Configuring communication between ASIL containers¶

Inter-process communication (IPC) allows processes, such as applications and services, to share data and synchronize resources through a common channel. Applications that communicate using IPC are either servers or clients. Servers actively listen to requests in the channel and respond to them. Clients connect to the channel to request data. Configuration stages related to ASIL applications belong in the rootfs pipeline of the manifest file, whereas configuration stages related to QM applications belong in the qm_rootfs pipeline.

Signals, shared files, network sockets, and D-Bus subsystems are mechanisms in the kernel that enable IPC. In AutoSD, IPC occurs by using UNIX domain sockets. UNIX domain sockets are virtual sockets similar to network sockets, but UNIX domain sockets are constrained to the same host operating system.

UNIX domain sockets use the file system as their address name space. The sockets are seen as files for other processes and can open the same socket to communicate. UNIX sockets do not use the file system to exchange data. They use the kernel’s memory buffers instead.

In this context, to enable IPC between ASIL applications, the applications’ containers require access to the same UNIX domain socket in the file system. The volume, or storage area, that contains the UNIX domain socket must be mounted inside the containers. For more information about socket creation, see Systemd sockets.

Prerequisites

A custom manifest file, such as the manifest file that you created in Embedding RPM packages in the AutoSD image
Two or more containers embedded in your image that you want to configure to communicate. In this example, these containers are the radio-service and engine-service containers from the sample apps repository.

Procedure

To configure communication between the radio-service and engine-service containers, mount the volumes that contain the UNIX domain sockets in /var/run/<your-dir>. In the example code, <your-dir> is asil-ipc-demo, but you can name the directory according to your own conventions.
1. Add this line to the engine.container file to mount the volume where the ASIL socket resides:
```
[Container]
Volume=/run/asil-ipc-demo:/run/asil-ipc-demo
```
2. Add this line to the radio.container file to mount the volume where the ASIL socket resides:
```
[Container]
Volume=/run/asil-ipc-demo:/run/asil-ipc-demo
```

Optional: Use systemd to create a UNIX socket file with the same name as the service with which the socket is associated, such as the radio service.

Create the systemd.socket file in files/root_fs/radio.socket:

[Unit]
Description=An example systemd unix socket

[Socket]
ListenStream=%t/asil-ipc-demo/asil_ipc.socket
RuntimeDirectory=asil-ipc-demo

[Install]
WantedBy=sockets.target

To enable the application to start after you create the socket, add these lines to the radio.container file to create a dependency with the systemd socket service:
```
[Unit]
After=radio.socket
Requires=radio.socket
```

To copy the radio.socket file to the /etc/systemd/system/ directory, create a new org.osbuild.copy stage in the rootfs pipeline of your manifest file:

- type: org.osbuild.copy
    inputs:
      inlinefile3:
        type: org.osbuild.files
        origin: org.osbuild.source
        mpp-embed:
          id: radio_socket
          path: ../files/root_fs/radio.socket

    options:
       paths:
       - from:
           mpp-format-string:
           input://inlinefile3/{embedded['radio_socket']}
         to: tree:///etc/systemd/system/radio.socket

Enable the service to ensure that the socket is created at boot. Create a new org.osbuild.systemd stage in the rootfs pipeline of your manifest file:
```
- type: org.osbuild.systemd
  options:
    enabled_services:
    - radio.socket
```

Next steps

Now that you configured communication between ASIL containers, you can build your AutoSD image. For more information, see Building an AutoSD image.
Alternatively, you can continue customizing your image. For more information, see Containerizing applications.

Additional resources

Sample custom OSBuild manifest