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In Elixir OTP, a GenServer implements a process-facing service, a supervisor starts and monitors child processes and applies their restart rules, and an OTP application packages functionality with a lifecycle that commonly starts a top-level supervision tree. These pieces work together, but they solve different problems: a process is not automatically needed for every value or calculation.
Examples here target Elixir v1.20.4, which the official documentation marked stable on October 4, 2026. That release lists Erlang/OTP 27, 28, and 29 as supported; the installation guide says Elixir v1.20.4 requires Erlang 27.0 or later. Check the official documentation index and installation guide against your installed toolchain, since compatibility changes over time.
What is a GenServer in Elixir?
A GenServer is Elixir’s generic server behavior: it provides standard process-loop and interaction machinery, while you define the service’s callbacks and domain behavior. The official API describes it as “A behaviour module for implementing the server of a client-server relation.” A GenServer process can own state and handle messages, and the behavior gives it familiar callback conventions, tracing and error-reporting support, and compatibility with supervision trees. See the GenServer API documentation.
A typical server initializes state in init/1, handles synchronous requests in handle_call/3, and handles asynchronous casts in handle_cast/2. Of these callbacks, only init/1 is required. For example, a stack server might remove an item and return it from handle_call/3, while its handle_cast/2 adds an item without returning a result to the sender.
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Callers should generally use a small public API on the server module rather than invoke callbacks directly. That keeps users of the service independent of its message and callback details.
When does a GenServer make sense?
Use one when the design benefits from a long-lived process identity, message handling, serialized access to state, or supervision and restart behavior. A GenServer is not a requirement for all mutable data: if a value can be passed into an ordinary function and returned as an updated value, that simpler design may be enough.
What is the difference between GenServer.call and GenServer.cast?
The practical difference is whether the caller needs a reply and must coordinate around it. A call is synchronous: it waits for the server’s reply, which is produced by handle_call/3. A cast is asynchronous: it sends a message handled by handle_cast/2, but gives the caller no reply confirming that application-level work completed.
| Operation | Server callback | What the caller gets | Use when |
|---|---|---|---|
GenServer.call/2 (or another call arity) |
handle_call/3 |
A reply; the caller waits for it. | The caller needs a result or must coordinate with the server before continuing. |
GenServer.cast/2 (or another cast arity) |
handle_cast/2 |
No application-level reply; the caller continues without waiting for completion. | The caller can send a message without needing a result from that operation. |
| Raw process message | handle_info/2 |
Not a GenServer call-and-reply exchange. | The process must handle messages sent outside the call and cast interfaces. |
These are different coordination contracts, not a universal performance ranking. A cast is not proof that the server received, processed, or completed the requested work.
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What does a supervisor do in Elixir?
A supervisor is a process that starts and monitors child processes. Its child specifications describe how children start and how they should be shut down or restarted. When a child terminates, the supervisor applies both the child’s restart setting and the supervisor’s strategy.
Supervision is fault recovery, not automatic restoration of a crashed process’s in-memory state. If a GenServer restarts, it initializes again; unless state is recovered from some other source, it starts from its initial state. For example, a counter that restarts at its configured initial value has not recovered the count it held before crashing. The official Supervisor documentation explains: “Supervision trees provide fault-tolerance and encapsulate how our applications start and shutdown.”
What is a supervision tree?
A supervision tree is the hierarchy of supervisors and the processes they supervise. A top-level supervisor can supervise other supervisors as well as service processes, allowing related components to have explicit startup, shutdown, and recovery relationships. This hierarchy is commonly the process structure started by an application’s callback.
What is the difference between one_for_one and one_for_all?
A supervisor strategy determines how sibling children are affected when one child terminates. Choose it according to actual lifecycle dependencies: are children independent, ordered, or dependent on one another?
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| Strategy | Effect when a child terminates | Best fit |
|---|---|---|
:one_for_one |
Restart the failed child without restarting its siblings. | Children are independent enough to recover separately. |
:rest_for_one |
Restart the failed child and children started after it in the supervisor’s child order. | Later children depend on earlier children, so they should be restarted when an earlier dependency fails. |
:one_for_all |
Restart all children when one child terminates. | Children form a mutually dependent unit that must be restarted together. |
For example, if a connection process is started before a worker that uses that connection, :rest_for_one can express that failure of the connection requires restarting the later worker. With :one_for_one, the worker would not be restarted as part of that failure response; with :one_for_all, every sibling would restart. These strategies are not interchangeable defaults: their effect should match the relationships among the children.
What do child specs and restart values mean?
A child specification tells a supervisor which child it is managing and how to start, shut down, and restart it. Elixir accepts a child-spec map or a module or {module, argument} form when the module supplies child_spec/1. Using use GenServer defines a child_spec/1 for that module, so it can be passed to a supervisor as a child.
The child’s :restart value is separate from the supervisor strategy. The restart value says whether that child should be restarted for a given exit reason; the strategy says how its siblings are affected.
| Restart value | Behavior | Typical intent |
|---|---|---|
:permanent |
Always restart the child. | A service intended to remain running under supervision. |
:temporary |
Never restart the child. | A child whose termination ends its supervised work. |
:transient |
Restart after abnormal termination, but not after normal termination or a shutdown reason. | A child that should recover from failure but can finish or shut down deliberately. |
For instance, a :permanent GenServer child can be restarted after termination, while :one_for_one governs whether its siblings are also restarted. Consult the current Supervisor reference when setting child-spec details for a particular kind of process.
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How do I start a GenServer under a supervisor?
Define a GenServer module, then include its child specification in a supervisor’s children list. The supervisor starts the process and applies its child and tree policies if it terminates.
-
Implement the server with
use GenServerand aninit/1callback to establish initial state. Addhandle_call/3and/orhandle_cast/2for the messages the service needs. -
In the supervisor’s
init/1, include the module in its children list, for examplechildren = [MyApp.Cache], when that module provideschild_spec/1. -
Return a supervisor configuration, such as
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-
Start the supervisor, commonly from an OTP application’s
start/2callback. The supervisor starts the GenServer child as part of its tree.
For a child whose start arguments or restart behavior need customization, provide an explicit child-spec map or an appropriate child-spec override rather than assuming the default settings fit every service.
What is an OTP application?
An OTP application is a standardized package of functionality with configuration and a lifecycle; it is not an operating-system process. Applications can be loaded, started, and stopped. In a Mix project, application information is declared by the project, and an application callback module can define start/2. That callback commonly starts and returns the top-level supervisor, which starts the application’s supervised process tree.
The official Application API documentation summarizes the lifecycle: “Applications are loaded, started, and stopped.” It also describes each application’s environment as a keyword list mapping atoms to terms. This application environment is not the operating system’s environment, and the API cautions libraries against using it as general-purpose global storage.
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At a high level, an OTP application is loaded, then started; its callback starts the top-level supervisor, which starts its children according to their specifications. When the application stops, its supervised processes are shut down through the supervision structure and the application’s stop callback can perform application-level cleanup. The precise lifecycle belongs to the OTP application and supervisor machinery, rather than to a single GenServer.
How should Mix configuration fit into startup?
Mix distinguishes build-time configuration from runtime configuration. In a Mix project, config/config.exs is loaded at build time, while config/runtime.exs is loaded at runtime just before the application starts. Put values that must be supplied or adjusted at deployment startup in runtime configuration rather than assuming build-time settings will be reevaluated there. See the Application documentation for application configuration and lifecycle details.
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