Idiomatic Elixir - Speaker Deck

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IDIOMATIC ELIXIR © Erlang Solu,ons Ltd, 2016 1

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Hello! © Erlang Solu,ons Ltd, 2016 2

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Claudio Ortolina Head of Elixir at Erlang Solu!ons Ltd. @cloud8421 claudio.ortolina@erlang-solu!ons.com © Erlang Solu,ons Ltd, 2016 3

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IDIOMATIC ELIXIR © Erlang Solu,ons Ltd, 2016 4

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IDIOMATIC ELIXIR © Erlang Solu,ons Ltd, 2016 5

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FLEXIBLE ELIXIR © Erlang Solu,ons Ltd, 2016 6

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DEFINITION 1 flexible adjec!ve - /ˈflek.sə.bəl/ • Able to change or be changed easily according to the situa!on • Able to bend or to be bent easily without breaking 1 h!p:/ /dic"onary.cambridge.org/dic"onary/english/flexible © Erlang Solu,ons Ltd, 2016 7

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WARNING: CODE AHEAD © Erlang Solu,ons Ltd, 2016 8

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1. HANDLING VOLATILE DATA © Erlang Solu,ons Ltd, 2016 9

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CACHE STORE defmodule CacheStore do @type key :: term @type value :: term @type reason :: term @spec get(key) :: {:ok, value} | {:error, reason} @spec set(key, value) :: :ok @spec start_link() :: {:ok, pid} | {:error, reason} end © Erlang Solu,ons Ltd, 2016 10

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v1: AGENT © Erlang Solu,ons Ltd, 2016 11

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defmodule MyApp do def start(_type, _args) do children = [ worker(CacheStore, []) ] ... end end defmodule CacheStore do def start_link() do initial = fn() -> %{} end Agent.start_link(initial, name: __MODULE__) end end © Erlang Solu,ons Ltd, 2016 12

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defmodule CacheStore do def set(key, val) do Agent.cast(__MODULE__, fn(store) -> Dict.put(store, key, val) end) end def get(key) do Agent.get(__MODULE__, fn(store) -> case Dict.get(store, key) do :error -> {:error, :not_found} value -> {:ok, value} end end) end end © Erlang Solu,ons Ltd, 2016 13

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! SIMPLE © Erlang Solu,ons Ltd, 2016 14

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! STRICTLY CONSISTENT © Erlang Solu,ons Ltd, 2016 15

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! ONE OPERATION AT A TIME © Erlang Solu,ons Ltd, 2016 16

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! IF AGENT CRASHES, DATA IS LOST © Erlang Solu,ons Ltd, 2016 17

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v2: GenServer + ETS © Erlang Solu,ons Ltd, 2016 18

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GenServer • Generic server behaviour • Used to model client-server rela!onship • Server holds data and api to access/mutate it • Used to implement Agent © Erlang Solu,ons Ltd, 2016 19

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ETS • Erlang Term Storage • Built-in • Large, in-memory data storage • Constant !me access © Erlang Solu,ons Ltd, 2016 20

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defmodule CacheStore do use GenServer def start_link do GenServer.start_link(__MODULE__, [], name: __MODULE__) end def init([]) do :ets.new(__MODULE__, [:named_table, :bag, :public, read_concurrency: true]) {:ok, []} end end © Erlang Solu,ons Ltd, 2016 21

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defmodule CacheStore do ... def set(key, value) do GenServer.cast(__MODULE__, {:set, key, value}) end def handle_cast({:set, key, value}, state) do true = :ets.insert(__MODULE__, {key, value}) {:noreply, state} end end © Erlang Solu,ons Ltd, 2016 22

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defmodule CacheStore do ... def get(key) do case :ets.lookup(__MODULE__, key) do [] -> {:error, :not_found} [{^key, result}] -> {:ok, result} end end def handle_cast({:set, key, value}, state) do true = :ets.insert(__MODULE__, {key, value}) {:noreply, state} end end © Erlang Solu,ons Ltd, 2016 23

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! CONCURRENT WRITES AND READS © Erlang Solu,ons Ltd, 2016 24

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! EVENTUALLY CONSISTENT © Erlang Solu,ons Ltd, 2016 25

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! IF SERVER CRASHES, DATA IS GONE © Erlang Solu,ons Ltd, 2016 26

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v3: GenServer + ETS + App © Erlang Solu,ons Ltd, 2016 27

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defmodule CacheStore do def create_cache_table do :ets.new(__MODULE__, [:named_table, :duplicate_bag, :public, read_concurrency: true]) end end © Erlang Solu,ons Ltd, 2016 28

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defmodule MyApp do def start(_type, _args) do # The app process owns the table now CacheStore.create_cache_table children = [ worker(CacheStore, []) ] ... end end © Erlang Solu,ons Ltd, 2016 29

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! ONLY AN APP CRASH CAN AFFECT THE CACHED DATA © Erlang Solu,ons Ltd, 2016 30

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2. LONG RUNNING PROCESSES © Erlang Solu,ons Ltd, 2016 31

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GAME PROCESS • Game session in a process • Data is self-contained • Exposes expensive opera!ons © Erlang Solu,ons Ltd, 2016 32

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v1: GenServer © Erlang Solu,ons Ltd, 2016 33

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defmodule Game do use GenServer defmodule State do defstruct score: 0, players: [], ... end def player_avatars(pid) do GenServer.call(pid, :player_avatars) end def handle_call(:player_avatars, _from, state) do ids = Enum.map(state.players, fn(p) -> p.id end) {:ok, avatars} = slow_fetch_urls(ids) {:reply, avatars, state} end end © Erlang Solu,ons Ltd, 2016 34

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! IT WORKS © Erlang Solu,ons Ltd, 2016 35

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! WHILE FETCHING, THE GAME IS BLOCKED © Erlang Solu,ons Ltd, 2016 36

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! IF FETCH FAILS, THE GAME DIES © Erlang Solu,ons Ltd, 2016 37

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2. GenServer.reply © Erlang Solu,ons Ltd, 2016 38

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defmodule Game do ... def player_avatars(pid) do GenServer.call(pid, :player_avatars) end def handle_call(:player_avatars, from, state) do spawn(fn() -> ids = Enum.map(state.players, fn(p) -> p.id end) {:ok, avatars} = slow_fetch_urls(ids) GenServer.reply(from, avatars) end) {:noreply, state} end end © Erlang Solu,ons Ltd, 2016 39

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! FETCH HAPPENS IN DIFFERENT PROCESS © Erlang Solu,ons Ltd, 2016 40

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! WHEN FETCH FAILS, GAME IS UNAFFECTED © Erlang Solu,ons Ltd, 2016 41

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! SAME GUARANTEES FOR THE CALLER © Erlang Solu,ons Ltd, 2016 42

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! EVENTUAL CONSISTENCY © Erlang Solu,ons Ltd, 2016 43

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3. FUNCTIONAL CHAINS © Erlang Solu,ons Ltd, 2016 44

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DIGEST USER INPUT • Incoming data • Whitelist, validate, store • Each step may fail • Computa!on must return a value © Erlang Solu,ons Ltd, 2016 45

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v1: PURE PIPELINE © Erlang Solu,ons Ltd, 2016 46

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defmodule CreateUser do @allowed_params ~w(email username) def run(params) do params |> whitelist |> validate |> store end def whitelist(params) do Map.take(params, @allowed_params) end end © Erlang Solu,ons Ltd, 2016 47

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defmodule MyApp.UserController do def create(params) do case CreateUser.run(params) do {:ok, _user} -> resp(conn, 201, :created) _error -> resp(conn, 406, :malformed) end end end © Erlang Solu,ons Ltd, 2016 48

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defmodule CreateUser do @allowed_params ~w(email username) def run(params) do try do params |> whitelist |> validate |> store rescue error -> error end end def whitelist(params) do Map.take(params, @allowed_params) end end © Erlang Solu,ons Ltd, 2016 52

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defmodule CreateUser do @allowed_params ~w(email username) def run(params) do with {:ok, whitelisted} <- whitelist(params), {:ok, valid} <- validate(whitelisted) do store(valid) else error -> error end end def whitelist(params) do {:ok, Map.take(params, @allowed_params)} end end © Erlang Solu,ons Ltd, 2016 56

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Claudio Ortolina Head of Elixir at Erlang Solu!ons Ltd. @cloud8421 claudio.ortolina@erlang-solu!ons.com © Erlang Solu,ons Ltd, 2016 67