Extending OTP with custom behaviours

Transcript

1. EXTENDING OTP WITH CUSTOM
   BEHAVIOURS
   Looking beyond a gen_server
   

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

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

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7. THE EXPERIMENT
   

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9. WHO USED ELIXIR?
   

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10. WHO USED SPAWN & RECEIVE FOR PMAP?
    

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11. WHO WROTE A GENSERVER
    IMPLEMENTATION?
    

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12. WHO WROTE A BEHAVIOUR?
    

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13. BEHAVIOUR
    

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

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16. 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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17. TWO USE CASES FOR BEHAVIOURS
    message driven data driven
    behaviour spectrum
    

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18. GENSERVER IS EMERGENT
    

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

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

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

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

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

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

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31. 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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33. 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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35. GEN_SERVER TO THE RESCUE!
    

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

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38. 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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39. 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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40. 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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42. 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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43. COWBOY_MIDDLEWARE AND PLUG
    

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

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

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

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48. 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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49. THERE ARE TWO WAYS TO DO BEHAVIOURS
    

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51. MAC ROS
    

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52. 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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53. 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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54. 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
    

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55. BEHAVIOURS ARE POWERFUL ABSTRACTIONS
    

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56. THEY ARE GREAT OPPORTUNITIES FOR
    LIBRARIES
    

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57. DON’T USE MACROS TO SPECIALISE
    GENSERVER
    

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58. EXTENDING OTP WITH CUSTOM
    BEHAVIOURS
    Looking beyond a gen_server
    

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