# Configure an Application to Launch at Start-up

## Pre-requisites

- You must already know how to install (deploy) your app on the robot. See *Deploying ROS 2 packages on your robot*.
- We recommend you to also read about the [robot's start-up configuration](application_management.md).

## Adding Your Own Application to the Module Manager

In addition to the system applications, you can easily add your own custom modules to the Module Manager. To do so, register the new module in the `CMakeLists.txt` of a ROS package. For example, if you have a package called `hello_world` with a node called `hello_world_node`, add the following lines to `CMakeLists.txt`:

```cmake
find_package(pal_module_cmake QUIET)

if(pal_module_cmake_FOUND)
  pal_register_modules(
    module/hello_world.yaml
  )
endif()
```

The module file `hello_world.yaml` should be placed in the `module/` directory of the package and would look something like this:

```yaml
hello_world:
  run: "hello_world hello_world_node"
```

Then, deploy the package to the robot as described in *Deploying ROS 2 packages on your robot*. After a reboot of the robot the module is available and can be started using:

```bash
pal module start hello_world
```

## Adding Your Own Application to the Startup Sequence

To start the module at startup, a module set has to be created. Register a new module set in the `CMakeLists.txt` of the `hello_world` ROS package:

```cmake
if(pal_module_cmake_FOUND)
  pal_register_module_sets(
    module/custom_module_set.yaml
  )
endif()
```

The module set file `custom_module_set.yaml` should be placed in the `module/` directory of the package and would look something like this:

```yaml
custom_modules:
  - hello_world
```

Then, deploy the package to the robot as described in *Deploying ROS 2 packages on your robot*. After a reboot of the robot the module `hello_world` starts automatically.

## Customizing an Existing Application in the Startup Sequence

To customize an existing application in the startup sequence, you can create a new module that overwrites the existing one. First, retrieve the filename of the existing module by running:

```bash
pal module info foo_module
```

In this example the filename is `00_foo_module.yaml`.

Then, register a new module in the `CMakeLists.txt` of a ROS package. The new module file should be placed in the `module/` directory of the package and would look something like this:

```yaml
foo_module:
  run: "foo_package different_node"
```

The Module Manager sorts all registered modules based on their filename in lexicographical order. This means that to overwrite the existing module in `00_foo_module.yaml` you have to register the new module in a file with a name that comes after it lexicographically — for example `10_foo_module.yaml`.

```{note}
The registered modules are sorted based on their filename in lexicographical order. Take this into account when naming the module file. A module file named `00_foo_module.yaml` will be overwritten by a module file named `10_foo_module.yaml`. To overwrite `bar_module.yaml`, the new module file should be named `bar_module_01.yaml`.
```

Finally, deploy the package to the robot. After a reboot, the updated version of `foo_module` is used. To verify, run:

```bash
pal module info foo_module
```

## Disabling an Existing Application in the Startup Sequence

To disable a module in the startup sequence, use the Module Manager command line interface:

```bash
pal module disable foo_module   # disable a module
pal module enable foo_module    # re-enable a module
```

Then, to apply the changes, restart the startup sequence:

```bash
pal module_manager restart
```

## See Also

- [The Startup Process of the Robot and Application Management](application_management.md)