Your System Isn't Fault-Tolerant. It's Just Waiting to Fail.
For decades, building reliable, concurrent systems has been one of the hardest problems in software engineering. Most languages approach this with complex, error-prone primitives like threads, locks, and mutexes. The Erlang virtual machine (BEAM), and the Elixir language built on top of it, offer a radically different and more powerful model. By embracing the Actor model, with lightweight, isolated processes that communicate via messages, Elixir allows developers to build systems that are concurrent by default and resilient by design. The philosophy is simple: "let it crash." Instead of trying to prevent every possible failure, you build supervisors that automatically restart failed processes, creating self healing systems.
However, this paradigm is completely alien to developers steeped in traditional object-oriented or procedural languages. An engineer who approaches Elixir with a Ruby on Rails or Python mindset will not build a fault-tolerant system. They will fight the functional nature of the language, struggle with the concept of immutable state, and fail to grasp the power of the Open Telecom Platform (OTP) behaviors like GenServer and Supervisor. They will write Elixir code, but they will not build an Elixir system.
This playbook explains how Axiom Cortex finds the developers who have made the mental leap to "thinking in processes," enabling them to build the kind of soft real time systems that power a huge portion of the world's telecommunications and messaging infrastructure.
Traditional Vetting and Vendor Limitations
A nearshore vendor sees "Elixir" on a résumé and, if they recognize it at all, treat it as just another backend language. Their interview will consist of a generic coding problem that does nothing to test the candidate's understanding of OTP, concurrency, or fault tolerance. This superficial vetting completely misses the point of why a company chooses Elixir.
The predictable and painful results are common in teams new to Elixir:
- Ignoring OTP: The team writes all their logic in simple modules and functions, completely ignoring OTP behaviors. They have no supervisors, no GenServers, and thus no fault tolerance. They have built a Node.js application in Elixir syntax.
- The Monolithic GenServer: All the state and logic for a major piece of functionality is crammed into a single, massive GenServer, which becomes a bottleneck and a single point of failure, defeating the purpose of a lightweight process architecture.
- Blocking the Actor: A developer performs a long running, synchronous operation inside a GenServer's `handle_call`, blocking the process and preventing it from responding to any other messages.
- Mutable-Mindset Bugs: The developer struggles with immutability, leading to convoluted code that is hard to reason about and performs poorly due to excessive data copying.
How Axiom Cortex Evaluates Elixir Developers
Axiom Cortex is designed to find engineers who have internalized the Actor model and the "let it crash" philosophy. We test for the practical skills and system design architectural discipline that are essential for building production grade Elixir applications. We evaluate candidates across four critical dimensions.
Dimension 1: OTP Mastery (The Heart of Elixir)
A developer who does not have a deep, intuitive understanding of OTP is not an Elixir developer. This dimension tests a candidate's ability to use OTP's core behaviors to build resilient systems.
We present a stateful, concurrent problem and evaluate their ability to:
- Design a Supervision Tree: Can they design a hierarchy of supervisors with appropriate strategies (`one_for_one`, `one_for_all`, etc.) to manage the lifecycle of their worker processes?
- Implement a GenServer: Can they correctly implement a GenServer to manage state, handling calls, casts, and info messages?
- Choose the Right Behavior: Can they explain when to use a GenServer vs. an Agent vs. a Task?
Dimension 2: Concurrency and the BEAM
This dimension tests a candidate's understanding of what makes Elixir's concurrency model so powerful.
We evaluate if they can:
- Reason About Processes: Can they explain how Elixir's lightweight processes are different from OS threads and why this allows the system to handle millions of concurrent connections?
- Avoid Common Pitfalls: Do they understand the dangers of unbounded message queues and how to design systems with backpressure?
Dimension 3: Functional Programming and Immutability
This dimension tests a candidate's fluency in the functional programming paradigm that underpins Elixir.
We evaluate their ability to:
- Use Pattern Matching: A high scoring candidate will use pattern matching extensively in function heads and `case` statements to write clean, declarative code.
- Embrace Immutability: Can they write code that transforms data using functions, rather than mutating it in place?
Dimension 4: The Broader Ecosystem (Phoenix, Ecto)
An elite Elixir developer is proficient with the key frameworks in the ecosystem.
Axiom Cortex assesses a candidate's familiarity with:
- Phoenix: Can they build a real time web application using Phoenix Channels and LiveView?
- Ecto: Are they proficient in using Ecto to interact with a database? Can they write Ecto schemas and Changesets to validate and persist data?
From Fragile Scripts to Fault-Tolerant Systems
When you staff your team with engineers who have passed the Elixir Axiom Cortex assessment, you are investing in a team that can build systems with a level of concurrency and fault tolerance that is nearly impossible to achieve in other languages.
A communications platform company was struggling to build a chat service that could handle hundreds of thousands of concurrent users. Their initial prototype, built on Node.js, was struggling with state management and connection handling. By augmenting their team with two elite nearshore Elixir engineers, they were able to rebuild the core service on the BEAM. The new system, built with OTP and Phoenix Channels, handled over two million concurrent connections on a handful of servers, a feat that would have required a massive and complex infrastructure in their previous stack.