AGL Advanced Audio Architecture (4a)
4a is an audio framework made to handle automotive use cases like stream priority and interruptions. It does not deal with the audio by itself but rather with the policies and signaling around it. This means that 4a must be used in conjunction with an other API that is in charge of the audio stuff, like alsa or pulseaudio.
This detailed diagram shows the interactions between all components.
This diagram shows the global architecture that any application playing audio should follow. As a developer, when you want to play audio, you rely on APIs such as alsa, pulseaudio or higher level API(s). To play audio you usually have some kind of device URI that you have to open in order to write audio data to it.
4a does not change anything about that. You still have to do all of this. 4a provides permissions, signals and policies. This means that all compliant applications have to ask 4a for a ‘role’ when they want to play audio. If 4a grants them the permission, it returns a device URI that applications have to open.
Nothing prevent an application to directly open a device, but in this case no policies can be applied and access permissions are handled by system policies.
Even if applications only deal with the high level API, 4a is made of multiple components that rely on each other.
The high level API uses the hal-manager to list enabled HALs then it uses directly those HALs.
HALs rely directly on drivers and/or softmixer.