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Debugging techniques in Elixir

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debug |dēˈbəɡ| the process of identifying and removing errors from computer hardware or software.

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https://twitter.com/cerealvelocity/status/575826348414885888

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"Elixir leverages the Erlang VM" Erlang |> Elixir

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Erlang/OTP has applications to debug and maintain your application

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What I learned in my journey… • Tools used to debug applications. • When I can use each tool. • How I can analyze the system information. • How I connect to different nodes.

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@erichkist

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Sao Paulo - Brazil https://flic.kr/p/fyybDP

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Organize the tour to catch pokémons and back all of us with life

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consulting and software engineering Elixir coaching Elixir design review Custom development

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http://elixir-lang.org/getting-started/introduction.html

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http://elixir-lang.org/getting-started/introduction.html

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IO.puts/2

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iex(1)> IO.puts 1 1 :ok

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iex(1)> IO.puts {:a, :b} ** (Protocol.UndefinedError) protocol String.Chars not implemented for {:a, :b} (elixir) lib/string/chars.ex:3: String.Chars.impl_for!/1 (elixir) lib/string/chars.ex:17: String.Chars.to_string/ 1 (elixir) lib/io.ex:500: IO.puts/2

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IO.inspect/2

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iex(1)> IO.inspect {:a, :b} {:a, :b} {:a, :b} Win! \o/

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IO.puts vs IO.inspect

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IO.puts: Chardata strings or lists of characters and strings

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IO.inspect: Inspect protocol transform any data structure into a readable textual representation

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http://elixir-lang.org/getting-started/protocols.html

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“From now on I'll only use inspect"

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|> Pipeability

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1..10 |> Enum.take_random(3) |> Enum.map(&(&1 * 2))

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list = Enum.take_random(1..10, 3) IO.inspect list list |> Enum.map(&(&1 * 2))

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1..10 |> Enum.take_random(3) |> IO.inspect |> Enum.map(&(&1 * 2)) Awesome!!!

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1..10 |> Enum.take_random(3) |> IO.inspect |> Enum.map(&(&1 * 2)) |> IO.inspect |> … |> IO.inspect |> … Awesome!!!

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Inspect.Opts

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1..10 |> Enum.take_random(3) |> IO.inspect(label: "list") |> Enum.map(&(&1 * 2)) list: [10, 2, 3] [20, 4, 6] Label

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1..10 |> Enum.take_random(3) |> IO.inspect(label: “list") |> Enum.map(&(&1 * 2)) |> IO.inspect(label: “after multiply") |> … |> IO.inspect(label: “after foo") |> … Label

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https://github.com/elixir-lang/elixir/commit/d6513a7 Label whatyouhide michalmuskala Thank you guys <3 Released?

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IO.inspect conn Limit

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%Plug.Conn{adapter: {Plug.Adapters.Cowboy.Conn, :...}, assigns: %{}, before_send: [#Function<0.7834419/1 in Plug.CSRFProtection.call/2>, #Function<4.100417920/1 in Phoenix.Controller.fetch_flash/2>, #Function<0.82590416/1 in Plug.Session.before_send/2>, #Function<1.75806487/1 in Plug.Logger.call/2>, #Function<0.96656268/1 in Phoenix.LiveReloader.before_send_inject_reloader/1>], body_params: %{}, cookies: %{}, halted: false, host: "localhost", method: "GET", owner: #PID<0.381.0>, params: %{}, path_info: [], peer: {{127, 0, 0, 1}, 63732}, port: 4000, private: %{Chat.Router => {[], %{}}, :phoenix_action => :index, :phoenix_controller => Chat.PageController, :phoenix_endpoint => Chat.Endpoint, :phoenix_flash => %{}, :phoenix_format => "html", :phoenix_layout => {Chat.LayoutView, :app}, :phoenix_pipelines => [:browser], :phoenix_route => #Function<1.12450572/1 in Chat.Router.match_route/4>, :phoenix_router => Chat.Router, :phoenix_view => Chat.PageView, :plug_session => %{}, :plug_session_fetch => :done}, query_params: %{}, query_string: "", remote_ip: {127, 0, 0, 1}, req_cookies: %{}, req_headers: [{"host", "localhost:4000"}, {"connection", "keep-alive"}, {"cache-control", "max-age=0"}, {"upgrade-insecure-requests", "1"}, {"user-agent", "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36"}, {"accept", "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8"}, {"accept-encoding", "gzip, deflate, sdch"}, Limit

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IO.inspect conn, limit: 5 %Plug.Conn{adapter: {Plug.Adapters.Cowboy.Conn, :...}, assigns: %{}, before_send: [#Function<0.7834419/1 in Plug.CSRFProtection.call/2>, #Function<4.100417920/1 in Phoenix.Controller.fetch_flash/ 2>, ...], body_params: %{}, cookies: %{}, ...} Limit

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http://elixir-lang.org/docs/stable/elixir/Inspect.Opts.html

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“From now on I’m SURE I'll only use inspect"

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Code debuging

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IEx.pry/0 Inspect the state of a particular process

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defmodule Example do def double(list) do list |> Enum.map(&(&1 * 2)) end end

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require IEx + IEx.pry

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require IEx defmodule Example do def double(list) do IEx.pry list |> Enum.map(&(&1 * 2)) end end

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Things to keep in mind: 1) lexical scope (its bindings and process info) 2) after allowed the pry, shell will be reset 3) a new shell will be started after respawn

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IEx.pry with Phoenix

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chrismccord/phoenix_chat_example

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defmodule Chat.RoomChannel do ... require IEx def handle_in("new:msg", msg, socket) do IEx.pry broadcast! socket, "new:msg", %{user: msg["user"], body: msg["body"]} {:reply, {:ok, %{msg: msg["body"]}}, assign(socket, :user, msg["user"])} end end web/channels/room_channel.ex

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fail $ mix phoenix.server Cannot pry #PID<0.336.0> at web/channels/ room_channel.ex:44. Is an IEx shell running?

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$ iex -S mix phoenix.server success

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IEx.pry with tests

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$ mix test Cannot pry #PID<0.244.0> at test/ chat_test.exs:7. Is an IEx shell running? fail

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$ iex -S mix test success

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It blocks the process Don’t use it in production

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IEx.pry isn’t a tradicional debugger

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:debugger Erlang Debugger for debugging and testing

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iex(1)> :debugger.start() {:ok, #PID<0.87.0>}

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You can set almost anything in the graphical tool

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iex(2)> :int.ni(Chat.RoomChannel) {:module, Chat.RoomChannel}

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It blocks the process Don’t use in production environment

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Unlike some debuggers, you can’t change all the values of the bound variables…

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pid, reference, binary, or port values cannot be changed

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development & production mode

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Code & Behavior debugging

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https://flic.kr/p/8k9JhZ

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Erlang VM

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Multicore, Schedulers and Reductions

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Symmetric Multi-Processing SMP takes full advantage of multiple CPU architectures

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For every core, the BEAM virtual machine starts a thread that runs a scheduler

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https://hamidreza-s.github.io/erlang/scheduling/real-time/preemptive/migration/2016/02/09/erlang-scheduler- details.html

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Preemptive and reduction count

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Scheduler working Queue Scheduler Done

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Process done Queue Scheduler Done

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Scheduler runs new process Queue Scheduler Done

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Scheduler will run the process Process reduction count: 0 Scheduler reduction limit: 500 Scheduler fake values

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process is running Process reduction count: 123 Scheduler reduction limit: 500 Scheduler fake values

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process is running Process reduction count: 123 Scheduler reduction limit: 500 Scheduler cheap function fake values

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process is running Process reduction count: 999 Scheduler reduction limit: 500 Scheduler expensive function fake values

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Process is preempted Queue Scheduler Done

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Scheduler working Queue Scheduler Done

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Scheduler working Queue Scheduler Done

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:observer Tracing and investigation of distributed systems

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iex(1)> :observer.start() :ok

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Common errors

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CPU bottleneck

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CPU is very hard to profile

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The VM does a lot of work unrelated to processes when it comes to scheduling

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To avoid going to sleep when work is low, the threads that control the Erlang schedulers will do busy looping

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High reduction count usually means a high amount of CPU usage

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Memory leak

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Memory types

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3 mistakes to avoid

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ETS tables are never garbage collected Only removing records manually will reclaim memory

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Don’t use dynamic atoms Atoms go in a global table and are cached forever!

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Message queue the most common cause of failure

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http://learnyousomeerlang.com/the-hitchhikers-guide-to-concurrency

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Identifying errors

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Crash Dump Travel machine!

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erl_crash.dump

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:observer > File > Examine Crashdump

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Tracing

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:et, :dbg, :ttb (:observer) Some tracing modules in the Erlang

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andytill/erlyberly

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Connect to a remote node

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It isn’t hard! but… you may find troubles.

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module :observer can be not available Releases are self contained packages without unnecessary applications

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iex(node@remote)1> :observer.start ** (UndefinedFunctionError) undefined function :observer.start/0 (module :observer is not available) :observer.start()

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:runtime_tools low footprint tracing/debugging tools in a production system

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def application do [mod: {Chat, []}, applications: [:runtime_tools, …]] end mix.exs

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server without graphical interface

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Name your remote node

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## Name of the node -sname my_app ## Cookie for distributed erlang -setcookie my_cookie running-config/vm.args

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Erlang Port Mapper Daemon daemon acts as a name server on all hosts involved in distributed Erlang computations

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EPMD Port: 4369

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$ iex --sname my_app@localhost --cookie my_cookie -S mix phoenix.server

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EPMD Port: 4369 Port: 64722 my_app

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$ epmd -names epmd: up and running on port 4369 with data: name my_app at port 64722

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Run :observer in a standalone node to minimize the impact of the system being observed

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$ iex --sname observer@localhost --cookie my_cookie --hidden -e ":observer.start"

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EPMD Port: 4369 my_app Port: 64722 :observer Port: 64749

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$ epmd -names epmd: up and running on port 4369 with data: name observer at port 64749 name my_app at port 64722

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SSH Port forwarding local EPMD port -> remote EPMD port

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http://blog.plataformatec.com.br/2016/05/ tracing-and-observing-your-remote-node/

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SASL System Architecture Support Libraries

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Redirects supervisor, crash and progress reports to Logger

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09:27:51.872 [info] Child Logger.ErrorHandler of Supervisor Logger.Supervisor started Pid: #PID<0.78.0> Start Call: Logger.Watcher.watcher(:error_logger, Logger.ErrorHandler, {true, true, 500}, :link) Restart: :permanent Shutdown: 5000 Type: :worker 09:27:51.884 [info] Application logger started at :nonode@nohost 09:27:52.174 [info] Child Mix.State of Supervisor Mix.Supervisor started Pid: #PID<0.86.0> Start Call: Mix.State.start_link() Restart: :permanent Shutdown: 5000 Type: :worker

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$ iex --logger-sasl-reports true -S mix

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def application do [mod: {Chat, []}, applications: [:sasl, …]] end mix.exs

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Recap!

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Some tools we can use only for development..

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System information is different…

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crash dump is your friend

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You can run observer (and others tools) locally connected in your remote nodes!

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Next steps

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http://erlang.org/doc/applications.html

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h4cc/awesome-elixir

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"Measuring your Elixir Application” Talk by Renan Ranelli - Track 2 - 3:30PM

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https://www.youtube.com/watch?v=4u6c2FNauYE

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https://www.youtube.com/watch?v=mrKwM9g6baQ

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https://www.erlang-in-anger.com/

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Thank you! @erichkist https://speakerdeck.com/erichkist