2019-11-08 18:14:29 +00:00
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---
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title: "How to debug a GenServer like Sherlock using IEx"
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subtitle: "A Sherlock Holmes approved guide 🕵"
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date: 2019-11-08
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draft: false
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2024-10-20 16:10:08 +00:00
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author: Wilfried
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2019-11-08 18:14:29 +00:00
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tags: [dev, programming, elixir, production, debug]
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---
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# Elixir, the path to functional programming ⚗️
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With my [**Rust** rediscovery](https://blog.papey.fr/post/03-bites-the-rust/)
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a lot of _functional programming_ concepts became less obscure. In order to
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go deeper into this paradigm I chose _Elixir_ a **powerful** general purpose, _functional
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programming_ language, compiled and executed inside the _Erlang_ virtual
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machine (**BEAM**).
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But why _Elixir_, if _Erlang_ exists ? Because :
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Elixir, is Erlang with underpants - Athoune
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Unlike _Erlang_, the _Elixir_ syntax is clear and elegant. The language comes
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with everything you need, included : interactive shell, deps tooling, building tooling
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and more…
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Even if _José Valim_, the _Elixir_ creator was a core _Ruby_ dev, _Elixir_ is
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completely different since this is a _functional programming_ language. No
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mutation, no inheritance, no classes, only **pure** functions. Pray your only
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true god, the pipe operator `|>` operator, used to compose functions.
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Now, I know, the only thing you want is code and examples, so let's take a
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quick look at some basic _Elixir_ stuff using the interactive shell **IEx**.
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A simple addition :
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{{< highlight exs >}}
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iex(1)> 2 + 2
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4
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{{< /highlight >}}
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A simple addition, inside a list
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{{< highlight exs >}}
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iex(1)> 2 + 2
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iex(5)> list = [1,2,3,4][1, 2, 3, 4]
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iex(6)> Enum.map(list, fn e -> e + 1 end)
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[2, 3, 4, 5]
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{{< /highlight >}}
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List splitting (head and tail)
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{{< highlight exs >}}
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iex(7)> [head | tail] = list
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[1, 2, 3, 4]
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iex(8)> head
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1
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iex(9)> tail
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[2, 3, 4]
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{{< /highlight >}}
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Another important aspect of Elixir is _pattern matching_ used to match
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values, data structures and much more, let's try it :
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{{< highlight exs >}}
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iex(11)> x = {:this, :is, :a, :test}
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{:this, :is, :a, :test}
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iex(12)> {a, b, c, d} = x
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{:this, :is, :a, :test}
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iex(13)> a
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:this
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iex(14)> b
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:is
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iex(15)> c
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:a
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iex(16)> d
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:test
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{{< /highlight >}}
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Using _pattern matching_ you can assign values but also destructure data to
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simplify interaction with it.
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Feels the hype growing ? Nice !
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As usual, I like a real project to experiment on something. In the _Elixir_
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case, I started a Discord bot project called
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[o2m](https://github.com/papey/o2m). The main idea of this bot is to send
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alert messages on a specific channel when a new episode from a selected
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podcast is available. Today, I was struggling with a bug on a production
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instance of _o2m_. The last episode was not fetched correctly and the action
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that write a _"There is a new episode_" message was not triggered correctly.
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To debug and inspect the current state of the application I used IEx **in
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production**.
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# Base ingredient of a good Elixir : Mix 🧙
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Combined with IEx, there is _Mix_. Shipped with _Elixir_, this a build tool
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used for the following application related tasks :
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- creating
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- setting up needed deps
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- testing
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- compiling
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The combo killer here is to start IEx inside the _Mix_ project, in order to have all
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the dependencies imported inside the interactive shell
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{{< highlight exs >}}
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iex -S mix
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{{< /highlight >}}
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After that, there is a lot of useful commands like `recompile` to refresh and
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recompile new code **while your code is running**. Yes, this is something
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that _Elixir_ do by default, hot code reloading.
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When your code is ready, you want to deploy it. With _Mix_ the standard way
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is to used the `release` command
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{{< highlight sh >}}
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MIX_ENV=production mix release
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{{< /highlight >}}
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This will create a precompile and packaged unit, with runtime included. With
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this, you do not have to install _Erlang_ or _Elixir_ on your production
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server. Boom.
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If you look carefully, there is an interesting message and the end of the command output :
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{{< highlight sh >}}
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Release created at \_build/prod/rel/o2m!
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# To start your system
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_build/prod/rel/o2m/bin/o2m start
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Once the release is running:
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# To connect to it remotely
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_build/prod/rel/o2m/bin/o2m remote
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# To stop it gracefully (you may also send SIGINT/SIGTERM)
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_build/prod/rel/o2m/bin/o2m stop
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To list all commands:
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_build/prod/rel/o2m/bin/o2m
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{{< /highlight >}}
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Particularly,
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To connect to it remotely
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This means I can have access to an interactive shell on production while my
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code is running, hooray 🎆
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# Sherlocking a GenServer 🔎
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Did you read the title ? We are talking about _GenServer_ here ! So _what the fuck_ is this ?
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Taken from the [documentation](https://hexdocs.pm/elixir/GenServer.html) :
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A GenServer is a process like any other Elixir process and it can be used
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to keep state, execute code asynchronously and so on. The advantage of using
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a generic server process (GenServer) implemented using this module is that it
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will have a standard set of interface functions and include functionality for
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tracing and error reporting. It will also fit into a supervision tree.
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In _Elixir_, this is the default and common module used to implement client-server behaviors.
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Here is the example from the documentation
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{{< highlight elixir >}}
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defmodule Stack do
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use GenServer
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# Callbacks
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@impl true
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def init(stack) do
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{:ok, stack}
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end
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@impl true
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def handle_call(:pop, \_from, [head | tail]) do
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{:reply, head, tail}
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end
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@impl true
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def handle_cast({:push, element}, state) do
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{:noreply, [element | state]}
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end
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end
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{{< / highlight >}}
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Basically, this is a data structure with a state responding to triggers using
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handlers that update or retrieve the current state
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To launch and interact with the server, in IEx :
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{{< highlight exs >}}
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iex(41)> {:ok, pid} = GenServer.start_link(Stack, [:hello])
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iex(42)> GenServer.call(pid, :pop)
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:hello
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iex(43)> GenServer.cast(pid, {:push, :world})
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:ok
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iex(44)> GenServer.call(pid, :pop)
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:world
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{{< /highlight >}}
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What if, I want to access the current state, of the GenServer ?
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In _Erlang_, there is the `sys` module with the `get_state/2`
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[function](http://erlang.org/doc/man/sys.html#get_state-2) available
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But, this is _Erlang_, and we use _Elixir_ ! We're doomed ? No ! Because
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everything available in _Erlang_ is available inside _Elixir_ using the `:`
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operator.
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{{< highlight erl >}}
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iex(18)> :io.format("Hello World~n")
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Hello World
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:ok
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{{< /highlight >}}
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So, with our `sys` example :
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{{< highlight exs >}}
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iex(19)> :sys.get_state(pid)
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[:!, :world]
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{{< /highlight >}}
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Nice ! Now we can get state of a GenServer using the PID !
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Now, let's move from local machine to prod. In this case, processus are
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handled by a **Supervisor** but first things first, we want a IEx shell
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inside our running app. In order to illustrate this last part, I will be
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using one of my prod instances of _o2m_
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{{< highlight sh >}}
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o2m@16557a733b59:/opt/o2m\$ ./prod/rel/o2m/bin/o2m remote
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Erlang/OTP 22 [erts-10.5.3][source] [64-bit][smp:2:2] [ds:2:2:10][async-threads:1] [hipe]
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Interactive Elixir (1.9.2) - press Ctrl+C to exit (type h() ENTER for help)
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iex(o2m@16557a733b59)1>
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{{< /highlight >}}
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Ok but, what is the _pid_ of the _GenServer_ I want to inspect ? I don't know !
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Here the first solution could be outputting _pid_ to _stdout_ but, imagine that
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our application kills and restart GenServer, on demand or our application is
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flooding _stdout_ because of some errors, this is not a viable solution.
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First, we could list all processus,
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{{< highlight exs >}}
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iex(o2m@16557a733b59)3> Supervisor.which_children(O2M.Supervisor)
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[
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{O2M, #PID<0.2368.0>, :worker, [O2M]},
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{"jobs-https://feed.ausha.co/bj5li17QPONy", #PID<0.2365.0>, :worker, [Jobs]},
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{"jobs-https://feed.ausha.co/yJeEUGlLVq0o", #PID<0.2362.0>, :worker, [Jobs]},
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{"jobs-https://anchor.fm/s/b3b7468/podcast/rss", #PID<0.2358.0>, :worker,
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[Jobs]}
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]
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{{< /highlight >}}
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Where `O2M.Supervisor` is the dedicated _Supervisor_ of my application
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Now I can identify the GenServer I want to inspect, let's take the
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`jobs-https://anchor.fm/s/b3b7468/podcast/rss` one, with associated pid
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`0.2358.0` (the last element of the list, here), here comes the fun
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{{< highlight exs >}}
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iex(o2m@16557a733b59)13> {_, pid, _, \_} = Supervisor.which_children(O2M.Supervisor) |> List.last
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{"jobs-https://anchor.fm/s/b3b7468/podcast/rss", #PID<0.2358.0>, :worker,
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[Jobs]}
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iex(o2m@16557a733b59)14> pid
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#PID<0.2358.0>
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{{< /highlight >}}
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Then, I can use the pid value to get _GenServer_ state and inspect it to see
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if everything is ok :
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{{< highlight exs >}}
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iex(o2m@16557a733b59)15> :sys.get_state(pid)
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{"https://anchor.fm/s/b3b7468/podcast/rss",
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%{
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date: "Mon, 04 Nov 2019 09:00:00 GMT",
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show: "Harry Cover, le podcast des meilleures reprises",
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title: "Yes we can work it out !",
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url: "https://anchor.fm/leotot8/episodes/Yes-we-can-work-it-out-e8n4d3"
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}}
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{{< /highlight >}}
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How ! Impressive ! It was a little bit Sherlock Holmes oriented debugging but
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that was fun. Keep in mind that this is a really simple operation here, from
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IEx everything is possible from starting new GenServers to modify state of a
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specific one and much more.
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I said it at the beginning, **Elixir is powerful**.
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