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What OTP means in an Elixir application
OTP is not a single library or a replacement for Elixir. It is a framework of behaviours and conventions for structuring concurrent systems and managing their lifecycles. A GenServer is a common way to implement a process that handles messages and requests; a supervisor starts and monitors child processes; and an OTP application is a runtime unit that can be started or stopped as a whole.
Mix projects and OTP applications are related, but they are not the same thing. Mix manages development work such as compiling and testing a project. The runtime starts the OTP application and its configured dependencies. An application callback can return a supervisor as its starting process, putting the supervision tree at the top of the application’s process structure. Elixir’s guide to supervisors and applications walks through this arrangement.
How GenServer calls and casts differ
Use GenServer.call/3 when the caller needs a reply: it waits for the server to respond. Use GenServer.cast/2 when sending a message without requiring a response; the cast returns without confirming that the server handled the work. That asynchronous contract is not inherently unsafe, but the immediate return from a cast is not evidence that the operation completed. A later successful call can establish that the server is available and has processed earlier work, as described in Elixir’s GenServer guide.
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What a supervisor does when a child fails
A supervisor starts, stops, and monitors child processes according to their child specifications. It starts children in the order specified and terminates them in reverse order. When a child terminates, the supervisor applies the configured restart strategy, which determines how broadly the failure affects its siblings.
Choose a restart strategy based on sibling relationships
| Strategy | Failure scope | When it fits |
|---|---|---|
:one_for_one |
Restarts only the child that terminated. | Use when sibling processes can recover independently. |
:one_for_all |
Terminates and restarts all children. | Use when siblings form a tightly coupled unit that must restart together. |
:rest_for_one |
Terminates and restarts the failed child and children started after it. | Use when later children depend on earlier children in start order. |
These descriptions follow the Erlang/OTP Design Principles guide. The right choice depends on actual process dependencies: a start-order relationship matters for :rest_for_one only when later siblings rely on earlier ones.
Restart intensity limits repeated failure
A supervisor also has a restart-intensity limit: a maximum number of restarts within a configured time window. If that limit is exceeded, the supervisor terminates its children and itself. A higher-level supervisor can then take responsibility for recovery. This prevents a child that repeatedly fails for the same reason from triggering an endless restart loop.
Check compatibility for your Elixir and Erlang/OTP versions
At the time of the current Elixir documentation snapshot, the official site labels Elixir v1.20.4 stable and lists Erlang/OTP 27, 28, and 29 as supported. Compatibility changes over time, so check the official documentation for the Elixir version you install. The site provides versioned documentation; when working in an older project, consult the documentation for that release rather than assuming every latest-version API or recommendation applies unchanged.
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Where to continue learning
The official Elixir Learning page collects books, courses, videos, and other learning resources. For a book-length treatment, the publisher’s contents for Elixir in Action, Third Edition include GenServer, fault tolerance and supervisors, supervision trees, dynamic supervision, OTP applications, releases, and distributed systems.
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