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Troubleshooting OTP Errors

We're in the weeds now! Sorry for the pain, but hey, backing over a cliff is what we do as developers isn't it? You *will find yourself completely stuck* when working with Elixir and I need you to be able to reason your way out of this mess.

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Unraveling This Ball of Suck

What we’ve just run into is an OTP process not being able to talk to another OTP process. Let this error seer into your eyes, because this is not the last time you’ll see it:

** (EXIT) no process

I believe I have spent days on no process errors, and only now have I begun to understand how to troubleshoot them. Let’s walk through this together.

The first thing to do is to sit back and visualize the application you’ve created. We know that at the top level we have an application called Physics and that it requires other applications - we set this up in our mix.exs file:

applications: [:logger, :timex, :postgrex, :ecto]

When our application starts, it will, in turn, startup Logger, Timex, Postgrex and Ecto. This will start our Repo and off we go. But that’s not what’s happening here.

Looking at the exact error, we see that :erlang is trying to send a message to Physics.Repo.Pool:

 ** (EXIT) no process
    :erlang.send(Physics.Repo.Pool, {:"$gen_cast", {:cancel_waiting, #Reference<>}}, [:noconnect]) # example 1
    (stdlib) gen_server.erl:416: :gen_server.do_send/2
    (stdlib) gen_server.erl:232: :gen_server.do_cast/2
    src/poolboy.erl:58: :poolboy.checkout/3
    lib/db_connection/poolboy.ex:36: DBConnection.Poolboy.checkout/2
    lib/db_connection.ex:811: DBConnection.checkout/2
    lib/db_connection.ex:421: DBConnection.query/4

Reading this stack trace from bottom to top, we can reason that:

  • DBConnection is trying to execute a query. We’ve made it this far, which is good!
  • Upon trying to checkout something, a library called Poolboy is trying use a GenServer without success.
  • The error is not coming from Ecto or Postgrex, the driver. This is critical - it’s coming from Erlang which stops everything saying “I don’t have the process you want”.

The thing we need to figure out is this: what process is Erlang trying to find?


The Code

Code for this video (and for the entire series) can be found in the production GitHub repo.

  • Welcome to Red:4

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    Elixir syntax can be familiar and weird at the same time, especially if you're coming from Ruby. To prepare for this section, let's clear our minds - let go of familiar syntax cues and 'the way it's done' in the languages you already know.

  • Functional Programming Primer

    Functional programming has been around forever; it's nothing new. Elixir is a functional language, as is its progenitor, Erlang. If you're coming from an Object-oriented language like Java, Ruby, C#, Python or JavaScript, functional programming will take some getting used to.

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  • Calculating Escape Velocity

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  • Solar Flare Project Setup

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  • List and Enumeration Basics

    Functional programming is all about transforming data and if we're going to work with Solar Flares, we need to understand how Lists work - how we query, slice, dice, add and remove. That's what we're going to do now.

  • Recursion and Refactoring Our Solar Flares

    Most developers know about recursion and have used it once or twice in their careers; perhaps more. It can be quite difficult to get right - this is not the case with Elixir. With a combination of pattern matching and the head/tail functionality of Lists, recursively iterating over lists is straightforward.

  • if, unless, and cond

    Here's something weird to think about: in a high-level functional language such as Elixir, writing conditional code is not exactly idiomatic. Using a combination of pattern matching and syntax rules we can write extremely clean code free of if statements and conditionals in general

  • A Quick Look at OTP

    You've seen a lot of Elixir so far and have learned some new concepts, especially if you're new to functional programming. No doubt that along the way you've probably been wondering, however: *how is this all supposed to work together? how can I actually build something?*. I've showed you a few little hints, but it's time to get explicit on this

  • Working With Ecto and PostgreSQL

    You have to store data somewhere and Elixir (and Erlang in general) give you quite a few tools to do so. In this video we'll see how to push data into PostgreSQL using Ecto, the ORM created by the Elixir team.

  • Troubleshooting OTP Errors

    We're in the weeds now! Sorry for the pain, but hey, backing over a cliff is what we do as developers isn't it? You *will find yourself completely stuck* when working with Elixir and I need you to be able to reason your way out of this mess.

  • Bombs Away!

    Developers often confuse *asynchronous* execution with *concurrent* or parallel execution. I do this often! It's something I learn and unlearn, all the time. Let's take a small tangent and discuss the difference.

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