Creating Push Tasks

This page describes how to create tasks and place them in push queues. When you want to process a task, you must create a new task object and place it on a queue. You can explicitly specify the service and handler that process the task, and optionally pass task-specific data along to the handler. You can also fine-tune the configuration for the task, like scheduling a time in the future when it should be executed or limiting the number of times you want the task to be retried if it fails.

Creating a new task

To create and enqueue a task, call the taskqueue.Add function.

import("google.golang.org/appengine/taskqueue")

Specifying the worker service

When a task is popped off its queue, the Task Queue service sends it on to a worker service. Every task has a target and a url, which determine what service and handler will ultimately perform the task.

target

The target specifies the service that will receive the HTTP request to perform the task. It is a string that specifies a service/version/instance in any one of the canonical forms. The most often-used forms are:

    service
    version.service
    instance.version.service

The target string is prepended to the domain name of your app. There are three ways to set the target for a task:

  • Declare the target when you construct the task. You can set the target explicitly when creating the Task object by setting the Host header: 

    h := http.Header{}
h.Add("Host", "versionHostname")
task := taskqueue.Task{
	Header: h,
}

  • Include a target directive when you define a queue in the queue.yaml, as in the definition of queue-blue. All tasks added to a queue with a target will use that target, even if a different target was assigned to the task at construction time.

  • If no target is specified according to either of the previous two methods, then the task's target is the version of the service that enqueues it. Note that if you enqueue a task from the default service and version in this manner, and the default version changes before the task executes, it will run in the new default version.

url

The url selects one of the handlers in the target service, which will perform the task.

The url should match one of the handler URL patterns in the target service. The url can include query parameters if the method specified in the task is GET or PULL. If no url is specified the default URL /_ah/queue/[QUEUE_NAME] is used, where [QUEUE_NAME] is the name of the task's queue.

Passing data to the handler

You can pass data to the handler as query parameters in the task's URL, but only if the method specified in the task is GET or PULL.

The NewPOSTTask function has a positional argument for query_data. The data is usually a dictionary of key-value pairs. If the task's method is POST or PUT, the data is added to the payload of the HTTP request. If the method is GET it is added to the URL as query parameters.

Naming a task

When you create a new task, App Engine assigns the task a unique name by default. However, you can assign your own name to a task by using the name parameter. An advantage of assigning your own task names is that named tasks are de-duplicated, which means you can use task names to guarantee that a task is only added once. De-duplication continues for 9 days after the task is completed or deleted.

Note that de-duplication logic introduces significant performance overhead, resulting in increased latencies and potentially increased error rates associated with named tasks. These costs can be magnified significantly if task names are sequential, such as with timestamps. So, if you assign your own names, we recommend using a well-distributed prefix for task names, such as a hash of the contents.

If you assign your own names to tasks, note that the maximum name length is 500 characters, and the name can contain uppercase and lowercase letters, numbers underscores, and hyphens.

Enqueuing tasks in Cloud Datastore transactions

You can enqueue a task as part of a Datastore transaction, such that the task is only enqueued—and guaranteed to be enqueued—if the transaction is committed successfully. Tasks added in a transaction are considered to be a part of it and have the same level of isolation and consistency.

An application cannot insert more than five transactional tasks into task queues during a single transaction. Transactional tasks must not have user-specified names.

The following code sample demonstrates how to insert transactional tasks into a push queue as part of a Datastore transaction:

import (
	"net/url"

	"golang.org/x/net/context"

	"google.golang.org/appengine/datastore"
	"google.golang.org/appengine/taskqueue"
)

func f(ctx context.Context) {
	err := datastore.RunInTransaction(ctx, func(ctx context.Context) error {
		t := taskqueue.NewPOSTTask("/worker", url.Values{
			// ...
		})
		// Use the transaction's context when invoking taskqueue.Add.
		_, err := taskqueue.Add(ctx, t, "")
		if err != nil {
			// Handle error
		}
		// ...
		return nil
	}, nil)
	if err != nil {
		// Handle error
	}
	// ...
}

Using the delayed package instead of a worker service

Setting up a handler for each distinct task (as described in the previous sections) can be cumbersome, as can serializing and deserializing complex arguments for the task—particularly if you have many diverse but small tasks that you want to run on the queue. The Go SDK includes a package (appengine/delay) exposing a simple API that allows you to bypass all the work of setting up dedicated task handlers and serializing and deserializing your parameters.

To use the delay package:

var expensiveFunc = delay.Func("some-arbitrary-key", func(ctx context.Context, a string, b int) {
	// do something expensive!
})

// Somewhere else
expensiveFunc.Call(ctx, "Hello, world!", 42)

The delay package serializes your function call and its arguments, then adds it to the task queue. When the task is executed, the delay package executes the function.

For more information about using the delay package, refer to its documentation.

Working with tasks in a multi-tenant application

By default, push queues use the current namespace as set in the namespace manager at the time the task is created. If your application uses multitenancy, see the Namespaces Go 1.11 API.

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