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The Android IPC system

January 3rd, 2011 7 comments

The information below comes from a number of sources, including my own experiments with the Android IPC and some disparate internet sources.

The overall architecture of the Android IPC system is shown in the diagram below. It consists of four major blocks; one in kernel space, and the other three in user space. The dashed lines represent the logical RPC calls. The solid lines represent the actual data flow.

  • BinderDriver: This is the core of IPC system. It passes data between a ServiceProvider(s) and a ServiceUser(s). This kernel component is provided by Android.
  • ServiceProvider: Provides some kind of service. It parses the received RPC data from the BinderDriver and does the real work. Application developers will either make use of existing service providers (such as the Camera or AudioFlinger), or in some cases will write their own.
  • ServiceManager: This is a special singleton ServiceProvider that provides service manager services for other service providers. This component is provided by Android.
  • ServiceUser: This is the client. It remote calls the ServiceProvider by generating an RPC and sending it to the BinderDriver. Application developers typically write their own ServiceUser as part of their application.

Here is a typical flow of events for a fictitious MultServiceProvider (a service provider that multiplies two numbers for a client) and a MultServiceUser client which doesn’t know how to do multiplication (maybe because the numbers are quaternions) and needs to use the MultServiceProvider:

  1. ServiceManager runs first (at power-up) and registers a special node (node O) with the BinderDriver.
  2. The MultServiceProvider gets an IServiceManager proxy object for the special node O by calling the global defaultServiceManager() function.
  3. The MultServiceProvider then calls defaultServiceManager()->addService("Multiplier", new MultServiceProvider()) to add itself as a service provider and then waits in an infinite loop for someone to request its services. The addService RPC call is routed to the ServiceManager through the BinderDriver.
  4. The BinderDriver notices that the RPC is for the ServiceManager to add a new service, so besides routing the RPC to the ServiceManager it generates another node (let’s call it node M), for the new MultServiceProvider.
  5. The ServiceManager reads the data from the BinderDriver and processes the IServiceManager::addService RPC call.
  6. The MultServiceUser client process gets an IServiceManager proxy object for the special node O (again by using defaultServiceManager()).
  7. The client does an IServiceManager::getService("Multiplier") RPC call to get the MultServiceProvider. This call is routed to the ServiceManager through the BinderDriver.
  8. The ServiceManager reads the RPC data from the BinderDriver, processes the IServiceManager::getService request and returns back the node representing the MultServiceProvider.
  9. MultServiceUser calls MultServiceProvider::multiply(a, b). This call is routed to  the MultServiceProvider by the BinderDriver.
  10. The MultServiceProvider handles the MultServiceProvider::multiply RPC call and sends the product of the 2 numbers in a reply to the BinderDriver.
  11. The BinderDriver routes the reply back to the client.
  12. The client reads the data from the BinderDriver which contains the result of “a * b”.

In a future post I hope to discuss the whole architecture in more detail, with concrete code examples for how to use IBinder, IInterface, BBinder, BpInterface, BnInterface, etc… to create a ServiceProvider and a ServiceUser all in native C++ code on Android.

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