Extending OTP with custom behaviours

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EXTENDING OTP WITH CUSTOM BEHAVIOURS Looking beyond a gen_server

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MICHAŁ MUSKAŁA http://michal.muskala.eu/ https://github.com/michalmuskala/ @michalmuskala

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THE MASTERPLAN • The GenServer • OTP special processes • OTP behaviours • Custom behaviours

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THE EXPERIMENT

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WHO USED ELIXIR?

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WHO USED SPAWN & RECEIVE FOR PMAP?

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WHO WROTE A GENSERVER IMPLEMENTATION?

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WHO WROTE A BEHAVIOUR?

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BEHAVIOUR

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OTP BEHAVIOURS • application • gen_server • gen_fsm • gen_statem • gen_event • supervisor

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http://blog.plataformatec.com.br/2015/10/mocks-and-explicit-contracts/ “Furthermore, we have already deﬁned three implementations of the Twitter API, so we better make it all explicit. In Elixir we do so by deﬁning a behaviour with callback functions (…)”

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TWO USE CASES FOR BEHAVIOURS message driven data driven behaviour spectrum

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GENSERVER IS EMERGENT

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defmodule Calculator do def start do spawn(__MODULE__, :loop, [[]]) end def add(pid, x, y) do send(pid, {:add, self(), x, y}) receive do {:result, x} -> x end end def loop(state) do receive do {:add, from, x, y} -> send(from, {:result, x + y}) end loop(state) end end

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MESSAGE HANDLER PROCESS • waits for a message • parses the message • handles the message • sends a reply

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MESSAGE HANDLER PROCESS • waits for a message • parses the message • handles the message • sends a reply

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defmodule Calculator do use GenServer def start do GenServer.start_link(__MODULE__, []) end def add(pid, x, y) do GenServer.call(pid, {:add, x, y}) end def handle_call({:add, x, y}, from, state) do {:reply, x + y, state} end end

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GEN_SERVER • init/1 • handle_cast/2 • handle_call/3 • handle_info/2 • terminate/2 • code_change/3

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GEN_SERVER • init/1 • handle_cast/2 • handle_call/3 • handle_info/2 • terminate/2 • code_change/3 • format_status/2

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FORMAT_STATUS • :sys.get_status/2 • abnormal termination and logging

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GenServer Callback module start_link call multi_call cast abcast init handle_cast handle_call handle_info terminate

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HOW IS GEN_SERVER IMPLEMENTED? HOW CAN WE DO SOMETHING SIMILAR?

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OTP SPECIAL PROCESSES • process ﬁts into a supervision tree • support for the :sys debug utilities • process system messages • OTP design principles  http://erlang.org/doc/design_principles/des_princ.html

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SPECIAL PROCESS IMPLEMENTATION • start with :proc_lib.start_link/3 • initialise debug with :sys.debug_options/1 • respond to start_link with :proc_lib.init_ack/2 • use :sys.handle_system_msg/6 for {:system, from, request} • implement system_continue/3 and system_terminate/4 callbacks

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GEN_SERVER TO THE RESCUE!

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GEN_SERVER FOR EVERYTHING? • pooling • database connection • web server • chat room • data streams

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CONNECTION • init/1, handle_call/3, handle_cast/2, handle_info/2, teminate/2, code_change/3 • connect/2 • disconnect/2 • :backoff, :connect, :disconnect,

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DBCONNECTION • connect/1, disconnect/2, ping/1 • out of process state: checkout/1, checkin/1 • transaction: handle_begin/2, handle_commit/2, handle_rollback/2 • queries: handle_prepare/3, handle_execute/4, handle_close/3 • coursors: handle_declare/4, handle_ﬁrst/4, handle_next/4, handle_deallocate/4

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ECTO.ADAPTER • ensure_all_started/2, child_spec/3 • loaders/2, dumpers/2, autogenerate/1 • prepare/2, execute/6, insert_all/7, insert/6, update/6, delete/4 • transaction/3, in_transaction?/1, rollback/2 • extensions for storage, migrations, dumps

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RANCH_TRANSPORT • acceptor: listen/1, accept/2, accept_ack/2 • connect/3, connect/4 • communication: recv/3, send/2, sendfile/2, sendfile/4, sendfile/5 • config: setopts/2, controlling_process/2, • read settings: name/0, secure/0, messages/0, peername/1, sockname/1 • shutdown/2, close/1

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COWBOY_MIDDLEWARE AND PLUG

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COWBOY_MIDDLEWARE AND PLUG • cowboy: execute/2 • plug: call/2

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PHOENIX.CHANNEL • join/2 • handle_in/3 • handle_out/3 (*) • handle_info/2 • terminate/2 • code_change/3

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HACKNEY_POOL_HANDLER • start/0 • checkout/4 • checkin/2

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GENSTAGE • init/1 • handle_demand/2 • handle_subscribe/4, handle_cancel/3 • handle_events/3 • handle_call/3, handle_cast/2, handle_info/2, teminate/2, code_change/3

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THERE ARE TWO WAYS TO DO BEHAVIOURS

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MAC ROS

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defmacro __using__(_) do quote do use GenServer def handle_call({:foo, args}, _, state) do {reply, state} = some_callback(args, state) Some.more_logic(reply, state) {:reply, reply, state} end defp some_callback(args, state), do: {:ok, state} defoverridable [some_callback: 2] end end

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use GenServer @callback init(term) :: {:ok, state :: term} | ... @callback some_callback(term) :: {reply :: term, state :: term} def start_link(module, args, opts) do GenServer.start_link(__MODULE__, {module, args, opts}, opts) end def init({mod, args, opts}) do case mod.init(args) do {:ok, int} -> %{mod: mod, int: int} other -> other end end

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def handle_call({:foo, arg}, _from, %{mod: mod, int: int} = state) do {reply, int} = mod.some_callback(arg, int) {:reply, reply, %{state | internal: int}} end def format_status(:normal, [pdict, %{mod: mod, int: int}]) do [{:data, [{'State', int}]}] end def format_status(:terminate, [pdict, %{int: int}]) do int end