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Documentation | 2025.5

# Workers

P4 Code Review uses background processes called workers to respond to events in the P4 Server. The default number of workers is three, and each worker waits to process events for up to 10 minutes before terminating. When a worker terminates, a new one is spawned.

A worker processes the longest-pending task from the queue and triggers an event for that task. Each event has two parameters:

- id – The unique identifier of the task

- time – The timestamp of the event.

The name of the event is task.type (e.g. task.change).

Immediately after processing a task, a worker attempts to process another task from the queue. If there are no task in the queue, the worker sleeps for one second and then checks the queue for another task.

When the worker's running time exceeds their lifetime, it terminates to avoid potential memory leakage. By default, a worker's lifetime is 10 minutes . To adjust this lifetime, see Worker configuration.

A given task can run for a long time and consume as much memory as is available (to accommodate large changes for instance). Workers run in the background by default.

Each worker maintains a connection to the P4 Server for the duration of its lifetime. This may impact your P4 Server management practices.

To view P4 Code Review task queue information, as an admin or super user, navigate to the User id dropdown menu, select System Information, and open the Queue Info tab. If the task queue starts to build up, consider increasing the number of workers.

## Worker status

To determine the current status of workers, do one of the following:

- As an admin or super user, navigate to the User id dropdown menu, select System Information, and open the Queue Info tab.

or

- Visit the URL: https://myswarm.url/queue/status.

The response is formatted in JSON, and looks like this:

{"tasks":0,"futureTasks":1,"workers":3,"maxWorkers":3,"workerLifetime":"595s"}

During normal use of P4 Code Review, the following error message appears for logged-in users when P4 Code Review detects that no workers are running:

## Worker configuration

If you make a configuration change, P4 Code Review will not use it until the configuration cache has been reloaded, this forces P4 Code Review to use the new configuration.

You must be an admin or super user to reload the P4 Code Review config cache. Navigate to the User id dropdown menu, select System Information, click the Cache Info tab, and click the Reload Configuration button.

To adjust the configuration for workers, add a configuration block to the `SWARM_ROOT/data/config.php` file:

    <?php
        // this block should be a peer of 'p4'
        'queue'  => array(
            'workers'             => 3,    // defaults to 3
            'worker_lifetime'     => 595,  // defaults to 10 minutes (less 5 seconds)
            'worker_task_timeout' => 1800, // defaults to 30 minutes
            'worker_memory_limit' => '1G', // defaults to 1 gigabyte
        ),

where:

- workers specifies the number of worker processes that should be available. The default is 3. The cron job ensures that new worker processes are started when necessary. If the limit is reached or exceeded, new worker processes are not started. Workers do not communicate with each other. If the task queue starts to build up, consider increasing the number of workers.
- worker_lifetime specifies the amount of time in seconds that a worker process should run for. The default is 595 seconds (10 minutes less 5 seconds). If a worker process exceeds this limit while processing a task, it will complete the active task and then terminate. `worker_lifetime` does not cause tasks to terminate mid-processing.
- worker_task_timeout specifies the maximum amount of time in seconds that a worker process can spend processing a single task. The default is 1800 seconds (30 minutes). This is useful for terminating workers that might get stalled in a variety of situations.
- worker_memory_limit specifies the maximum amount of memory that a worker process is allowed to use while processing a task. The default is 1G (1 gigabyte).

## Manually start workers

To kick off a new worker process, do one of the following:

- As an admin or super user, navigate to the User id dropdown menu, select System Information, open the Queue Info tab, and click Start a worker.

or

- Visit the URL: https://myswarm.url/queue/worker.

If the number of workers running matches the configured limit, the requested worker process is not started.

This technique does start a worker, but it lasts only for its configured lifetime. Typically, you would always want at least one worker running. See Set up a recurring task to spawn workers for details.

## Manually restart workers

To restart an idle worker process, remove its lock file:

rm data/queue/workers/worker_id

A worker process that is busy processing a task will continue operation until its task is complete. Immediately afterwards, if the worker notices that its lock file is missing it exits.

If you have a recurring task to start workers, the recurring task starts a fresh worker, if necessary. See Set up a recurring task to spawn workers for details.

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