GenServer Callbacks: handle_continue and handle_call

The handle_continue callback function

The handle_continue/2 callback is a recent addition to GenServer. Often GenServer processes do complex work as soon as they start. Rather than blocking the whole application from starting, we return {:ok, state, {:continue, term}} from the init/1 callback and use handle_continue/2.

Return values

Accepted return values for handle_continue/2 include the following:

• {:noreply, new_state}

• {:noreply, new_state, {:continue, term}}

• {:stop, reason, new_state}

{:noreply, new_state}

Since the handle_continue/2 callback receives the latest state, we can use it to update the GenServer with new information by returning {:noreply, new_state}. For example, we can write something like this:

def handle_continue(:fetch_from_database, state) do 
    # get `users` from the database
    {:noreply, Map.put(state, :users, users)}
end

{:noreply, new_state, {:continue, term}} and {:stop, reason, new_state}

The other return values are similar to the ones we already covered for init/1, but it’s interesting to note that handle_continue/2 can also return {:continue, term}, which triggers another handle_continue/2. We can use this to break down work into several steps when needed. Although handle_continue/2 is often used in conjunction with init/1, other callbacks can also return {:continue, term}.

Send process messages

One of the highlights of GenServer processes is that we can interact with them while they run. This is done by sending messages to the process. If we want to get some information back from the process, we use GenServer.call/3. When we don’t need a result back, we use GenServer.cast/2. Both functions accept the process identifier as their first argument. The second argument is the message to send to the process. Messages could be any Elixir term.

When the cast/2 and call/3 functions are used, the handle_cast/2 and handle_call/3 callbacks are invoked, respectively. Let’s see how they work in practice.

The handle_call callback function

Let’s implement an interface to get the current process state and start sending emails. We add the following code to send_server.ex: