# Application/Module Template

An application can be written to run as either a _task_ (a module with its own stack and process priority) or as a _work queue task_ (a module that runs on a work queue thread, sharing the stack and thread priority with other tasks on the work queue). In most cases a work queue task can be used, as this minimizes resource usage.

:::note [Architectural Overview > Runtime Environment](../../concept/architecture#runtime-environment) provides more information about tasks and work queue tasks. :::

:::note All the things learned in the [First Application Tutorial](hello_sky) are relevant for writing a full application. :::

## Work Queue Task

PX4-Autopilot contains a template for writing a new application (module) that runs as a _work queue task_: [src/examples/work\_item](https://github.com/PX4/PX4-Autopilot/tree/main/src/examples/work_item).

A work queue task application is just the same as an ordinary (task) application, except that it needs to specify that it is a work queue task, and schedule itself to run during initialisation.

The example shows how. In summary:

1.  Specify the dependency on the work queue library in the cmake definition file ([CMakeLists.txt](https://github.com/PX4/PX4-Autopilot/blob/main/src/examples/work_item/CMakeLists.txt)):

    ```
    ...
    DEPENDS
       px4_work_queue
    ```
2. In addition to `ModuleBase`, the task should also derive from `ScheduledWorkItem` (included from [ScheduledWorkItem.hpp](https://github.com/PX4/PX4-Autopilot/blob/main/platforms/common/include/px4_platform_common/px4_work_queue/ScheduledWorkItem.hpp))
3.  Specify the queue to add the task to in the constructor initialisation. The [work\_item](https://github.com/PX4/PX4-Autopilot/blob/main/src/examples/work_item/WorkItemExample.cpp#L42) example adds itself to the `wq_configurations::test1` work queue as shown below:

    ```cpp
    WorkItemExample::WorkItemExample() :
        ModuleParams(nullptr),
        ScheduledWorkItem(MODULE_NAME, px4::wq_configurations::test1)
    {
    }
    ```

    :::note The available work queues (`wq_configurations`) are listed in [WorkQueueManager.hpp](https://github.com/PX4/PX4-Autopilot/blob/main/platforms/common/include/px4_platform_common/px4_work_queue/WorkQueueManager.hpp#L49). :::
4. Implement the `ScheduledWorkItem::Run()` method to perform "work".
5. Implement the `task_spawn` method, specifying that the task is a work queue (using the `task_id_is_work_queue` id.
6. Schedule the work queue task using one of the scheduling methods (in the example we use `ScheduleOnInterval` from within the `init` method).

## Tasks

PX4/PX4-Autopilot contains a template for writing a new application (module) that runs as a task on its own stack: [src/templates/template\_module](https://github.com/PX4/PX4-Autopilot/tree/main/src/templates/template_module).

The template demonstrates the following additional features/aspects that are required or are useful for a full application:

* Accessing parameters and reacting to parameter updates.
* uORB subscriptions and waiting for topic updates.
* Controlling the task that runs in the background via `start`/`stop`/`status`. The `module start [<arguments>]` command can then be directly added to the [startup script](../concept/system_startup).
* Command-line argument parsing.
* Documentation: the `PRINT_MODULE_*` methods serve two purposes (the API is documented [in the source code](https://github.com/PX4/PX4-Autopilot/blob/v1.8.0/src/platforms/px4_module.h#L381)):
  * They are used to print the command-line usage when entering `module help` on the console.
  * They are automatically extracted via script to generate the [Modules & Commands Reference](../modules_main) page.
