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Exercise: OTP-Supervisors-2

Rework your stack server to use a supervision tree with a separate stash process to hold that state. Verify that it works and that when you crash the server the state is retained across a restart.

Solution

See the stack directory for the full project.

It is a little hard to tell if the state is preserved when we cause an error by popping an empty stack. So, I changed the push implementation to the following so we could observe a couple scenarios:

def handle_cast({:push, item}, {current_stack, stash_pid} = state) do
  case item do
    1 ->
      raise RuntimeError
    2 ->
      {:stop, :normal, state}
    _ ->
      {:noreply, {[item | current_stack], stash_pid}}
  end
end

If 1 is pushed onto the stack, we will raise a RuntimeError. If 2 is pushed, we will stop the server normally; let's see if it restarts:

First let's try the case where we pop from an empty stack:

iex> Stack.Server.pop
1
iex> Stack.Server.pop
2
iex> Stack.Server.pop
3
iex> Stack.Server.pop
Stack.Server.terminate - Reason: {{:badmatch, []}, [{Stack.Server, :handle_call, 3, [file: 'lib/stack/server.ex', line: 39]}, {:gen_server, :try_handle_call, 4, [file: 'gen_server.erl', line: 615]}, {:gen_server, :handle_msg, 5, [file: 'gen_server.erl', line: 647]}, {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 247]}]}; State: {[], #PID<0.126.0>}
** (exit) exited in: GenServer.call(MyStack, :pop, 5000)
    ** (EXIT) an exception was raised:
        ** (MatchError) no match of right hand side value: []
            (stack) lib/stack/server.ex:39: Stack.Server.handle_call/3
            (stdlib) gen_server.erl:615: :gen_server.try_handle_call/4
            (stdlib) gen_server.erl:647: :gen_server.handle_msg/5
            (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3

09:03:20.456 [error] GenServer MyStack terminating
** (MatchError) no match of right hand side value: []
    (stack) lib/stack/server.ex:39: Stack.Server.handle_call/3
    (stdlib) gen_server.erl:615: :gen_server.try_handle_call/4
    (stdlib) gen_server.erl:647: :gen_server.handle_msg/5
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: :pop
State: {[], #PID<0.126.0>}
    (elixir) lib/gen_server.ex:564: GenServer.call/3
iex> Stack.Server.pop
Stack.Server.terminate - Reason: {{:badmatch, []}, [{Stack.Server, :handle_call, 3, [file: 'lib/stack/server.ex', line: 39]}, {:gen_server, :try_handle_call, 4, [file: 'gen_server.erl', line: 615]}, {:gen_server, :handle_msg, 5, [file: 'gen_server.erl', line: 647]}, {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 247]}]}; State: {[], #PID<0.126.0>}
** (exit) exited in: GenServer.call(MyStack, :pop, 5000)
    ** (EXIT) an exception was raised:
        ** (MatchError) no match of right hand side value: []
            (stack) lib/stack/server.ex:39: Stack.Server.handle_call/3
            (stdlib) gen_server.erl:615: :gen_server.try_handle_call/4
            (stdlib) gen_server.erl:647: :gen_server.handle_msg/5
            (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3

09:03:23.443 [error] GenServer MyStack terminating
** (MatchError) no match of right hand side value: []
    (stack) lib/stack/server.ex:39: Stack.Server.handle_call/3
    (stdlib) gen_server.erl:615: :gen_server.try_handle_call/4
    (stdlib) gen_server.erl:647: :gen_server.handle_msg/5
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: :pop
State: {[], #PID<0.126.0>}
    (elixir) lib/gen_server.ex:564: GenServer.call/3

The stack server was restarted and the stack was still empty, so popping again caused it to fail again. Previously, the stack would have been restarted with its initial state [1, 2, 3].

Now let's try causing an error by pushing 1:

iex> Stack.Server.push 3
:ok
iex> Stack.Server.push 4
:ok
iex> Stack.Server.pop  
4
iex> Stack.Server.push 1
:ok
iex> Stack.Server.terminate - Reason: {%RuntimeError{message: "runtime error"}, [{Stack.Server, :handle_cast, 2, [file: 'lib/stack/server.ex', line: 30]}, {:gen_server, :try_dispatch, 4, [file: 'gen_server.erl', line: 601]}, {:gen_server, :handle_msg, 5, [file: 'gen_server.erl', line: 667]}, {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 247]}]}; State: {[3], #PID<0.126.0>}

09:04:05.751 [error] GenServer MyStack terminating
** (RuntimeError) runtime error
    (stack) lib/stack/server.ex:30: Stack.Server.handle_cast/2
    (stdlib) gen_server.erl:601: :gen_server.try_dispatch/4
    (stdlib) gen_server.erl:667: :gen_server.handle_msg/5
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: {:"$gen_cast", {:push, 1}}
State: {[3], #PID<0.126.0>}
iex> Stack.Server.pop   
3

Even after the stack terminates and restarts, the next number we pop is 3, which is the last successful push (That wasn't popped) before the server died.

Lastly, let's try stopping the server to see it the supervisor restarts it:

iex> Stack.Server.push 2
:ok
iex> Stack.Server.terminate - Reason: :normal; State: {[1, 2, 3], #PID<0.113.0>}

nil
iex> Stack.Server.pop
1

So, when the server is stopped with the reason :normal, the supervisor still restarts it!