Hiring guide for Elixir OTP Engineers

Elixir OTP Developer Hiring Guide

Elixir is a dynamic, functional language designed for building scalable and maintainable applications. It leverages the Erlang VM, known for running low-latency, distributed and fault-tolerant systems. Elixir was created by José Valim in 2011 to enable higher extensibility and productivity in the Erlang VM while maintaining compatibility with its ecosystem (OTP). OTP or Open Telecom Platform is a collection of middleware modules originally designed to develop telecom applications but has evolved into a general-purpose development tool. Its robust concurrency model makes it particularly useful for handling many simultaneous connections, making it ideal for web servers or databases.

Ask the right questions secure the right Elixir OTP talent among an increasingly shrinking pool of talent.

First 20 minutes

General Elixir OTP app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in Elixir OTP application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

How would you define Elixir OTP?
Elixir OTP is a set of tools and libraries used for building scalable, fault-tolerant and distributed systems. It provides abstractions for creating concurrent and distributed applications, such as servers, state machines, and supervisors.
What are the key components of Elixir OTP?
The key components of Elixir OTP are GenServer, Supervisor, Application, Agent, and Task. These components provide abstractions for creating concurrent and distributed applications.
Describe the difference between GenServer and Agent in Elixir OTP.
GenServer is a behaviour module for implementing the server of a client-server relation. An Agent is a process that holds state. While both can hold state, GenServer is more flexible and can handle a variety of generic server behaviours, whereas Agent is specifically designed for maintaining state.
How would you handle errors in Elixir OTP?
In Elixir OTP, errors are handled by using the 'let it crash' philosophy. Supervisors are used to monitor processes and handle errors. If a process crashes, the supervisor can decide whether to restart it, stop the entire application, or do something else.
What is the role of a Supervisor in Elixir OTP?
A Supervisor is a process which supervises other processes, called child processes. Its role is to start, stop, and monitor its child processes. It can also restart child processes when they terminate abnormally.
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What you’re looking for early on

Does the candidate have a solid understanding of Elixir and OTP?
Has the candidate demonstrated problem-solving skills?
Is the candidate familiar with concurrent and distributed programming?
Can the candidate effectively communicate their thoughts and ideas?

Next 20 minutes

Specific Elixir OTP development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

How would you implement a GenServer in Elixir OTP?
To implement a GenServer, you would define a module that uses the GenServer behaviour and implement the required callback functions. You would then start the GenServer using the GenServer.start_link function.
What are the benefits of using Elixir OTP?
Elixir OTP provides benefits such as fault-tolerance, scalability, and concurrency. It allows developers to build distributed and concurrent applications with relative ease. It also provides a set of behaviours and conventions that make it easier to design and structure your code.
Describe the difference between synchronous and asynchronous calls in Elixir OTP.
Synchronous calls in Elixir OTP block the caller until the server has finished processing the request, while asynchronous calls send a message to the server and return immediately. GenServer.call is used for synchronous calls and GenServer.cast is used for asynchronous calls.
How would you use Tasks in Elixir OTP?
Tasks in Elixir OTP are used for better concurrency. You can use Task.start to start a new task, Task.async to start a task that can be awaited on later, and Task.await to wait for a task to finish and retrieve its result.
What is the role of an Application in Elixir OTP?
An Application in Elixir OTP is a component that is packaged and configured. It can be started and stopped as a unit, and it can be re-used in other systems. It also has its own configuration and can depend on other applications.
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The ideal back-end app developer

What you’re looking to see on the Elixir OTP engineer at this point.

At this point, a skilled Elixir OTP engineer should demonstrate strong problem-solving abilities, proficiency in Elixir OTP programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with Elixir OTP.

What does this simple Elixir function do?
defmodule Greet do
  def hello(name) do
    IO.puts "Hello, #{name}"
This code defines a module named 'Greet' with a function 'hello'. The function takes one argument 'name' and prints a greeting message to the console using the 'IO.puts' function.
What will be the output of this Elixir code?
list = [1, 2, 3, 4]
IO.inspect Enum.map(list, fn x -> x * 2 end)
This code will output a list [2, 4, 6, 8]. It uses the 'Enum.map' function to iterate over each element in the list and multiplies it by 2.
What does this Elixir code do?
defmodule Collection do
  def sum_list(list) do
    Enum.reduce(list, 0, &(&1 + &2))
This code defines a module 'Collection' with a function 'sum_list'. The function takes a list as an argument and uses the 'Enum.reduce' function to sum all elements in the list.
What does this Elixir code do?
defmodule Concurrency do
  def start do
    spawn_link(fn -> loop end)

  defp loop do
    receive do
      msg -> IO.puts msg
This code defines a module 'Concurrency' with a 'start' function. The function starts a new process with the 'spawn_link' function, which executes a private function 'loop'. This function waits for a message, prints it to the console, and then calls itself again, creating an infinite loop.

Wrap-up questions

Final candidate for Elixir OTP Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle Elixir OTP application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

How would you handle state in a GenServer?
State in a GenServer is handled by passing the state as an argument to the handle_call, handle_cast, or handle_info functions and returning it as part of the tuple. The new state will be passed in to the next function call.
What are the different strategies a Supervisor can use to manage child processes?
A Supervisor can use one of three strategies to manage child processes: :one_for_one, where if a child process terminates, only that process is restarted; :rest_for_one, where if a child process terminates, that process and the ones started after it are restarted; and :one_for_all, where if a child process terminates, all other child processes are restarted.
How would you implement a Supervisor in Elixir OTP?
To implement a Supervisor, you would define a module that uses the Supervisor behaviour and implement the init function to define the child processes and supervision strategy. You would then start the Supervisor using the Supervisor.start_link function.

Elixir OTP application related

Product Perfect's Elixir OTP development capabilities

Beyond hiring for your Elixir OTP engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in Elixir OTP projects, and can engage in multiple capacities.