<br />

Task queue functions take advantage of Google
[Cloud Tasks](https://cloud.google.com/tasks/docs/dual-overview)
to help your app run time-consuming, resource-intensive, or bandwidth-limited
tasks asynchronously, outside your main application flow.

For example, imagine that you want to create backups of a large set of image
files that are currently hosted on an API with a rate limit. In order to be a
responsible consumer of that API, you need to respect their rate limits. Plus,
this kind of long-running job could be vulnerable to failure due to timeouts and
memory limits.

To mitigate this complexity, you can write a task queue function that sets basic
task options like `scheduleTime`, and `dispatchDeadline`, and then hands the
function off to a queue in Cloud Tasks. The Cloud Tasks
environment is designed specifically to ensure effective congestion control and
retry policies for these kinds of operations.

The Firebase SDK for Cloud Functions for Firebase v3.20.1 and higher interoperates
with Firebase Admin SDK v10.2.0 and higher to support task queue functions.

Using task queue functions with Firebase can result in charges for
Cloud Tasks processing. See
[Cloud Tasks pricing](https://cloud.google.com/tasks/pricing)
for more information.

## Create task queue functions

To use task queue functions, follow this workflow:

1. Write a task queue function using the Firebase SDK for Cloud Functions.
2. Test your function by triggering it with an HTTP request.
3. Deploy your function with the Firebase CLI. When deploying your task queue function for the first time, the CLI will create a task queue in Cloud Tasks with options (rate limiting and retry) specified in your source code.
4. Add tasks to the newly created task queue, passing along parameters to set up an execution schedule if needed. You can achieve this by writing the code using the Admin SDK and deploying it to Cloud Functions for Firebase.

### Write task queue functions

Code samples in this section are based on an app that sets
up a service that backs up all images from NASA's
[Astronomy Picture of the Day](https://apod.nasa.gov/apod/astropix.html).
To get started, import the required modules:  

### Node.js

    // Dependencies for task queue functions.
    const {onTaskDispatched} = require("firebase-functions/tasks");
    const {onRequest, HttpsError} = require("firebase-functions/https");
    const {getFunctions} = require("firebase-admin/functions");
    const {logger} = require("firebase-functions");

    // Dependencies for image backup.
    const path = require("path");
    const {initializeApp} = require("firebase-admin/app");
    const {getStorage} = require("firebase-admin/storage");
    const {GoogleAuth} = require("google-auth-library");  
    https://github.com/firebase/functions-samples/blob/a6ae4cbd3cf2fff3e2b97538081140ad9befd5d8/Node/taskqueues-backup-images/functions/index.js#L18-L28

### Python

    # Dependencies for task queue functions.
    from google.cloud import tasks_v2
    import requests
    from firebase_functions.options import RetryConfig, RateLimits, SupportedRegion

    # Dependencies for image backup.
    from datetime import datetime, timedelta
    import json
    import pathlib
    from urllib.parse import urlparse
    from firebase_admin import initialize_app, storage, functions
    from firebase_functions import https_fn, tasks_fn, params
    import google.auth
    from google.auth.transport.requests import AuthorizedSession  
    https://github.com/firebase/functions-samples/blob/a6ae4cbd3cf2fff3e2b97538081140ad9befd5d8/Python/taskqueues-backup-images/functions/main.py#L16-L29

Use [`onTaskDispatched`](https://firebase.google.com/docs/reference/functions/2nd-gen/node/firebase-functions.tasks#tasksontaskdispatched)
or [`on_task_dispatched`](https://firebase.google.com/docs/reference/functions/2nd-gen/python/firebase_functions.tasks_fn)
for task queue functions. When writing a task queue function
you can set per-queue retry and rate- limiting configuration.

#### Configure task queue functions

Task queue functions come with a powerful set of configuration settings
to precisely control rate limits and retry behavior of a task queue:  

### Node.js

    exports.backupapod = onTaskDispatched(
        {
          retryConfig: {
            maxAttempts: 5,
            minBackoffSeconds: 60,
          },
          rateLimits: {
            maxConcurrentDispatches: 6,
          },
        }, async (req) => {  
    https://github.com/firebase/functions-samples/blob/a6ae4cbd3cf2fff3e2b97538081140ad9befd5d8/Node/taskqueues-backup-images/functions/index.js#L42-L51

### Python

    @tasks_fn.on_task_dispatched(retry_config=RetryConfig(max_attempts=5, min_backoff_seconds=60),
                                 rate_limits=RateLimits(max_concurrent_dispatches=10))
    def backupapod(req: tasks_fn.CallableRequest) -> str:
        """Grabs Astronomy Photo of the Day (APOD) using NASA's API."""  
    https://github.com/firebase/functions-samples/blob/a6ae4cbd3cf2fff3e2b97538081140ad9befd5d8/Python/taskqueues-backup-images/functions/main.py#L43-L46

- `retryConfig.maxAttempts=5`: Each task in the task queue is automatically
  retried up to 5 times. This helps mitigate transient errors like network
  errors or temporary service disruption of a dependent, external service.

- `retryConfig.minBackoffSeconds=60`: Each task is retried at least 60 seconds
  apart from each attempt. This provides a large buffer between each attempt
  so we don't rush to exhaust the 5 retry attempts too quickly.

- `rateLimits.maxConcurrentDispatch=6`: At most 6 tasks are dispatched at a
  given time. This helps ensure a steady stream of requests to the underlying
  function and helps reduce the number of active instances and cold starts.

### Test task queue functions

For most cases, the Cloud Functions emulator is the best way to test task
queue functions. See the Emulator Suite documentation to learn how to
[instrument your app for task queue functions emulation](https://firebase.google.com/docs/functions/local-emulator#instrument-tq).

Additionally, task queue functions are exposed as simple
HTTP functions in the Firebase Local Emulator Suite.
You can test an emulated task function by sending an HTTP POST
request with a JSON data payload:  

     # start the Local Emulator Suite
     firebase emulators:start

     # trigger the emulated task queue function
     curl \
      -X POST                                            # An HTTP POST request...
      -H "content-type: application/json" \              # ... with a JSON body
      http://localhost:$PORT/$PROJECT_ID/$REGION/$NAME \ # ... to function url
      -d '{"data": { ... some data .... }}'              # ... with JSON encoded data

### Deploy task queue functions

Deploy task queue function using the Firebase CLI:  

    $ firebase deploy --only functions:backupapod

When deploying a task queue function for the first time, the CLI creates a
task queue in Cloud Tasks with options (rate limiting and retry) specified in your source code.

If you encounter permissions errors when deploying functions, make sure that the
appropriate [IAM roles](https://firebase.google.com/docs/projects/iam/permissions#functions)
are assigned to the user running the deployment commands.

## Enqueue task queue functions

Task queue functions can be enqueued in Cloud Tasks from a trusted
server environment like Cloud Functions for Firebase using the Firebase Admin SDK for
Node.js or Google Cloud libraries for Python.
If you're new to the Admin SDKs, see
[Add Firebase to a server](https://firebase.google.com/docs/admin/setup) to get started.

A typical flow creates a new task, enqueues it in
Cloud Tasks, and sets the configuration for the task:  

### Node.js

    exports.enqueuebackuptasks = onRequest(
        async (_request, response) => {
          const queue = getFunctions().taskQueue("backupapod");
          const targetUri = await getFunctionUrl("backupapod");

          const enqueues = [];
          for (let i = 0; i <= BACKUP_COUNT; i += 1) {
            const iteration = Math.floor(i / HOURLY_BATCH_SIZE);
            // Delay each batch by N * hour
            const scheduleDelaySeconds = iteration * (60 * 60);

            const backupDate = new Date(BACKUP_START_DATE);
            backupDate.setDate(BACKUP_START_DATE.getDate() + i);
            // Extract just the date portion (YYYY-MM-DD) as string.
            const date = backupDate.toISOString().substring(0, 10);
            enqueues.push(
                queue.enqueue({date}, {
                  scheduleDelaySeconds,
                  dispatchDeadlineSeconds: 60 * 5, // 5 minutes
                  uri: targetUri,
                }),
            );
          }
          await Promise.all(enqueues);
          response.sendStatus(200);
        });  
    https://github.com/firebase/functions-samples/blob/a6ae4cbd3cf2fff3e2b97538081140ad9befd5d8/Node/taskqueues-backup-images/functions/index.js#L132-L157

### Python

    @https_fn.on_request()
    def enqueuebackuptasks(_: https_fn.Request) -> https_fn.Response:
        """Adds backup tasks to a Cloud Tasks queue."""
        task_queue = functions.task_queue("backupapod")
        target_uri = get_function_url("backupapod")

        for i in range(BACKUP_COUNT):
            batch = i // HOURLY_BATCH_SIZE

            # Delay each batch by N hours
            schedule_delay = timedelta(hours=batch)
            schedule_time = datetime.now() + schedule_delay

            dispatch_deadline_seconds = 60 * 5  # 5 minutes

            backup_date = BACKUP_START_DATE + timedelta(days=i)
            body = {"data": {"date": backup_date.isoformat()[:10]}}
            task_options = functions.TaskOptions(schedule_time=schedule_time,
                                                 dispatch_deadline_seconds=dispatch_deadline_seconds,
                                                 uri=target_uri)
            task_queue.enqueue(body, task_options)
        return https_fn.Response(status=200, response=f"Enqueued {BACKUP_COUNT} tasks")  
    https://github.com/firebase/functions-samples/blob/a6ae4cbd3cf2fff3e2b97538081140ad9befd5d8/Python/taskqueues-backup-images/functions/main.py#L96-L117

- The sample code tries to spread out execution of
  tasks by associating a delay of Nth minutes for the Nth task. This
  translates to triggering \~ 1 task/minute. Note that you can also use
  `scheduleTime` (Node.js) or `schedule_time` (Python) if you want
  Cloud Tasks to trigger a task at a specific time.

- The sample code sets the maximum amount of time
  Cloud Tasks will wait
  for a task to complete. Cloud Tasks will retry
  the task following the retry
  configuration of the queue or until this deadline is reached. In the sample,
  the queue is configured to retry the task up to 5 times, but the task is
  automatically cancelled if the whole process (including retry attempts)
  takes more than 5 minutes.

| **Important:** Note that this configuration set in the Admin SDK is applied on a per-task basis, unlike retry and rate limit configuration, which are applied per-queue. You can enqueue multiple tasks with the Admin SDK for the same function with different configurations (`scheduleTime`) and so on.

## Troubleshooting

### Turn on Cloud Tasks logging

Logs from Cloud Tasks contain useful diagnostic information like the
status of the request associated with a task. By default, logs from
Cloud Tasks are turned off due to the large volume of logs it can
potentially generate on your project. We recommend you turn on the debug logs
while you are actively developing and debugging your task queue functions. See
[Turning on
logging](https://cloud.google.com/tasks/docs/logging#turning_on_logging).

### IAM Permissions

You may see `PERMISSION DENIED` errors when enqueueing tasks or when
Cloud Tasks tries to invoke your task queue functions. Ensure that your
project has the following IAM bindings:

- The identity used to enqueue tasks to Cloud Tasks needs
  `cloudtasks.tasks.create` IAM permission.

  In the sample, this is the [App Engine default service account](https://cloud.google.com/functions/docs/securing/function-identity#runtime_service_account)

    gcloud projects add-iam-policy-binding $PROJECT_ID \
      --member=serviceAccount:${PROJECT_ID}@appspot.gserviceaccount.com \
      --role=roles/cloudtasks.enqueuer

- The identity used to enqueue tasks to Cloud Tasks needs permission
  to use the service account associated with a task in Cloud Tasks.

  In the sample, this is the [App Engine default service account](https://cloud.google.com/functions/docs/securing/function-identity#runtime_service_account).

See [Google Cloud IAM documentation](https://cloud.google.com/iam/docs/impersonating-service-accounts#impersonate-sa-%0Alevel)
for instructions on how to add the App Engine default service account
as a user of the App Engine default service account.

- The identity used to trigger the task queue function needs
  `cloudfunctions.functions.invoke` permission.

  In the sample, this is the [App Engine default service account](https://cloud.google.com/functions/docs/securing/function-identity#runtime_service_account)

    gcloud functions add-iam-policy-binding $FUNCTION_NAME \
      --region=us-central1 \
      --member=serviceAccount:${PROJECT_ID}@appspot.gserviceaccount.com \
      --role=roles/cloudfunctions.invoker