José Valim
3b77086073
One common concern about using Elixir libraries is that "use SomeModule" does not make clear how it impacts the caller. Most of the time those changes are minor but it is impossible to know without traversing the source code. This pull request suggests defining a summary alongside each module that succintly explains the impact of using it. This is documented as best practice in our guides and we added the note to all of Elixir modules.
1268 lines
46 KiB
Elixir
1268 lines
46 KiB
Elixir
defmodule GenServer do
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@moduledoc """
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A behaviour module for implementing the server of a client-server relation.
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A GenServer is a process like any other Elixir process and it can be used
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to keep state, execute code asynchronously and so on. The advantage of using
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a generic server process (GenServer) implemented using this module is that it
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will have a standard set of interface functions and include functionality for
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tracing and error reporting. It will also fit into a supervision tree.
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## Example
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The GenServer behaviour abstracts the common client-server interaction.
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Developers are only required to implement the callbacks and functionality
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they are interested in.
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Let's start with a code example and then explore the available callbacks.
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Imagine we want to implement a service with a GenServer that works
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like a stack, allowing us to push and pop elements. We'll customize a
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generic GenServer with our own module by implementing three callbacks.
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`c:init/1` transforms our initial argument to the initial state for the
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GenServer. `c:handle_call/3` fires when the server receives a synchronous
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`pop` message, popping an element from the stack and returning it to the
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user. `c:handle_cast/2` will fire when the server receives an asynchronous
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`push` message, pushing an element onto the stack:
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defmodule Stack do
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use GenServer
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# Callbacks
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@impl true
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def init(elements) do
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initial_state = String.split(elements, ",", trim: true)
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{:ok, initial_state}
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end
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@impl true
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def handle_call(:pop, _from, state) do
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[to_caller | new_state] = state
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{:reply, to_caller, new_state}
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end
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@impl true
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def handle_cast({:push, element}, state) do
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new_state = [element | state]
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{:noreply, new_state}
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end
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end
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We leave the process machinery of startup, message passing, and the message
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loop to the GenServer behaviour and focus only on the stack
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implementation. We can now use the GenServer API to interact with
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the service by creating a process and sending it messages:
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# Start the server
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{:ok, pid} = GenServer.start_link(Stack, "hello,world")
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# This is the client
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GenServer.call(pid, :pop)
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#=> "hello"
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GenServer.cast(pid, {:push, "elixir"})
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#=> :ok
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GenServer.call(pid, :pop)
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#=> "elixir"
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We start our `Stack` by calling `start_link/2`, passing the module
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with the server implementation and its initial argument with a
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comma-separated list of elements. The GenServer behaviour calls the
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`c:init/1` callback to establish the initial GenServer state. From
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this point on, the GenServer has control so we interact with it by
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sending two types of messages on the client. **call** messages expect
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a reply from the server (and are therefore synchronous) while **cast**
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messages do not.
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Each call to `GenServer.call/3` results in a message
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that must be handled by the `c:handle_call/3` callback in the GenServer.
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A `cast/2` message must be handled by `c:handle_cast/2`. `GenServer`
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supports 8 callbacks, but only `c:init/1` is required.
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> #### `use GenServer` {: .info}
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>
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> When you `use GenServer`, the `GenServer` module will
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> set `@behaviour GenServer` and define a `child_spec/1`
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> function, so your module can be used as a child
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> in a supervision tree.
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## Client / Server APIs
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Although in the example above we have used `GenServer.start_link/3` and
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friends to directly start and communicate with the server, most of the
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time we don't call the `GenServer` functions directly. Instead, we wrap
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the calls in new functions representing the public API of the server.
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These thin wrappers are called the **client API**.
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Here is a better implementation of our Stack module:
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defmodule Stack do
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use GenServer
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# Client
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def start_link(default) when is_binary(default) do
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GenServer.start_link(__MODULE__, default)
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end
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def push(pid, element) do
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GenServer.cast(pid, {:push, element})
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end
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def pop(pid) do
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GenServer.call(pid, :pop)
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end
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# Server (callbacks)
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@impl true
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def init(elements) do
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initial_state = String.split(elements, ",", trim: true)
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{:ok, initial_state}
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end
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@impl true
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def handle_call(:pop, _from, state) do
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[to_caller | new_state] = state
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{:reply, to_caller, new_state}
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end
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@impl true
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def handle_cast({:push, element}, state) do
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new_state = [element | state]
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{:noreply, new_state}
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end
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end
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In practice, it is common to have both server and client functions in
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the same module. If the server and/or client implementations are growing
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complex, you may want to have them in different modules.
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## How to supervise
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A `GenServer` is most commonly started under a supervision tree.
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When we invoke `use GenServer`, it automatically defines a `child_spec/1`
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function that allows us to start the `Stack` directly under a supervisor.
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To start a default stack of `["hello", "world"]` under a supervisor,
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we can do:
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children = [
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{Stack, "hello,world"}
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]
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Supervisor.start_link(children, strategy: :one_for_all)
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Note that specifying a module `MyServer` would be the same as specifying
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the tuple `{MyServer, []}`.
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`use GenServer` also accepts a list of options which configures the
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child specification and therefore how it runs under a supervisor.
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The generated `child_spec/1` can be customized with the following options:
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* `:id` - the child specification identifier, defaults to the current module
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* `:restart` - when the child should be restarted, defaults to `:permanent`
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* `:shutdown` - how to shut down the child, either immediately or by giving it time to shut down
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For example:
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use GenServer, restart: :transient, shutdown: 10_000
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See the "Child specification" section in the `Supervisor` module for more
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detailed information. The `@doc` annotation immediately preceding
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`use GenServer` will be attached to the generated `child_spec/1` function.
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When stopping the GenServer, for example by returning a `{:stop, reason, new_state}`
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tuple from a callback, the exit reason is used by the supervisor to determine
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whether the GenServer needs to be restarted. See the "Exit reasons and restarts"
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section in the `Supervisor` module.
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## Name registration
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Both `start_link/3` and `start/3` support the `GenServer` to register
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a name on start via the `:name` option. Registered names are also
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automatically cleaned up on termination. The supported values are:
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* an atom - the GenServer is registered locally (to the current node)
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with the given name using `Process.register/2`.
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* `{:global, term}` - the GenServer is registered globally with the given
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term using the functions in the [`:global` module](`:global`).
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* `{:via, module, term}` - the GenServer is registered with the given
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mechanism and name. The `:via` option expects a module that exports
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`register_name/2`, `unregister_name/1`, `whereis_name/1` and `send/2`.
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One such example is the [`:global` module](`:global`) which uses these functions
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for keeping the list of names of processes and their associated PIDs
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that are available globally for a network of Elixir nodes. Elixir also
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ships with a local, decentralized and scalable registry called `Registry`
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for locally storing names that are generated dynamically.
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For example, we could start and register our `Stack` server locally as follows:
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# Start the server and register it locally with name MyStack
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{:ok, _} = GenServer.start_link(Stack, "hello", name: MyStack)
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# Now messages can be sent directly to MyStack
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GenServer.call(MyStack, :pop)
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#=> "hello"
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Once the server is started, the remaining functions in this module (`call/3`,
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`cast/2`, and friends) will also accept an atom, or any `{:global, ...}` or
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`{:via, ...}` tuples. In general, the following formats are supported:
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* a PID
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* an atom if the server is locally registered
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* `{atom, node}` if the server is locally registered at another node
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* `{:global, term}` if the server is globally registered
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* `{:via, module, name}` if the server is registered through an alternative
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registry
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If there is an interest to register dynamic names locally, do not use
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atoms, as atoms are never garbage-collected and therefore dynamically
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generated atoms won't be garbage-collected. For such cases, you can
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set up your own local registry by using the `Registry` module.
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## Receiving "regular" messages
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The goal of a `GenServer` is to abstract the "receive" loop for developers,
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automatically handling system messages, supporting code change, synchronous
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calls and more. Therefore, you should never call your own "receive" inside
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the GenServer callbacks as doing so will cause the GenServer to misbehave.
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Besides the synchronous and asynchronous communication provided by `call/3`
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and `cast/2`, "regular" messages sent by functions such as `send/2`,
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`Process.send_after/4` and similar, can be handled inside the `c:handle_info/2`
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callback.
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`c:handle_info/2` can be used in many situations, such as handling monitor
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DOWN messages sent by `Process.monitor/1`. Another use case for `c:handle_info/2`
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is to perform periodic work, with the help of `Process.send_after/4`:
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defmodule MyApp.Periodically do
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use GenServer
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def start_link(_) do
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GenServer.start_link(__MODULE__, %{})
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end
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@impl true
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def init(state) do
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# Schedule work to be performed on start
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schedule_work()
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{:ok, state}
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end
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@impl true
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def handle_info(:work, state) do
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# Do the desired work here
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# ...
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# Reschedule once more
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schedule_work()
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{:noreply, state}
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end
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defp schedule_work do
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# We schedule the work to happen in 2 hours (written in milliseconds).
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# Alternatively, one might write :timer.hours(2)
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Process.send_after(self(), :work, 2 * 60 * 60 * 1000)
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end
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end
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## Timeouts
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The return value of `c:init/1` or any of the `handle_*` callbacks may include
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a timeout value in milliseconds; if not, `:infinity` is assumed.
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The timeout can be used to detect a lull in incoming messages.
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The `timeout()` value is used as follows:
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* If the process has any message already waiting when the `timeout()` value
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is returned, the timeout is ignored and the waiting message is handled as
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usual. This means that even a timeout of `0` milliseconds is not guaranteed
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to execute (if you want to take another action immediately and unconditionally,
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use a `:continue` instruction instead).
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* If any message arrives before the specified number of milliseconds
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elapse, the timeout is cleared and that message is handled as usual.
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* Otherwise, when the specified number of milliseconds have elapsed with no
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message arriving, `handle_info/2` is called with `:timeout` as the first
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argument.
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## When (not) to use a GenServer
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So far, we have learned that a `GenServer` can be used as a supervised process
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that handles sync and async calls. It can also handle system messages, such as
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periodic messages and monitoring events. GenServer processes may also be named.
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A GenServer, or a process in general, must be used to model runtime characteristics
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of your system. A GenServer must never be used for code organization purposes.
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In Elixir, code organization is done by modules and functions, processes are not
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necessary. For example, imagine you are implementing a calculator and you decide
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to put all the calculator operations behind a GenServer:
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def add(a, b) do
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GenServer.call(__MODULE__, {:add, a, b})
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end
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def subtract(a, b) do
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GenServer.call(__MODULE__, {:subtract, a, b})
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end
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def handle_call({:add, a, b}, _from, state) do
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{:reply, a + b, state}
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end
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def handle_call({:subtract, a, b}, _from, state) do
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{:reply, a - b, state}
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end
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This is an anti-pattern not only because it convolutes the calculator logic but
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also because you put the calculator logic behind a single process that will
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potentially become a bottleneck in your system, especially as the number of
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calls grow. Instead just define the functions directly:
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def add(a, b) do
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a + b
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end
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def subtract(a, b) do
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a - b
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end
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If you don't need a process, then you don't need a process. Use processes only to
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model runtime properties, such as mutable state, concurrency and failures, never
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for code organization.
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## Debugging with the :sys module
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GenServers, as [special processes](https://www.erlang.org/doc/design_principles/spec_proc.html),
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can be debugged using the [`:sys` module](`:sys`).
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Through various hooks, this module allows developers to introspect the state of
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the process and trace system events that happen during its execution, such as
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received messages, sent replies and state changes.
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Let's explore the basic functions from the
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[`:sys` module](`:sys`) used for debugging:
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* `:sys.get_state/2` - allows retrieval of the state of the process.
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In the case of a GenServer process, it will be the callback module state,
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as passed into the callback functions as last argument.
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* `:sys.get_status/2` - allows retrieval of the status of the process.
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This status includes the process dictionary, if the process is running
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or is suspended, the parent PID, the debugger state, and the state of
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the behaviour module, which includes the callback module state
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(as returned by `:sys.get_state/2`). It's possible to change how this
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status is represented by defining the optional `c:GenServer.format_status/2`
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callback.
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* `:sys.trace/3` - prints all the system events to `:stdio`.
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* `:sys.statistics/3` - manages collection of process statistics.
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* `:sys.no_debug/2` - turns off all debug handlers for the given process.
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It is very important to switch off debugging once we're done. Excessive
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debug handlers or those that should be turned off, but weren't, can
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seriously damage the performance of the system.
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* `:sys.suspend/2` - allows to suspend a process so that it only
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replies to system messages but no other messages. A suspended process
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can be reactivated via `:sys.resume/2`.
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Let's see how we could use those functions for debugging the stack server
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we defined earlier.
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iex> {:ok, pid} = Stack.start_link([])
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iex> :sys.statistics(pid, true) # turn on collecting process statistics
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iex> :sys.trace(pid, true) # turn on event printing
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iex> Stack.push(pid, 1)
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*DBG* <0.122.0> got cast {push,1}
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*DBG* <0.122.0> new state [1]
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:ok
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iex> :sys.get_state(pid)
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[1]
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iex> Stack.pop(pid)
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*DBG* <0.122.0> got call pop from <0.80.0>
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*DBG* <0.122.0> sent 1 to <0.80.0>, new state []
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1
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iex> :sys.statistics(pid, :get)
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{:ok,
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[
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start_time: {{2016, 7, 16}, {12, 29, 41}},
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current_time: {{2016, 7, 16}, {12, 29, 50}},
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reductions: 117,
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messages_in: 2,
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messages_out: 0
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]}
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iex> :sys.no_debug(pid) # turn off all debug handlers
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:ok
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iex> :sys.get_status(pid)
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{:status, #PID<0.122.0>, {:module, :gen_server},
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[
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[
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"$initial_call": {Stack, :init, 1}, # process dictionary
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"$ancestors": [#PID<0.80.0>, #PID<0.51.0>]
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],
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:running, # :running | :suspended
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#PID<0.80.0>, # parent
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[], # debugger state
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[
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header: 'Status for generic server <0.122.0>', # module status
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data: [
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{'Status', :running},
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{'Parent', #PID<0.80.0>},
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{'Logged events', []}
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],
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data: [{'State', [1]}]
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]
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]}
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## Learn more
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If you wish to find out more about GenServers, the Elixir Getting Started
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guide provides a tutorial-like introduction. The documentation and links
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in Erlang can also provide extra insight.
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* [GenServer - Elixir's Getting Started Guide](https://elixir-lang.org/getting-started/mix-otp/genserver.html)
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* [`:gen_server` module documentation](`:gen_server`)
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* [gen_server Behaviour - OTP Design Principles](https://www.erlang.org/doc/design_principles/gen_server_concepts.html)
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* [Clients and Servers - Learn You Some Erlang for Great Good!](http://learnyousomeerlang.com/clients-and-servers)
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"""
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@doc """
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Invoked when the server is started. `start_link/3` or `start/3` will
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block until it returns.
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`init_arg` is the argument term (second argument) passed to `start_link/3`.
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Returning `{:ok, state}` will cause `start_link/3` to return
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`{:ok, pid}` and the process to enter its loop.
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Returning `{:ok, state, timeout}` is similar to `{:ok, state}`,
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except that it also sets a timeout. See the "Timeouts" section
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in the module documentation for more information.
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Returning `{:ok, state, :hibernate}` is similar to `{:ok, state}`
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except the process is hibernated before entering the loop. See
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`c:handle_call/3` for more information on hibernation.
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Returning `{:ok, state, {:continue, continue_arg}}` is similar to
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`{:ok, state}` except that immediately after entering the loop,
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the `c:handle_continue/2` callback will be invoked with `continue_arg`
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as the first argument and `state` as the second one.
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Returning `:ignore` will cause `start_link/3` to return `:ignore` and
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the process will exit normally without entering the loop or calling
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`c:terminate/2`. If used when part of a supervision tree the parent
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supervisor will not fail to start nor immediately try to restart the
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`GenServer`. The remainder of the supervision tree will be started
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and so the `GenServer` should not be required by other processes.
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It can be started later with `Supervisor.restart_child/2` as the child
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specification is saved in the parent supervisor. The main use cases for
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this are:
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* The `GenServer` is disabled by configuration but might be enabled later.
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* An error occurred and it will be handled by a different mechanism than the
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`Supervisor`. Likely this approach involves calling `Supervisor.restart_child/2`
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after a delay to attempt a restart.
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Returning `{:stop, reason}` will cause `start_link/3` to return
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`{:error, reason}` and the process to exit with reason `reason` without
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entering the loop or calling `c:terminate/2`.
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"""
|
|
@callback init(init_arg :: term) ::
|
|
{:ok, state}
|
|
| {:ok, state, timeout | :hibernate | {:continue, continue_arg :: term}}
|
|
| :ignore
|
|
| {:stop, reason :: any}
|
|
when state: any
|
|
|
|
@doc """
|
|
Invoked to handle synchronous `call/3` messages. `call/3` will block until a
|
|
reply is received (unless the call times out or nodes are disconnected).
|
|
|
|
`request` is the request message sent by a `call/3`, `from` is a 2-tuple
|
|
containing the caller's PID and a term that uniquely identifies the call, and
|
|
`state` is the current state of the `GenServer`.
|
|
|
|
Returning `{:reply, reply, new_state}` sends the response `reply` to the
|
|
caller and continues the loop with new state `new_state`.
|
|
|
|
Returning `{:reply, reply, new_state, timeout}` is similar to
|
|
`{:reply, reply, new_state}` except that it also sets a timeout.
|
|
See the "Timeouts" section in the module documentation for more information.
|
|
|
|
Returning `{:reply, reply, new_state, :hibernate}` is similar to
|
|
`{:reply, reply, new_state}` except the process is hibernated and will
|
|
continue the loop once a message is in its message queue. However, if a message is
|
|
already in the message queue, the process will continue the loop immediately.
|
|
Hibernating a `GenServer` causes garbage collection and leaves a continuous
|
|
heap that minimises the memory used by the process.
|
|
|
|
Hibernating should not be used aggressively as too much time could be spent
|
|
garbage collecting, which would delay the processing of incoming messages.
|
|
Normally it should only be used when you are not expecting new messages to
|
|
immediately arrive and minimising the memory of the process is shown to be
|
|
beneficial.
|
|
|
|
Returning `{:reply, reply, new_state, {:continue, continue_arg}}` is similar to
|
|
`{:reply, reply, new_state}` except that `c:handle_continue/2` will be invoked
|
|
immediately after with `continue_arg` as the first argument and
|
|
`state` as the second one.
|
|
|
|
Returning `{:noreply, new_state}` does not send a response to the caller and
|
|
continues the loop with new state `new_state`. The response must be sent with
|
|
`reply/2`.
|
|
|
|
There are three main use cases for not replying using the return value:
|
|
|
|
* To reply before returning from the callback because the response is known
|
|
before calling a slow function.
|
|
* To reply after returning from the callback because the response is not yet
|
|
available.
|
|
* To reply from another process, such as a task.
|
|
|
|
When replying from another process the `GenServer` should exit if the other
|
|
process exits without replying as the caller will be blocking awaiting a
|
|
reply.
|
|
|
|
Returning `{:noreply, new_state, timeout | :hibernate | {:continue, continue_arg}}`
|
|
is similar to `{:noreply, new_state}` except a timeout, hibernation or continue
|
|
occurs as with a `:reply` tuple.
|
|
|
|
Returning `{:stop, reason, reply, new_state}` stops the loop and `c:terminate/2`
|
|
is called with reason `reason` and state `new_state`. Then, the `reply` is sent
|
|
as the response to call and the process exits with reason `reason`.
|
|
|
|
Returning `{:stop, reason, new_state}` is similar to
|
|
`{:stop, reason, reply, new_state}` except a reply is not sent.
|
|
|
|
This callback is optional. If one is not implemented, the server will fail
|
|
if a call is performed against it.
|
|
"""
|
|
@callback handle_call(request :: term, from, state :: term) ::
|
|
{:reply, reply, new_state}
|
|
| {:reply, reply, new_state,
|
|
timeout | :hibernate | {:continue, continue_arg :: term}}
|
|
| {:noreply, new_state}
|
|
| {:noreply, new_state, timeout | :hibernate | {:continue, continue_arg :: term}}
|
|
| {:stop, reason, reply, new_state}
|
|
| {:stop, reason, new_state}
|
|
when reply: term, new_state: term, reason: term
|
|
|
|
@doc """
|
|
Invoked to handle asynchronous `cast/2` messages.
|
|
|
|
`request` is the request message sent by a `cast/2` and `state` is the current
|
|
state of the `GenServer`.
|
|
|
|
Returning `{:noreply, new_state}` continues the loop with new state `new_state`.
|
|
|
|
Returning `{:noreply, new_state, timeout}` is similar to `{:noreply, new_state}`
|
|
except that it also sets a timeout. See the "Timeouts" section in the module
|
|
documentation for more information.
|
|
|
|
Returning `{:noreply, new_state, :hibernate}` is similar to
|
|
`{:noreply, new_state}` except the process is hibernated before continuing the
|
|
loop. See `c:handle_call/3` for more information.
|
|
|
|
Returning `{:noreply, new_state, {:continue, continue_arg}}` is similar to
|
|
`{:noreply, new_state}` except `c:handle_continue/2` will be invoked
|
|
immediately after with `continue_arg` as the first argument and
|
|
`state` as the second one.
|
|
|
|
Returning `{:stop, reason, new_state}` stops the loop and `c:terminate/2` is
|
|
called with the reason `reason` and state `new_state`. The process exits with
|
|
reason `reason`.
|
|
|
|
This callback is optional. If one is not implemented, the server will fail
|
|
if a cast is performed against it.
|
|
"""
|
|
@callback handle_cast(request :: term, state :: term) ::
|
|
{:noreply, new_state}
|
|
| {:noreply, new_state, timeout | :hibernate | {:continue, continue_arg :: term}}
|
|
| {:stop, reason :: term, new_state}
|
|
when new_state: term
|
|
|
|
@doc """
|
|
Invoked to handle all other messages.
|
|
|
|
`msg` is the message and `state` is the current state of the `GenServer`. When
|
|
a timeout occurs the message is `:timeout`.
|
|
|
|
Return values are the same as `c:handle_cast/2`.
|
|
|
|
This callback is optional. If one is not implemented, the received message
|
|
will be logged.
|
|
"""
|
|
@callback handle_info(msg :: :timeout | term, state :: term) ::
|
|
{:noreply, new_state}
|
|
| {:noreply, new_state, timeout | :hibernate | {:continue, continue_arg :: term}}
|
|
| {:stop, reason :: term, new_state}
|
|
when new_state: term
|
|
|
|
@doc """
|
|
Invoked to handle continue instructions.
|
|
|
|
It is useful for performing work after initialization or for splitting the work
|
|
in a callback in multiple steps, updating the process state along the way.
|
|
|
|
Return values are the same as `c:handle_cast/2`.
|
|
|
|
This callback is optional. If one is not implemented, the server will fail
|
|
if a continue instruction is used.
|
|
"""
|
|
@callback handle_continue(continue_arg, state :: term) ::
|
|
{:noreply, new_state}
|
|
| {:noreply, new_state, timeout | :hibernate | {:continue, continue_arg}}
|
|
| {:stop, reason :: term, new_state}
|
|
when new_state: term, continue_arg: term
|
|
|
|
@doc """
|
|
Invoked when the server is about to exit. It should do any cleanup required.
|
|
|
|
`reason` is exit reason and `state` is the current state of the `GenServer`.
|
|
The return value is ignored.
|
|
|
|
`c:terminate/2` is useful for cleanup that requires access to the
|
|
`GenServer`'s state. However, it is **not guaranteed** that `c:terminate/2`
|
|
is called when a `GenServer` exits. Therefore, important cleanup should be
|
|
done using process links and/or monitors. A monitoring process will receive the
|
|
same exit `reason` that would be passed to `c:terminate/2`.
|
|
|
|
`c:terminate/2` is called if:
|
|
|
|
* the `GenServer` traps exits (using `Process.flag/2`) *and* the parent
|
|
process (the one which called `start_link/1`) sends an exit signal
|
|
|
|
* a callback (except `c:init/1`) does one of the following:
|
|
|
|
* returns a `:stop` tuple
|
|
|
|
* raises (via `raise/2`) or exits (via `exit/1`)
|
|
|
|
* returns an invalid value
|
|
|
|
If part of a supervision tree, a `GenServer` will receive an exit signal from
|
|
its parent process (its supervisor) when the tree is shutting down. The exit
|
|
signal is based on the shutdown strategy in the child's specification, where
|
|
this value can be:
|
|
|
|
* `:brutal_kill`: the `GenServer` is killed and so `c:terminate/2` is not called.
|
|
|
|
* a timeout value, where the supervisor will send the exit signal `:shutdown` and
|
|
the `GenServer` will have the duration of the timeout to terminate.
|
|
If after duration of this timeout the process is still alive, it will be killed
|
|
immediately.
|
|
|
|
For a more in-depth explanation, please read the "Shutdown values (:shutdown)"
|
|
section in the `Supervisor` module.
|
|
|
|
If the `GenServer` receives an exit signal (that is not `:normal`) from any
|
|
process when it is not trapping exits it will exit abruptly with the same
|
|
reason and so not call `c:terminate/2`. Note that a process does *NOT* trap
|
|
exits by default and an exit signal is sent when a linked process exits or its
|
|
node is disconnected.
|
|
|
|
`c:terminate/2` is only called after the `GenServer` finishes processing all
|
|
messages which arrived in its mailbox prior to the exit signal. If it
|
|
receives a `:kill` signal before it finishes processing those,
|
|
`c:terminate/2` will not be called. If `c:terminate/2` is called, any
|
|
messages received after the exit signal will still be in the mailbox.
|
|
|
|
There is no cleanup needed when the `GenServer` controls a `port` (for example,
|
|
`:gen_tcp.socket`) or `t:File.io_device/0`, because these will be closed on
|
|
receiving a `GenServer`'s exit signal and do not need to be closed manually
|
|
in `c:terminate/2`.
|
|
|
|
If `reason` is neither `:normal`, `:shutdown`, nor `{:shutdown, term}` an error is
|
|
logged.
|
|
|
|
This callback is optional.
|
|
"""
|
|
@callback terminate(reason, state :: term) :: term
|
|
when reason: :normal | :shutdown | {:shutdown, term} | term
|
|
|
|
@doc """
|
|
Invoked to change the state of the `GenServer` when a different version of a
|
|
module is loaded (hot code swapping) and the state's term structure should be
|
|
changed.
|
|
|
|
`old_vsn` is the previous version of the module (defined by the `@vsn`
|
|
attribute) when upgrading. When downgrading the previous version is wrapped in
|
|
a 2-tuple with first element `:down`. `state` is the current state of the
|
|
`GenServer` and `extra` is any extra data required to change the state.
|
|
|
|
Returning `{:ok, new_state}` changes the state to `new_state` and the code
|
|
change is successful.
|
|
|
|
Returning `{:error, reason}` fails the code change with reason `reason` and
|
|
the state remains as the previous state.
|
|
|
|
If `c:code_change/3` raises the code change fails and the loop will continue
|
|
with its previous state. Therefore this callback does not usually contain side effects.
|
|
|
|
This callback is optional.
|
|
"""
|
|
@callback code_change(old_vsn, state :: term, extra :: term) ::
|
|
{:ok, new_state :: term}
|
|
| {:error, reason :: term}
|
|
when old_vsn: term | {:down, term}
|
|
|
|
@doc """
|
|
Invoked in some cases to retrieve a formatted version of the `GenServer` status:
|
|
|
|
* one of `:sys.get_status/1` or `:sys.get_status/2` is invoked to get the
|
|
status of the `GenServer`; in such cases, `reason` is `:normal`
|
|
|
|
* the `GenServer` terminates abnormally and logs an error; in such cases,
|
|
`reason` is `:terminate`
|
|
|
|
This callback can be useful to control the *appearance* of the status of the
|
|
`GenServer`. For example, it can be used to return a compact representation of
|
|
the `GenServer`'s state to avoid having large state terms printed.
|
|
|
|
`pdict_and_state` is a two-elements list `[pdict, state]` where `pdict` is a
|
|
list of `{key, value}` tuples representing the current process dictionary of
|
|
the `GenServer` and `state` is the current state of the `GenServer`.
|
|
"""
|
|
@callback format_status(reason, pdict_and_state :: list) :: term
|
|
when reason: :normal | :terminate
|
|
|
|
@optional_callbacks code_change: 3,
|
|
terminate: 2,
|
|
handle_info: 2,
|
|
handle_cast: 2,
|
|
handle_call: 3,
|
|
format_status: 2,
|
|
handle_continue: 2
|
|
|
|
@typedoc "Return values of `start*` functions"
|
|
@type on_start :: {:ok, pid} | :ignore | {:error, {:already_started, pid} | term}
|
|
|
|
@typedoc "The GenServer name"
|
|
@type name :: atom | {:global, term} | {:via, module, term}
|
|
|
|
@typedoc "Options used by the `start*` functions"
|
|
@type options :: [option]
|
|
|
|
@typedoc "Option values used by the `start*` functions"
|
|
@type option ::
|
|
{:debug, debug}
|
|
| {:name, name}
|
|
| {:timeout, timeout}
|
|
| {:spawn_opt, [Process.spawn_opt()]}
|
|
| {:hibernate_after, timeout}
|
|
|
|
@typedoc "Debug options supported by the `start*` functions"
|
|
@type debug :: [:trace | :log | :statistics | {:log_to_file, Path.t()}]
|
|
|
|
@typedoc """
|
|
The server reference.
|
|
|
|
This is either a plain PID or a value representing a registered name.
|
|
See the "Name registration" section of this document for more information.
|
|
"""
|
|
@type server :: pid | name | {atom, node}
|
|
|
|
@typedoc """
|
|
Tuple describing the client of a call request.
|
|
|
|
`pid` is the PID of the caller and `tag` is a unique term used to identify the
|
|
call.
|
|
"""
|
|
@type from :: {pid, tag :: term}
|
|
|
|
@doc false
|
|
defmacro __using__(opts) do
|
|
quote location: :keep, bind_quoted: [opts: opts] do
|
|
@behaviour GenServer
|
|
|
|
unless Module.has_attribute?(__MODULE__, :doc) do
|
|
@doc """
|
|
Returns a specification to start this module under a supervisor.
|
|
|
|
See `Supervisor`.
|
|
"""
|
|
end
|
|
|
|
def child_spec(init_arg) do
|
|
default = %{
|
|
id: __MODULE__,
|
|
start: {__MODULE__, :start_link, [init_arg]}
|
|
}
|
|
|
|
Supervisor.child_spec(default, unquote(Macro.escape(opts)))
|
|
end
|
|
|
|
defoverridable child_spec: 1
|
|
|
|
# TODO: Remove this on v2.0
|
|
@before_compile GenServer
|
|
|
|
@doc false
|
|
def handle_call(msg, _from, state) do
|
|
proc =
|
|
case Process.info(self(), :registered_name) do
|
|
{_, []} -> self()
|
|
{_, name} -> name
|
|
end
|
|
|
|
# We do this to trick Dialyzer to not complain about non-local returns.
|
|
case :erlang.phash2(1, 1) do
|
|
0 ->
|
|
raise "attempted to call GenServer #{inspect(proc)} but no handle_call/3 clause was provided"
|
|
|
|
1 ->
|
|
{:stop, {:bad_call, msg}, state}
|
|
end
|
|
end
|
|
|
|
@doc false
|
|
def handle_info(msg, state) do
|
|
proc =
|
|
case Process.info(self(), :registered_name) do
|
|
{_, []} -> self()
|
|
{_, name} -> name
|
|
end
|
|
|
|
:logger.error(
|
|
%{
|
|
label: {GenServer, :no_handle_info},
|
|
report: %{
|
|
module: __MODULE__,
|
|
message: msg,
|
|
name: proc
|
|
}
|
|
},
|
|
%{
|
|
domain: [:otp, :elixir],
|
|
error_logger: %{tag: :error_msg},
|
|
report_cb: &GenServer.format_report/1
|
|
}
|
|
)
|
|
|
|
{:noreply, state}
|
|
end
|
|
|
|
@doc false
|
|
def handle_cast(msg, state) do
|
|
proc =
|
|
case Process.info(self(), :registered_name) do
|
|
{_, []} -> self()
|
|
{_, name} -> name
|
|
end
|
|
|
|
# We do this to trick Dialyzer to not complain about non-local returns.
|
|
case :erlang.phash2(1, 1) do
|
|
0 ->
|
|
raise "attempted to cast GenServer #{inspect(proc)} but no handle_cast/2 clause was provided"
|
|
|
|
1 ->
|
|
{:stop, {:bad_cast, msg}, state}
|
|
end
|
|
end
|
|
|
|
@doc false
|
|
def terminate(_reason, _state) do
|
|
:ok
|
|
end
|
|
|
|
@doc false
|
|
def code_change(_old, state, _extra) do
|
|
{:ok, state}
|
|
end
|
|
|
|
defoverridable code_change: 3, terminate: 2, handle_info: 2, handle_cast: 2, handle_call: 3
|
|
end
|
|
end
|
|
|
|
defmacro __before_compile__(env) do
|
|
unless Module.defines?(env.module, {:init, 1}) do
|
|
message = """
|
|
function init/1 required by behaviour GenServer is not implemented \
|
|
(in module #{inspect(env.module)}).
|
|
|
|
We will inject a default implementation for now:
|
|
|
|
def init(init_arg) do
|
|
{:ok, init_arg}
|
|
end
|
|
|
|
You can copy the implementation above or define your own that converts \
|
|
the arguments given to GenServer.start_link/3 to the server state.
|
|
"""
|
|
|
|
IO.warn(message, env)
|
|
|
|
quote do
|
|
@doc false
|
|
def init(init_arg) do
|
|
{:ok, init_arg}
|
|
end
|
|
|
|
defoverridable init: 1
|
|
end
|
|
end
|
|
end
|
|
|
|
@doc """
|
|
Starts a `GenServer` process linked to the current process.
|
|
|
|
This is often used to start the `GenServer` as part of a supervision tree.
|
|
|
|
Once the server is started, the `c:init/1` function of the given `module` is
|
|
called with `init_arg` as its argument to initialize the server. To ensure a
|
|
synchronized start-up procedure, this function does not return until `c:init/1`
|
|
has returned.
|
|
|
|
Note that a `GenServer` started with `start_link/3` is linked to the
|
|
parent process and will exit in case of crashes from the parent. The GenServer
|
|
will also exit due to the `:normal` reasons in case it is configured to trap
|
|
exits in the `c:init/1` callback.
|
|
|
|
## Options
|
|
|
|
* `:name` - used for name registration as described in the "Name
|
|
registration" section in the documentation for `GenServer`
|
|
|
|
* `:timeout` - if present, the server is allowed to spend the given number of
|
|
milliseconds initializing or it will be terminated and the start function
|
|
will return `{:error, :timeout}`
|
|
|
|
* `:debug` - if present, the corresponding function in the [`:sys` module](`:sys`) is invoked
|
|
|
|
* `:spawn_opt` - if present, its value is passed as options to the
|
|
underlying process as in `Process.spawn/4`
|
|
|
|
* `:hibernate_after` - if present, the GenServer process awaits any message for
|
|
the given number of milliseconds and if no message is received, the process goes
|
|
into hibernation automatically (by calling `:proc_lib.hibernate/3`).
|
|
|
|
## Return values
|
|
|
|
If the server is successfully created and initialized, this function returns
|
|
`{:ok, pid}`, where `pid` is the PID of the server. If a process with the
|
|
specified server name already exists, this function returns
|
|
`{:error, {:already_started, pid}}` with the PID of that process.
|
|
|
|
If the `c:init/1` callback fails with `reason`, this function returns
|
|
`{:error, reason}`. Otherwise, if it returns `{:stop, reason}`
|
|
or `:ignore`, the process is terminated and this function returns
|
|
`{:error, reason}` or `:ignore`, respectively.
|
|
"""
|
|
@spec start_link(module, any, options) :: on_start
|
|
def start_link(module, init_arg, options \\ []) when is_atom(module) and is_list(options) do
|
|
do_start(:link, module, init_arg, options)
|
|
end
|
|
|
|
@doc """
|
|
Starts a `GenServer` process without links (outside of a supervision tree).
|
|
|
|
See `start_link/3` for more information.
|
|
"""
|
|
@spec start(module, any, options) :: on_start
|
|
def start(module, init_arg, options \\ []) when is_atom(module) and is_list(options) do
|
|
do_start(:nolink, module, init_arg, options)
|
|
end
|
|
|
|
defp do_start(link, module, init_arg, options) do
|
|
case Keyword.pop(options, :name) do
|
|
{nil, opts} ->
|
|
:gen.start(:gen_server, link, module, init_arg, opts)
|
|
|
|
{atom, opts} when is_atom(atom) ->
|
|
:gen.start(:gen_server, link, {:local, atom}, module, init_arg, opts)
|
|
|
|
{{:global, _term} = tuple, opts} ->
|
|
:gen.start(:gen_server, link, tuple, module, init_arg, opts)
|
|
|
|
{{:via, via_module, _term} = tuple, opts} when is_atom(via_module) ->
|
|
:gen.start(:gen_server, link, tuple, module, init_arg, opts)
|
|
|
|
{other, _} ->
|
|
raise ArgumentError, """
|
|
expected :name option to be one of the following:
|
|
|
|
* nil
|
|
* atom
|
|
* {:global, term}
|
|
* {:via, module, term}
|
|
|
|
Got: #{inspect(other)}
|
|
"""
|
|
end
|
|
end
|
|
|
|
@doc """
|
|
Synchronously stops the server with the given `reason`.
|
|
|
|
The `c:terminate/2` callback of the given `server` will be invoked before
|
|
exiting. This function returns `:ok` if the server terminates with the
|
|
given reason; if it terminates with another reason, the call exits.
|
|
|
|
This function keeps OTP semantics regarding error reporting.
|
|
If the reason is any other than `:normal`, `:shutdown` or
|
|
`{:shutdown, _}`, an error report is logged.
|
|
"""
|
|
@spec stop(server, reason :: term, timeout) :: :ok
|
|
def stop(server, reason \\ :normal, timeout \\ :infinity) do
|
|
case whereis(server) do
|
|
nil ->
|
|
exit({:noproc, {__MODULE__, :stop, [server, reason, timeout]}})
|
|
|
|
pid when pid == self() ->
|
|
exit({:calling_self, {__MODULE__, :stop, [server, reason, timeout]}})
|
|
|
|
pid ->
|
|
try do
|
|
:proc_lib.stop(pid, reason, timeout)
|
|
catch
|
|
:exit, err ->
|
|
exit({err, {__MODULE__, :stop, [server, reason, timeout]}})
|
|
end
|
|
end
|
|
end
|
|
|
|
@doc """
|
|
Makes a synchronous call to the `server` and waits for its reply.
|
|
|
|
The client sends the given `request` to the server and waits until a reply
|
|
arrives or a timeout occurs. `c:handle_call/3` will be called on the server
|
|
to handle the request.
|
|
|
|
`server` can be any of the values described in the "Name registration"
|
|
section of the documentation for this module.
|
|
|
|
## Timeouts
|
|
|
|
`timeout` is an integer greater than zero which specifies how many
|
|
milliseconds to wait for a reply, or the atom `:infinity` to wait
|
|
indefinitely. The default value is `5000`. If no reply is received within
|
|
the specified time, the function call fails and the caller exits. If the
|
|
caller catches the failure and continues running, and the server is just late
|
|
with the reply, it may arrive at any time later into the caller's message
|
|
queue. The caller must in this case be prepared for this and discard any such
|
|
garbage messages that are two-element tuples with a reference as the first
|
|
element.
|
|
"""
|
|
@spec call(server, term, timeout) :: term
|
|
def call(server, request, timeout \\ 5000)
|
|
when (is_integer(timeout) and timeout >= 0) or timeout == :infinity do
|
|
case whereis(server) do
|
|
nil ->
|
|
exit({:noproc, {__MODULE__, :call, [server, request, timeout]}})
|
|
|
|
# TODO: remove this clause when we require Erlang/OTP 25+
|
|
pid when pid == self() ->
|
|
exit({:calling_self, {__MODULE__, :call, [server, request, timeout]}})
|
|
|
|
pid ->
|
|
try do
|
|
:gen.call(pid, :"$gen_call", request, timeout)
|
|
catch
|
|
:exit, reason ->
|
|
exit({reason, {__MODULE__, :call, [server, request, timeout]}})
|
|
else
|
|
{:ok, res} -> res
|
|
end
|
|
end
|
|
end
|
|
|
|
@doc """
|
|
Casts a request to the `server` without waiting for a response.
|
|
|
|
This function always returns `:ok` regardless of whether
|
|
the destination `server` (or node) exists. Therefore it
|
|
is unknown whether the destination `server` successfully
|
|
handled the request.
|
|
|
|
`server` can be any of the values described in the "Name registration"
|
|
section of the documentation for this module.
|
|
"""
|
|
@spec cast(server, term) :: :ok
|
|
def cast(server, request)
|
|
|
|
def cast({:global, name}, request) do
|
|
try do
|
|
:global.send(name, cast_msg(request))
|
|
:ok
|
|
catch
|
|
_, _ -> :ok
|
|
end
|
|
end
|
|
|
|
def cast({:via, mod, name}, request) do
|
|
try do
|
|
mod.send(name, cast_msg(request))
|
|
:ok
|
|
catch
|
|
_, _ -> :ok
|
|
end
|
|
end
|
|
|
|
def cast({name, node}, request) when is_atom(name) and is_atom(node),
|
|
do: do_send({name, node}, cast_msg(request))
|
|
|
|
def cast(dest, request) when is_atom(dest) or is_pid(dest), do: do_send(dest, cast_msg(request))
|
|
|
|
@doc """
|
|
Casts all servers locally registered as `name` at the specified nodes.
|
|
|
|
This function returns immediately and ignores nodes that do not exist, or where the
|
|
server name does not exist.
|
|
|
|
See `multi_call/4` for more information.
|
|
"""
|
|
@spec abcast([node], name :: atom, term) :: :abcast
|
|
def abcast(nodes \\ [node() | Node.list()], name, request)
|
|
when is_list(nodes) and is_atom(name) do
|
|
msg = cast_msg(request)
|
|
_ = for node <- nodes, do: do_send({name, node}, msg)
|
|
:abcast
|
|
end
|
|
|
|
defp cast_msg(req) do
|
|
{:"$gen_cast", req}
|
|
end
|
|
|
|
defp do_send(dest, msg) do
|
|
try do
|
|
send(dest, msg)
|
|
:ok
|
|
catch
|
|
_, _ -> :ok
|
|
end
|
|
end
|
|
|
|
@doc """
|
|
Calls all servers locally registered as `name` at the specified `nodes`.
|
|
|
|
First, the `request` is sent to every node in `nodes`; then, the caller waits
|
|
for the replies. This function returns a two-element tuple `{replies,
|
|
bad_nodes}` where:
|
|
|
|
* `replies` - is a list of `{node, reply}` tuples where `node` is the node
|
|
that replied and `reply` is its reply
|
|
* `bad_nodes` - is a list of nodes that either did not exist or where a
|
|
server with the given `name` did not exist or did not reply
|
|
|
|
`nodes` is a list of node names to which the request is sent. The default
|
|
value is the list of all known nodes (including this node).
|
|
|
|
## Examples
|
|
|
|
Assuming the `Stack` GenServer mentioned in the docs for the `GenServer`
|
|
module is registered as `Stack` in the `:"foo@my-machine"` and
|
|
`:"bar@my-machine"` nodes:
|
|
|
|
GenServer.multi_call(Stack, :pop)
|
|
#=> {[{:"foo@my-machine", :hello}, {:"bar@my-machine", :world}], []}
|
|
|
|
"""
|
|
@spec multi_call([node], name :: atom, term, timeout) ::
|
|
{replies :: [{node, term}], bad_nodes :: [node]}
|
|
def multi_call(nodes \\ [node() | Node.list()], name, request, timeout \\ :infinity) do
|
|
:gen_server.multi_call(nodes, name, request, timeout)
|
|
end
|
|
|
|
@doc """
|
|
Replies to a client.
|
|
|
|
This function can be used to explicitly send a reply to a client that called
|
|
`call/3` or `multi_call/4` when the reply cannot be specified in the return
|
|
value of `c:handle_call/3`.
|
|
|
|
`client` must be the `from` argument (the second argument) accepted by
|
|
`c:handle_call/3` callbacks. `reply` is an arbitrary term which will be given
|
|
back to the client as the return value of the call.
|
|
|
|
Note that `reply/2` can be called from any process, not just the GenServer
|
|
that originally received the call (as long as that GenServer communicated the
|
|
`from` argument somehow).
|
|
|
|
This function always returns `:ok`.
|
|
|
|
## Examples
|
|
|
|
def handle_call(:reply_in_one_second, from, state) do
|
|
Process.send_after(self(), {:reply, from}, 1_000)
|
|
{:noreply, state}
|
|
end
|
|
|
|
def handle_info({:reply, from}, state) do
|
|
GenServer.reply(from, :one_second_has_passed)
|
|
{:noreply, state}
|
|
end
|
|
|
|
"""
|
|
@spec reply(from, term) :: :ok
|
|
def reply(client, reply) do
|
|
:gen.reply(client, reply)
|
|
end
|
|
|
|
@doc """
|
|
Returns the `pid` or `{name, node}` of a GenServer process, `nil` otherwise.
|
|
|
|
To be precise, `nil` is returned whenever a `pid` or `{name, node}` cannot
|
|
be returned. Note there is no guarantee the returned `pid` or `{name, node}`
|
|
is alive, as a process could terminate immediately after it is looked up.
|
|
|
|
## Examples
|
|
|
|
For example, to lookup a server process, monitor it and send a cast to it:
|
|
|
|
process = GenServer.whereis(server)
|
|
monitor = Process.monitor(process)
|
|
GenServer.cast(process, :hello)
|
|
|
|
"""
|
|
@spec whereis(server) :: pid | {atom, node} | nil
|
|
def whereis(server)
|
|
|
|
def whereis(pid) when is_pid(pid), do: pid
|
|
|
|
def whereis(name) when is_atom(name) do
|
|
Process.whereis(name)
|
|
end
|
|
|
|
def whereis({:global, name}) do
|
|
case :global.whereis_name(name) do
|
|
pid when is_pid(pid) -> pid
|
|
:undefined -> nil
|
|
end
|
|
end
|
|
|
|
def whereis({:via, mod, name}) do
|
|
case apply(mod, :whereis_name, [name]) do
|
|
pid when is_pid(pid) -> pid
|
|
:undefined -> nil
|
|
end
|
|
end
|
|
|
|
def whereis({name, local}) when is_atom(name) and local == node() do
|
|
Process.whereis(name)
|
|
end
|
|
|
|
def whereis({name, node} = server) when is_atom(name) and is_atom(node) do
|
|
server
|
|
end
|
|
|
|
@doc false
|
|
def format_report(%{
|
|
label: {GenServer, :no_handle_info},
|
|
report: %{module: mod, message: msg, name: proc}
|
|
}) do
|
|
{~c"~p ~p received unexpected message in handle_info/2: ~p~n", [mod, proc, msg]}
|
|
end
|
|
end
|