Erlang: difference between using gen_server:cast/2 and standard message passing

Question

I was working though a problem and noticed some code where a previous programmer was passing messages using the standard convention of PID ! Message. I have been using gen_server:cast/2. I was wondering if somebody could explain to me the critical differences and considerations when choosing between the two?

Answer 1

There are a few minor differences:

• Obviously, the gen_server handles casts in handle_cast and "normal" messages in handle_info.
• A cast never fails; it always returns ok. Sending a message with ! fails with badarg if you are sending a message to an atom that is currently not registered by a process. (Sending a message to a pid never causes an error, even if the process is dead.)
• If the gen_server is running on a remote node that is not currently connected to the local node, then gen_server:cast will spawn a background process to establish the connection and send the message, and return immediately, while ! only returns once the connection is established. (See the code for gen_server:do_send.)

As for when to choose one or the other, it's mostly a matter of taste. I'd say that if the message could be thought of as an asynchronous API function for the gen_server, then it should use cast, and have a specific API function in the gen_server callback module. That is, instead of calling gen_server:cast directly, like this:

gen_server:cast(foo_proc, {some_message, 42})

make a function call:

foo_proc:some_message(42)

and implement that function like the direct cast above. That encapsulates the specific protocol of the gen_server inside its own module.

In my mind, "plain" messages would be used for events, as opposed to API calls. An example would be monitor messages, {'DOWN', Ref, process, Id, Reason}, and events of a similar kind that might happen in your system.