Bond is implemented in the following languages:
Bond exists to ensure that two processes can monitor each other's termination. This is useful in several situations.
Spawning nodelets/controllers/robots in gazebo
When spawning a nodelet (or anything else), two processes, the spawner and the container, communicate to bring the nodelet up, however, the current system does not cleanly deal with all termination possibilities (<<Ticket(ros-pkg 4221)>>). Creating a bond between the spawner and the container allows each to know when the other crashes and to implement appropriate recovery behaviors.
This applies to controllers (re-loading when the realtime_loop goes down) and to gazebo (re-creating the robot if gazebo crashes).
Bonds can be used to track resource ownership. The creation of a bond indicates a lease on the resource, and breaking a bond indicates that the lease is broken.
You could, for example, request that the laser scan at a particular frequency. By passing a bond id along with the request, the laser scanner controller will form a bond based on the request. When it receives a new request, it breaks the bond. Whether the laser controller receives a new request or the controller crashes, you will be informed that the laser is no longer scanning as you desired.