Revision and Interview Questions

Cheat sheet

Platform: Elixir on the BEAM (Erlang VM); lightweight isolated processes, message passing, preemptive scheduling, distribution, hot code upgrades; Erlang interop. Tools: Mix (mix new --sup, deps.get, test, format), Hex, HexDocs, IEx (h, i, iex -S mix), Livebook. Types: integers, floats (/ returns a float, div/rem), atoms, UTF-8 binary strings, charlists, lists, tuples, maps (%{m | k: v}), keyword lists; immutability and rebinding. Matching: = match operator, destructuring, pin ^, _, MatchError. Functions: def/defp, name and arity, multi-clause with guards, defaults \\, fn and & capture, module attributes, @doc, @spec. Pipelines: |>, Enum (eager), Stream (lazy), for comprehensions. Control: case, cond, if, with/else. Data: structs, @enforce_keys, protocols vs behaviours. Errors: {:ok, _}/{:error, _}, bang functions, raise/rescue, let it crash. OTP: spawn, send, receive, links, monitors, GenServer (call, cast, handle_info), Supervisors (one_for_one, one_for_all, rest_for_one), DynamicSupervisor, Registry, Task and async_stream, ETS, Agent. Macros: quote, unquote, use and __using__. Phoenix: endpoint, router, pipelines, plugs, controllers, HEEx, ~p, contexts, action_fallback, phx.gen.auth; Ecto: Repo, schemas, changesets, migrations, queries with ^, preload, Multi; LiveView, PubSub, Channels, Presence; ExUnit; releases and runtime.exs.

Common interview questions

Answer each with a short code example.

1. What makes BEAM processes different from OS threads?
2. Explain the match operator and the pin operator.
3. Why does Elixir use {:ok, value} and {:error, reason} tuples?
4. What is the difference between GenServer.call and GenServer.cast?
5. Explain supervision strategies and "let it crash".
6. Protocols vs behaviours: when do you use each?
7. Enum vs Stream: when does laziness matter?
8. How does with simplify error handling?
9. What is ETS and when would you use it instead of a GenServer?
10. How does Phoenix LiveView work under the hood?
11. What is an Ecto changeset, and why is it not part of the schema struct?
12. Why should you avoid String.to_atom on user input?

Talk about failure

Elixir interviews often focus on failure: what happens when a process crashes, how supervisors restart it, what state is lost and how the design keeps the system available. Explaining that clearly shows real OTP understanding.

Quick check: Which construct restarts crashed processes in an Elixir application?

• A protocol
• A changeset
• A plug
• A supervisor