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An Android 101 : An overview on Binder framework.

While the underlying platform makes use of Linux concepts, the Android applications are pretty unique. Unlike traditional Java apps, Android apps do not have a an entry point (like a main() function ). The framework could invoke a particular entry point within an application to achieve the requested functionality.
Each android application is a mixture of several components . The components can be broadly grouped into 4 ‘component types’. You could read more about components here.

[Image credit : Android binder thesis by Thorsten Schreiber]

The different components may need to exchange data and it is realized through inter component/ inter process communication .The communication works with so called intents.Intents are representations for actions to be performed. Each intent is made of a URI and an ‘action’. The URI identifies the destination and action defines the operation to be performed on the component identified by the URI. As we learn about components and interactions it becomes increasingly evident that IPC forms a major part of the Android framework.

The Binder framework :
Unlike traditional Linux machines, Android uses its own mechanism for IPC called the Binder. Binder was originally developed as OpenBinder and was used as the IPC in BeOS . The current Binder implementation in Android is a customized implementation of the OpenBinder. This was mainly to ensure the new implementation uses a license that is compatible with the Android’s user space code.

Each Binder object is an implementation of the IBinder implementation . An important difference between Binder and other IPC mechanisms is the use of Binder token. A binder token is value that uniquely identifies any Binder. This capability to uniquely identify a binder across processes allows the binder id to act as a ‘shared token’ across multiple processes.

Communication model:
The Binder framework communication is a client server model . Each client initiates communication and waits for response from the server. Each client would have a proxy object for the client side communication. The server side constitutes a pool of worker threads.The server shall spawn a new thread for each new Binder request from the client. The bridge between the client and the server process is the binder driver. The Binder driver is a character device that is part of kernel space. This module ensures the client reaches the appropriate destination across process boundaries.

[Image credit : Android binder thesis by Thorsten Schreiber]

AIDL : As with any other RPC , there is a need to write a proxy and stub class that would be interacting with the client and server. Developers write the interface for their remote services using AIDL .The AIDL parser takes care of generating Java classes for the proxy and stub and logic to convert data into parcels ,that the Binder middleware understands.
If you are in the business of writing AIDLs you should keep in mind that the Binder protocol is always ‘request gives response’ there is no inbuilt constructs to provide asynchronous functions. Asynchronous mechanisms have to be built on top of the Binder framework.

Binder vs SysV IPC: You may wonder why not reuse SysV IPC that comes with Linux. The Binder documentation has good explanation on why Google has stayed away from using SysV IPC.

One of the design requirements of Binder is to ensure there is no ‘resource leak’ when any service dies improperly . i.e.Ensure that any kernel level resources (like semaphores/binder objects) are cleared up in case the parent process is killed or crashes prematurely. There is no way to release a SysV semaphore if the process that created it is explicitly killed or it crashes. Killing processes automatically to make room for new ones is very common with the Android framework.

For an in-depth discussion on Binders, you could refer to some of the links below that I used to learn about Binders.

References:
1. eLinux Binder documentation : http://elinux.org/Android_Binder
2. A nice ppt with implementation details : http://blog.kmckk.com/archives/3676340.html
2. NDK reference documentation about SYS V IPC : <NDK INSTALL ROOT>\docs\system\libc\SYSV-IPC.html .
3. Comparison of IPC mechanisms : http://lwn.net/Articles/466304/
4. A good overview on OpenBinder : http://www.osnews.com/story/13674/Introduction-to-OpenBinder-and-Interview-with-Dianne-Hackborn/