2 Communication in Erlang
Communication in Erlang is conceptually performed using asynchronous signaling. All different executing entities, such as processes and ports, communicate through asynchronous signals. The most commonly used signal is a message. Other common signals are exit, link, unlink, monitor, and demonitor signals.
2.1 Passing of Signals
The amount of time that passes between a signal is sent and the arrival of the signal at the destination is unspecified but positive. If the receiver has terminated, the signal does not arrive, but it can trigger another signal. For example, a link signal sent to a non-existing process triggers an exit signal, which is sent back to where the link signal originated from. When communicating over the distribution, signals can be lost if the distribution channel goes down.
The only signal ordering guarantee given is the following: if an entity sends multiple signals to the same destination entity, the order is preserved; that is, if A sends a signal S1 to B, and later sends signal S2 to B, S1 is guaranteed not to arrive after S2.
2.2 Synchronous Communication
Some communication is synchronous. If broken down into pieces, a synchronous communication operation consists of two asynchronous signals; one request signal and one reply signal. An example of such a synchronous communication is a call to erlang:process_info/2 when the first argument is not self(). The caller sends an asynchronous signal requesting information, and then waits for the reply signal containing the requested information. When the request signal reaches its destination, the destination process replies with the requested information.
The implementation of different asynchronous signals in the virtual machine can vary over time, but the behavior always respects this concept of asynchronous signals being passed between entities as described above.
By inspecting the implementation, you might notice that some specific signal gives a stricter guarantee than described above. It is of vital importance that such knowledge about the implementation is not used by Erlang code, as the implementation can change at any time without prior notice.
Examples of major implementation changes:
- As from ERTS 5.5.2 exit signals to processes are truly asynchronously delivered.
- As from ERTS 5.10 all signals from processes to ports are truly asynchronously delivered.