Java applications define task queues in a configuration file called
queue.xml. This file is stored in the application's
WEB-INF directory. You can use
queue.xml to configure both push queues and pull queues. This configuration file is optional for push queues, which have a default queue. Pull queues must be specifically configured in
- About Queues
- Setting the Storage Limit for All Queues
- Defining Push Queues and Processing Rates
- Defining Pull Queues
- Configuring Retry Attempts for Failed Tasks
- Queue Definitions
- Updating Task Queue Configuration
A Java app can define task queues using a file named
queue.xml, in the app's
WEB-INF/ directory in the WAR. The file specifies an element named
queue. Within this element, you can name any number of individual queues and define their processing rates. If no
queue.xml file is provided, the app can still use the default queue returned by
The app's queue configuration applies to all versions of the app. If a given version of an app enqueues a task, the queue uses the task handler for that version of the app. To control this behaviour see the target parameter.
An app can only add tasks to queues defined in
queue.xml and the default queue. If you upload a new
queue.xml file that removes a queue, but that queue still has tasks, the queue is "paused" (its rate is set to 0) but not deleted. You can re-enable the deleted queue by uploading a new
queue.xml file with the queue defined.
Setting the Storage Limit for All Queues
You can use
queue.xml to define the total amount of storage that task data can consume over all queues. To define the total storage limit, include an element named
<total-storage-limit> at the first level under
<!--Change the total storage limit for all queues to 50MB--> <queue-entries> <total-storage-limit>50M</total-storage-limit> <queue> ... </queue> </queue-entries>
The value is a number followed by a unit:
B for bytes,
K for kilobytes,
M for megabytes,
G for gigabytes,
T for terabytes.
100K specifies a limit of 100 kilobytes. If adding a task would
cause the queue to exceed its storage limit, the call to add the task will fail. The default limit is
500M (500 megabytes) for free apps. For billed apps there is no limit until you
explicitly set one. You can use this limit to protect your app from a
fork bomb programming error
in which each task adds multiple other tasks during its execution. If your app is receiving
errors for insufficient quota when adding tasks, increasing the total storage
limit may help. If you are using this feature, we strongly recommend setting a limit that
corresponds to the storage required for several days' worth of tasks. In this way, your app is robust to its queues
being temporarily backed up and can continue to accept new tasks while working through the backlog while
still being protected from a fork bomb programming error.
Defining Push Queues and Processing Rates
You can define any number of individual queues by providing a queue
<name>. You can control the rate at which tasks are processed in each queue by defining other directives, such as
<max-concurrent-requests>. You can read more about these directives in the Queue Definitions section.
The task queue uses token buckets to control the rate of task execution. Each named queue has a token bucket that holds a certain number of tokens, defined by the
<bucket-size> directive. Each time your application executes a task, it uses a token. Your app continues processing tasks in the queue until the queue's bucket runs out of tokens. App Engine refills the bucket with new tokens continuously based on the
<rate> that you specified for the queue.
Configuring the Default Queue
All apps have a push queue named
default. This queue has a preset rate of 5 tasks per second, but you can change this rate by defining a default queue in
queue.xml. If you do not configure a default queue in
default queue doesn't display in the Administration Console until the first time it is used. You can customize the settings for this queue by defining a queue named
<queue-entries> <!--Change the refresh rate of the default queue to 1/s--> <queue> <name>default</name> <rate>1/s</rate> </queue> </queue-entries>
Configuring the Processing Rate
If your queue contains tasks to process, and the queue's bucket contains tokens, App Engine processes as many tasks as there are tokens remaining in the bucket. This can lead to bursts of processing, consuming system resources and competing with user-serving requests. You can adjust the bucket size and rate based on your application's needs and available resources as follows:
<queue-entries> <queue> <name>optimize-queue</name> <rate>20/s</rate> <bucket-size>40</bucket-size> </queue> <queue-entries>
Configuring the Maximum Number of Concurrent Requests
You can further control the processing rate by setting
<max-concurrent-requests>, which limits the number of tasks that can execute simultaneously.
If your application queue has a rate of 20/s and a bucket size of 40, tasks in that queue execute at a rate of 20/s and can burst up to 40/s briefly. These settings work fine if task latency is relatively low; however, if latency increases significantly, you'll end up processing significantly more concurrent tasks. This extra processing load can consume extra instances and slow down your application.
For example, let's assume that your normal task latency is 0.3 seconds. At this latency, you'll process at most around 40 tasks simultaneously. But if your task latency increases to 5 seconds, you could easily have over 100 tasks processing at once. This increase forces your application to consume more instances to process the extra tasks, potentially slowing down the entire application and interfering with user requests.
You can avoid this possibility by setting
<max-concurrent-requests> to a lower value. For example, if you set
<max-concurrent-requests> to 10, our example queue maintains about 20 tasks/second when latency is 0.3 seconds. However, when the latency increases over 0.5 seconds, this setting throttles the processing rate to ensure that no more than 10 tasks run simultaneously.
<queue-entries> <!--Set the number of max concurrent requests to 10--> <queue> <name>optimize-queue</name> <rate>20/s</rate> <bucket-size>40</bucket-size> <max-concurrent-requests>10</max-concurrent-requests> </queue> </queue-entries>
Defining Pull Queues
You can specify any named queue as a pull queue by adding the
<mode>pull</mode> element to
If you are using the Task Queue REST API, you also need to create an access control list (ACL) using the <acl> directive. This directive allows you to restrict access to user email addresses corresponding to an account hosted by Google.
acl element has two available parameters:
user-email: enables the user to list, get, lease, delete, and update tasks.
writer-email: enables the user to insert tasks.
In order to access all functions of the API, a developer's email address must be specified both as a
user_email and a
writer_email. The following code snippet creates a pull queue named pull-queue with two users in the ACL. The email account
firstname.lastname@example.org can access all API calls:
<queue-entries> <queue> <name>pull-queue</name> <mode>pull</mode> <acl> <user-email>email@example.com</user-email> <!-- can list, get, lease, delete, and update tasks --> <writer-email>firstname.lastname@example.org</writer-email> <!-- can insert tasks --> <writer-email>email@example.com</writer-email> <!-- can do all --> </acl> </queue> </queue-entries>
Configuring Retry Attempts for Failed Tasks
Tasks executing in the task queue can fail for many reasons. If a task fails to execute (by returning any HTTP status code outside of the range 200-299), App Engine retries the task until it succeeds. By default, the system gradually reduces the retry rate to avoid flooding your application with too many requests, but schedules retry attempts to recur at a maximum of once per hour until the task succeeds.
Push queues and pull queues differ in how they retry tasks.
Retrying Tasks in Push Queues
In push queues, you can specify your own scheme for task retries by adding the <retry-parameters> element in
queue.xml. This addition allows you to specify the maximum number of times to retry failed tasks in a specific queue. You can also set a time limit for retry attempts and control the interval between attempts.
The following example demonstrates various retry scenarios:
fooqueue, tasks are retried at least seven times and for up to two days from the first execution attempt. After both limits are passed, it fails permanently.
barqueue, App Engine attempts to retry tasks, increasing the interval linearly between each retry until reaching the maximum backoff and retrying indefinitely at the maximum interval (so the intervals between requests are 10s, 20s, 30s, ..., 190s, 200s, 200s, ...).
bazqueue, the interval increases to twice the minimum backoff and retries indefinitely at the maximum interval (so the intervals between requests are 10s, 20s, 40s, 80s, 120s, 160s, 200s, 200s, ...).
<queue-entries> <queue> <name>fooqueue</name> <rate>1/s</rate> <retry-parameters> <task-retry-limit>7</task-retry-limit> <task-age-limit>2d</task-age-limit> </retry-parameters> </queue> <queue> <name>barqueue</name> <rate>1/s</rate> <retry-parameters> <min-backoff-seconds>10</min-backoff-seconds> <max-backoff-seconds>200</max-backoff-seconds> <max-doublings>0</max-doublings> </retry-parameters> </queue> <queue> <name>bazqueue</name> <rate>1/s</rate> <retry-parameters> <min-backoff-seconds>10</min-backoff-seconds> <max-backoff-seconds>200</max-backoff-seconds> <max-doublings>2</max-doublings> </retry-parameters> </queue> </queue-entries>
Retrying Tasks in Pull Queues
In pull queues, you can specify the number of times to retry a task using the <retry-parameters> element with the <task-retry-limit> field. The system counts each time you lease a task using leaseTasks(). When the count exceeds the
<task-retry-limit>, the system deletes the task automatically. If you don't specify a
<task-retry-limit>, the system never deletes a task automatically.
The following code sample shows how to specify a pull queue limited to seven retry attempts:
<queue-entries> <queue> <name>pull-queue</name> <mode>pull</mode> <retry-parameters> <task-retry-limit>7</task-retry-limit> </retry-parameters> </queue> </queue-entries>
queue.xml file is an XML file whose root element is
<queue-entries>. This element contains zero or more
<queue> elements, one for each named queue.
<queue> element contains configuration for a queue. It can contain the following elements:
<acl>Experimental! (pull queues only)
Creates an access control list (ACL) for Pull Queues using the Task Queue REST API. The ACL is composed of the specified email addresses. Accepts email addresses only from a Google Account. Enter each email address on its own line as follows
<writer-email>firstname.lastname@example.org</writer-email>. For more information, please see Defining Pull Queues.
<bucket-size>(push queues only)
Task queues use a "token bucket" algorithm for dequeueing tasks. The bucket size limits how fast the queue is processed when many tasks are in the queue and the rate is high. This allows you to have a high rate so processing starts shortly after a task is enqueued, but still limit resource usage when many tasks are enqueued in a short period of time.
<bucket-size>is specified for a queue, the default value is 5.
For more information on the algorithm, see the Wikipedia article on token buckets.
<max-concurrent-requests>(push queues only)
Sets the maximum number of tasks that can be executed at any given time in the specified queue. The value is an integer. By default, this directive is unset and there is no limit on the maximum number of concurrent tasks.
Restricting the maximum number of concurrent tasks gives you more control over your queue's rate of execution. For example, you can constrain the number of instances that are running the queue's tasks. Limiting the number of concurrent requests in a given queue allows you to make resources available for other queues or online processing.
The name of the queue. This is the name you specify when you call
A queue name can contain letters, numbers, and hyphens.
<rate>(push queues only)
How often tasks are processed on this queue. The value is a number followed by a slash and a unit of time, where the unit is
hfor hours, or
dfor days. For example, the value
5/msays tasks will be processed at a rate of 5 times per minute.
If the number is
0/s), the queue is considered "paused," and no tasks are processed.
Configures retry attempts for failed tasks. This addition allows you to specify the maximum number of times to retry failed tasks in a specific queue. You can also set a time limit for retry attempts and control the interval between attempts.
The maximum number of retry attempts for a failed task. If specified with
task-age-limit, App Engine retries the task until both limits are reached.
- <task-age-limit> (push queues only)
The time limit for retrying a failed task, measured from when the task was first run. The value is a number followed by a unit of time, where the unit is
hfor hours, or
dfor days. For example, the value
5dspecifies a limit of five days after the task's first execution attempt. If specified with
task-retry-limit, App Engine retries the task until both limits are reached.
- <min-backoff-seconds> (push queues only)
- The minimum number of seconds to wait before retrying a task after it fails.
- <max-backoff-seconds> (push queues only)
- The maximum number of seconds to wait before retrying a task after it fails.
- <max-doublings> (push queues only)
The maximum number of times that the interval between failed task retries will be doubled before the increase becomes constant. The constant is:
2**(max-doublings - 1) * min-backoff-seconds.
<target>(push queues only)
A string naming a module/version, a frontend version, or a backend, on which to execute this task.
The string is prepended to the domain name of the tasks's HTTP request to your app. For example, if you app ID is
my-app.appspot.comand you set the target to
my-version.my-module, the URL hostname will be set to
If target is unspecified, then tasks are invoked on the same version of the application where they were enqueued. So, if you enqueued a task from the default application version without specifying a target on the queue, the task is invoked in the default application version. You can specify a backend as your target.
Updating Task Queue Configuration
The task queue configuration for the app is updated when you upload the application using
appcfg.sh update. You can update just the configuration for an app's task queues without uploading the full application. To upload the
queue.xml file, use the
appcfg.sh update_queues command:
./appengine-java-sdk/bin/appcfg.sh update_queues myapp/war
See Uploading an and Managing a Java App for more information.