Editing Erlang (programming language) (section)

===Supervisor trees===
A typical Erlang application is written in the form of a supervisor tree.  This architecture is based on a hierarchy of processes in which the top level process is known as a "supervisor".  The supervisor then spawns multiple child processes that act either as workers or more, lower level supervisors.  Such hierarchies can exist to arbitrary depths and have proven to provide a highly scalable and fault-tolerant environment within which application functionality can be implemented.

Within a supervisor tree, all supervisor processes are responsible for managing the lifecycle of their child processes, and this includes handling situations in which those child processes crash.  Any process can become a supervisor by first spawning a child process, then calling <code>erlang:monitor/2</code> on that process.  If the monitored process then crashes, the supervisor will receive a message containing a tuple whose first member is the atom <code>'DOWN'</code>.  The supervisor is responsible firstly for listening for such messages and for taking the appropriate action to correct the error condition.