Engine app with Pub/Sub can cause your app to incur costs.
Quotas
The Pub/Sub quotas are different from the quotas for Task Queues. Like with Task Queues, sending requests to your App Engine app from Pub/Sub might impact your App Engine request quotas.
Before migrating
The following sections discuss the setup steps before migrating your pull queues to Pub/Sub.
Enabling the Pub/Sub API
To enable the Pub/Sub API, click Enable on the Pub/Sub API in the API Library. If you see a Manage button instead of an Enable button, you have previously enabled the Pub/Sub API for your project and do not need to do so again.
Authenticating your app to the Pub/Sub API
You must authenticate your app to the 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 Google Cloud CLI
Download and install the Google Cloud CLI to use the gcloud CLI with the Pub/Sub API if you have not installed it previously. Run the following command from your terminal if you already have the Google Cloud CLI installed.
gcloud components update
Importing the Cloud Client Libraries
Follow the Pub/Sub instructions for installing the client library to use the client library with your app.Pub/Sub and pull queues
Feature comparison
Pub/Sub sends work to workers through a publisher/subscriber relationship. A pull subscription in 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 Pub/Sub.
| Task Queues feature | Pub/Sub feature |
|---|---|
| Queue | Topic |
| Task | Message |
| Worker | Subscriber |
To learn more about the 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 Pub/Sub.
| Task Queues workflow | 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 Pub/Sub
You can use a 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 Pub/Sub. To recreate your Task Queues pull queues as 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
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 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.