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The Elixir GenServer is a programming language module integral to the Elixir software. Stemming from Erlang, another high-level programming language, GenServer facilitates concurrent computing and aids in maintaining state over a duration of time. It functions as an abstraction for concurrent processes and provides built-in functionality for inter-process communication (source: Hexdocs.pm). Its robustness makes it popular among developers who need fault-tolerance and real-time processing capabilities in their applications (source: Pluralsight.com). Thus, Elixir's GenServer plays a crucial role in building scalable systems with high availability.

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

First 20 minutes

General Elixir GenServer app knowledge and experience

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

What are the basic components of a GenServer in Elixir?

The basic components of a GenServer in Elixir are the server, which contains the state and implements the functionality, and the client, which sends requests to the server and receives responses.

How would you start a GenServer process?

You can start a GenServer process by using the GenServer.start_link/3 function, which needs a module (where the GenServer callbacks are implemented), initial arguments, and options.

What is the role of the handle_call function in a GenServer?

The handle_call function is used to handle synchronous requests sent to the GenServer. It takes three arguments - the request, the from tuple, and the current state, and returns a tuple indicating the server's response and new state.

How would you handle asynchronous requests in GenServer?

Asynchronous requests are handled using the handle_cast function in GenServer. It takes two arguments - the request and the current state, and returns a tuple with an action atom and the new state.

Describe the difference between handle_call and handle_cast in GenServer.

The difference between handle_call and handle_cast in GenServer is that handle_call handles synchronous requests and sends a response back to the client, while handle_cast handles asynchronous requests and doesn't send any response.

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What you’re looking for early on

Does the candidate have a solid understanding of the Elixir language and its syntax?

Can the candidate explain the GenServer behaviour in Elixir and its usage?

Has the candidate demonstrated an understanding of OTP (Open Telecom Platform) principles?

Is the candidate capable of writing and understanding concurrent and distributed systems?

Next 20 minutes

Specific Elixir GenServer 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.

What are the main use cases of GenServer in Elixir?

GenServer in Elixir is mainly used for maintaining shared state, encapsulating concurrency, handling system messages, and providing a standard structure for building servers.

How would you stop a GenServer process?

To stop a GenServer process, you can use the GenServer.stop/3 function, which takes the server (the process ID), the reason, and options.

What are the different types of requests that a GenServer can handle?

A GenServer can handle two types of requests - synchronous requests using handle_call, and asynchronous requests using handle_cast.

How would you handle system messages in GenServer?

System messages in GenServer are handled using the handle_info function. This function is used to handle all messages that aren't sent via GenServer's API.

Describe the difference between GenServer.start_link and GenServer.start.

The difference between GenServer.start_link and GenServer.start is that start_link creates a linked process, which means that if either the parent or the child process terminates, the other one will too. On the other hand, start creates an unlinked process.

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At this point, a skilled Elixir GenServer engineer should demonstrate strong problem-solving abilities, proficiency in Elixir GenServer 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 GenServer.

What does this simple GenServer code do?
defmodule SimpleServer do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  end

  def init(:ok) do
    {:ok, %{} }
  end
end
This code defines a simple GenServer. The start_link function starts the GenServer with the current module as the callback module and :ok as the initial state. The init function is called when the GenServer starts, and it returns :ok and an empty map as the initial state of the GenServer.
What will be the output of this GenServer call?
defmodule KVStore do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, %{}, name: __MODULE__)
  end

  def init(state) do
    {:ok, state}
  end

  def handle_call({:get, key}, _from, state) do
    {:reply, Map.get(state, key), state}
  end
end

{:ok, pid} = KVStore.start_link()
GenServer.call(pid, {:get, :key})
This code will return nil. The GenServer is started with an empty map as its state. When the :get call is made, it tries to get the value of :key from the state, but since the state is an empty map, it returns nil.
What does this GenServer code do with the list?
defmodule ListServer do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  def init(state) do
    {:ok, state}
  end

  def handle_call(:pop, _from, [head | tail]) do
    {:reply, head, tail}
  end
end
This GenServer maintains a list as its state. The :pop call pops the first element from the list and returns it. The state of the GenServer is then updated to the rest of the list.
What does this GenServer code do with concurrency?
defmodule Counter do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, 0, name: __MODULE__)
  end

  def init(state) do
    {:ok, state}
  end

  def handle_cast(:increment, state) do
    {:noreply, state + 1}
  end
end
This GenServer maintains a counter as its state. The :increment cast message increments the counter. Since handle_cast is used, the increment operation is performed asynchronously, allowing for concurrent increments.

Wrap-up questions

Final candidate for Elixir GenServer 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 GenServer application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

What are the benefits of using GenServer in Elixir?

By using GenServer in Elixir, you can encapsulate state and behavior in a single process, handle system messages, and manage concurrency. It also provides a standard structure for building servers.

How would you handle state in a GenServer?

State in a GenServer is handled by returning it from the callback functions. The returned state from one function will be passed as an argument to the next function call.

What is the role of the init function in a GenServer?

The init function in a GenServer is called when the server is started. It initializes the state of the server and can also define the initial call.

Elixir GenServer application related

Product Perfect's Elixir GenServer development capabilities

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

Elixir GenServer Consultive assessments

Elixir GenServer Application Conversion Assessment Elixir GenServer Application Integration Assessment Elixir GenServer Data & Database Integration Assessment

Custom Elixir GenServer development projects

Custom Elixir GenServer App Conversion Elixir GenServer Custom App Development Process App UI Design for Elixir GenServer Facelifts

Hiring guide for Elixir GenServer Engineers

Elixir GenServer Developer Hiring Guide

What you’re looking for early on

The ideal back-end app developer

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

Elixir GenServer Consultive assessments

Custom Elixir GenServer development projects