View Source GenServer behaviour (Elixir v1.14.1)
A behaviour module for implementing the server of a client-server relation.
A GenServer is a process like any other Elixir process and it can be used to keep state, execute code asynchronously and so on. The advantage of using a generic server process (GenServer) implemented using this module is that it will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into a supervision tree.
Example
The GenServer behaviour abstracts the common client-server interaction. Developers are only required to implement the callbacks and functionality they are interested in.
Let's start with a code example and then explore the available callbacks. Imagine we want a GenServer that works like a stack, allowing us to push and pop elements:
defmodule Stack do
use GenServer
# Callbacks
@impl true
def init(stack) do
{:ok, stack}
end
@impl true
def handle_call(:pop, _from, [head | tail]) do
{:reply, head, tail}
end
@impl true
def handle_cast({:push, element}, state) do
{:noreply, [element | state]}
end
end
# Start the server
{:ok, pid} = GenServer.start_link(Stack, [:hello])
# This is the client
GenServer.call(pid, :pop)
#=> :hello
GenServer.cast(pid, {:push, :world})
#=> :ok
GenServer.call(pid, :pop)
#=> :world
We start our Stack
by calling start_link/2
, passing the module
with the server implementation and its initial argument (a list
representing the stack containing the element :hello
). We can primarily
interact with the server by sending two types of messages. call
messages expect a reply from the server (and are therefore synchronous)
while cast messages do not.
Every time you do a GenServer.call/3
, the client will send a message
that must be handled by the handle_call/3
callback in the GenServer.
A cast/2
message must be handled by handle_cast/2
. There are 8 possible
callbacks to be implemented when you use a GenServer
. The only required
callback is init/1
.
Client / Server APIs
Although in the example above we have used GenServer.start_link/3
and
friends to directly start and communicate with the server, most of the
time we don't call the GenServer
functions directly. Instead, we wrap
the calls in new functions representing the public API of the server.
Here is a better implementation of our Stack module:
defmodule Stack do
use GenServer
# Client
def start_link(default) when is_list(default) do
GenServer.start_link(__MODULE__, default)
end
def push(pid, element) do
GenServer.cast(pid, {:push, element})
end
def pop(pid) do
GenServer.call(pid, :pop)
end
# Server (callbacks)
@impl true
def init(stack) do
{:ok, stack}
end
@impl true
def handle_call(:pop, _from, [head | tail]) do
{:reply, head, tail}
end
@impl true
def handle_cast({:push, element}, state) do
{:noreply, [element | state]}
end
end
In practice, it is common to have both server and client functions in the same module. If the server and/or client implementations are growing complex, you may want to have them in different modules.
How to supervise
A GenServer
is most commonly started under a supervision tree.
When we invoke use GenServer
, it automatically defines a child_spec/1
function that allows us to start the Stack
directly under a supervisor.
To start a default stack of [:hello]
under a supervisor, one may do:
children = [
{Stack, [:hello]}
]
Supervisor.start_link(children, strategy: :one_for_all)
Note you can also start it simply as Stack
, which is the same as
{Stack, []}
:
children = [
Stack # The same as {Stack, []}
]
Supervisor.start_link(children, strategy: :one_for_all)
In both cases, Stack.start_link/1
is always invoked.
use GenServer
also accepts a list of options which configures the
child specification and therefore how it runs under a supervisor.
The generated child_spec/1
can be customized with the following options:
:id
- the child specification identifier, defaults to the current module:restart
- when the child should be restarted, defaults to:permanent
:shutdown
- how to shut down the child, either immediately or by giving it time to shut down
For example:
use GenServer, restart: :transient, shutdown: 10_000
See the "Child specification" section in the Supervisor
module for more
detailed information. The @doc
annotation immediately preceding
use GenServer
will be attached to the generated child_spec/1
function.
When stopping the GenServer, for example by returning a {:stop, reason, new_state}
tuple from a callback, the exit reason is used by the supervisor to determine
whether the GenServer needs to be restarted. See the "Exit reasons and restarts"
section in the Supervisor
module.
Name registration
Both start_link/3
and start/3
support the GenServer
to register
a name on start via the :name
option. Registered names are also
automatically cleaned up on termination. The supported values are:
an atom - the GenServer is registered locally (to the current node) with the given name using
Process.register/2
.{:global, term}
- the GenServer is registered globally with the given term using the functions in the:global
module.{:via, module, term}
- the GenServer is registered with the given mechanism and name. The:via
option expects a module that exportsregister_name/2
,unregister_name/1
,whereis_name/1
andsend/2
. One such example is the:global
module which uses these functions for keeping the list of names of processes and their associated PIDs that are available globally for a network of Elixir nodes. Elixir also ships with a local, decentralized and scalable registry calledRegistry
for locally storing names that are generated dynamically.
For example, we could start and register our Stack
server locally as follows:
# Start the server and register it locally with name MyStack
{:ok, _} = GenServer.start_link(Stack, [:hello], name: MyStack)
# Now messages can be sent directly to MyStack
GenServer.call(MyStack, :pop)
#=> :hello
Once the server is started, the remaining functions in this module (call/3
,
cast/2
, and friends) will also accept an atom, or any {:global, ...}
or
{:via, ...}
tuples. In general, the following formats are supported:
- a PID
- an atom if the server is locally registered
{atom, node}
if the server is locally registered at another node{:global, term}
if the server is globally registered{:via, module, name}
if the server is registered through an alternative registry
If there is an interest to register dynamic names locally, do not use
atoms, as atoms are never garbage-collected and therefore dynamically
generated atoms won't be garbage-collected. For such cases, you can
set up your own local registry by using the Registry
module.
Receiving "regular" messages
The goal of a GenServer
is to abstract the "receive" loop for developers,
automatically handling system messages, supporting code change, synchronous
calls and more. Therefore, you should never call your own "receive" inside
the GenServer callbacks as doing so will cause the GenServer to misbehave.
Besides the synchronous and asynchronous communication provided by call/3
and cast/2
, "regular" messages sent by functions such as send/2
,
Process.send_after/4
and similar, can be handled inside the handle_info/2
callback.
handle_info/2
can be used in many situations, such as handling monitor
DOWN messages sent by Process.monitor/1
. Another use case for handle_info/2
is to perform periodic work, with the help of Process.send_after/4
:
defmodule MyApp.Periodically do
use GenServer
def start_link(_) do
GenServer.start_link(__MODULE__, %{})
end
@impl true
def init(state) do
# Schedule work to be performed on start
schedule_work()
{:ok, state}
end
@impl true
def handle_info(:work, state) do
# Do the desired work here
# ...
# Reschedule once more
schedule_work()
{:noreply, state}
end
defp schedule_work do
# We schedule the work to happen in 2 hours (written in milliseconds).
# Alternatively, one might write :timer.hours(2)
Process.send_after(self(), :work, 2 * 60 * 60 * 1000)
end
end
Timeouts
The return value of init/1
or any of the handle_*
callbacks may include
a timeout value in milliseconds; if not, :infinity
is assumed.
The timeout can be used to detect a lull in incoming messages.
The timeout()
value is used as follows:
If the process has any message already waiting when the
timeout()
value is returned, the timeout is ignored and the waiting message is handled as usual. This means that even a timeout of0
milliseconds is not guaranteed to execute (if you want to take another action immediately and unconditionally, use a:continue
instruction instead).If any message arrives before the specified number of milliseconds elapse, the timeout is cleared and that message is handled as usual.
Otherwise, when the specified number of milliseconds have elapsed with no message arriving,
handle_info/2
is called with:timeout
as the first argument.
When (not) to use a GenServer
So far, we have learned that a GenServer
can be used as a supervised process
that handles sync and async calls. It can also handle system messages, such as
periodic messages and monitoring events. GenServer processes may also be named.
A GenServer, or a process in general, must be used to model runtime characteristics of your system. A GenServer must never be used for code organization purposes.
In Elixir, code organization is done by modules and functions, processes are not necessary. For example, imagine you are implementing a calculator and you decide to put all the calculator operations behind a GenServer:
def add(a, b) do
GenServer.call(__MODULE__, {:add, a, b})
end
def subtract(a, b) do
GenServer.call(__MODULE__, {:subtract, a, b})
end
def handle_call({:add, a, b}, _from, state) do
{:reply, a + b, state}
end
def handle_call({:subtract, a, b}, _from, state) do
{:reply, a - b, state}
end
This is an anti-pattern not only because it convolutes the calculator logic but also because you put the calculator logic behind a single process that will potentially become a bottleneck in your system, especially as the number of calls grow. Instead just define the functions directly:
def add(a, b) do
a + b
end
def subtract(a, b) do
a - b
end
If you don't need a process, then you don't need a process. Use processes only to model runtime properties, such as mutable state, concurrency and failures, never for code organization.
Debugging with the :sys module
GenServers, as special processes,
can be debugged using the :sys
module.
Through various hooks, this module allows developers to introspect the state of
the process and trace system events that happen during its execution, such as
received messages, sent replies and state changes.
Let's explore the basic functions from the
:sys
module used for debugging:
:sys.get_state/2
- allows retrieval of the state of the process. In the case of a GenServer process, it will be the callback module state, as passed into the callback functions as last argument.:sys.get_status/2
- allows retrieval of the status of the process. This status includes the process dictionary, if the process is running or is suspended, the parent PID, the debugger state, and the state of the behaviour module, which includes the callback module state (as returned by:sys.get_state/2
). It's possible to change how this status is represented by defining the optionalGenServer.format_status/2
callback.:sys.trace/3
- prints all the system events to:stdio
.:sys.statistics/3
- manages collection of process statistics.:sys.no_debug/2
- turns off all debug handlers for the given process. It is very important to switch off debugging once we're done. Excessive debug handlers or those that should be turned off, but weren't, can seriously damage the performance of the system.:sys.suspend/2
- allows to suspend a process so that it only replies to system messages but no other messages. A suspended process can be reactivated via:sys.resume/2
.
Let's see how we could use those functions for debugging the stack server we defined earlier.
iex> {:ok, pid} = Stack.start_link([])
iex> :sys.statistics(pid, true) # turn on collecting process statistics
iex> :sys.trace(pid, true) # turn on event printing
iex> Stack.push(pid, 1)
*DBG* <0.122.0> got cast {push,1}
*DBG* <0.122.0> new state [1]
:ok
iex> :sys.get_state(pid)
[1]
iex> Stack.pop(pid)
*DBG* <0.122.0> got call pop from <0.80.0>
*DBG* <0.122.0> sent 1 to <0.80.0>, new state []
1
iex> :sys.statistics(pid, :get)
{:ok,
[
start_time: {{2016, 7, 16}, {12, 29, 41}},
current_time: {{2016, 7, 16}, {12, 29, 50}},
reductions: 117,
messages_in: 2,
messages_out: 0
]}
iex> :sys.no_debug(pid) # turn off all debug handlers
:ok
iex> :sys.get_status(pid)
{:status, #PID<0.122.0>, {:module, :gen_server},
[
[
"$initial_call": {Stack, :init, 1}, # process dictionary
"$ancestors": [#PID<0.80.0>, #PID<0.51.0>]
],
:running, # :running | :suspended
#PID<0.80.0>, # parent
[], # debugger state
[
header: 'Status for generic server <0.122.0>', # module status
data: [
{'Status', :running},
{'Parent', #PID<0.80.0>},
{'Logged events', []}
],
data: [{'State', [1]}]
]
]}
Learn more
If you wish to find out more about GenServers, the Elixir Getting Started guide provides a tutorial-like introduction. The documentation and links in Erlang can also provide extra insight.
Link to this section Summary
Types
Debug options supported by the start*
functions
Tuple describing the client of a call request.
The GenServer name
Return values of start*
functions
Option values used by the start*
functions
Options used by the start*
functions
The server reference.
Callbacks
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.
Invoked in some cases to retrieve a formatted version of the GenServer
status
Invoked to handle asynchronous cast/2
messages.
Invoked to handle continue instructions.
Invoked to handle all other messages.
Invoked when the server is started. start_link/3
or start/3
will
block until it returns.
Invoked when the server is about to exit. It should do any cleanup required.
Functions
Casts all servers locally registered as name
at the specified nodes.
Makes a synchronous call to the server
and waits for its reply.
Sends an asynchronous request to the server
.
Calls all servers locally registered as name
at the specified nodes
.
Replies to a client.
Starts a GenServer
process without links (outside of a supervision tree).
Starts a GenServer
process linked to the current process.
Synchronously stops the server with the given reason
.
Returns the pid
or {name, node}
of a GenServer process, nil
otherwise.
Link to this section Types
@type debug() :: [:trace | :log | :statistics | {:log_to_file, Path.t()}]
Debug options supported by the start*
functions
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.
The GenServer name
Return values of start*
functions
@type option() :: {:debug, debug()} | {:name, name()} | {:timeout, timeout()} | {:spawn_opt, [Process.spawn_opt()]} | {:hibernate_after, timeout()}
Option values used by the start*
functions
@type options() :: [option()]
Options used by the start*
functions
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.
Link to this section Callbacks
@callback code_change(old_vsn, state :: term(), extra :: term()) :: {:ok, new_state :: term()} | {:error, reason :: term()} when old_vsn: term() | {:down, term()}
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 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 format_status(reason, pdict_and_state :: list()) :: term() when reason: :normal | :terminate
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 theGenServer
; 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 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()
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.
Returning {:reply, reply, new_state, {:continue, continue_arg}}
is similar to
{:reply, reply, new_state}
except that handle_continue/2
will be invoked
immediately after with continue_arg
as the first argument and
state
as the second one.
Hibernating should not be used aggressively as too much time could be spent garbage collecting. Normally it should only be used when a message is not expected soon and minimising the memory of the process is shown to be beneficial.
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 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_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()
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 handle_call/3
for more information.
Returning {:noreply, new_state, {:continue, continue_arg}}
is similar to
{:noreply, new_state}
except 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 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_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()
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 handle_cast/2
.
This callback is optional. If one is not implemented, the server will fail if a continue instruction is used.
@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()
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 handle_cast/2
.
This callback is optional. If one is not implemented, the received message will be logged.
@callback init(init_arg :: term()) :: {:ok, state} | {:ok, state, timeout() | :hibernate | {:continue, continue_arg :: term()}} | :ignore | {:stop, reason :: any()} when state: any()
Invoked when the server is started. start_link/3
or start/3
will
block until it returns.
init_arg
is the argument term (second argument) passed to start_link/3
.
Returning {:ok, state}
will cause start_link/3
to return
{:ok, pid}
and the process to enter its loop.
Returning {:ok, state, timeout}
is similar to {:ok, state}
,
except that it also sets a timeout. See the "Timeouts" section
in the module documentation for more information.
Returning {:ok, state, :hibernate}
is similar to {:ok, state}
except the process is hibernated before entering the loop. See
handle_call/3
for more information on hibernation.
Returning {:ok, state, {:continue, continue_arg}}
is similar to
{:ok, state}
except that immediately after entering the loop,
the handle_continue/2
callback will be invoked with continue_arg
as the first argument and state
as the second one.
Returning :ignore
will cause start_link/3
to return :ignore
and
the process will exit normally without entering the loop or calling
terminate/2
. If used when part of a supervision tree the parent
supervisor will not fail to start nor immediately try to restart the
GenServer
. The remainder of the supervision tree will be started
and so the GenServer
should not be required by other processes.
It can be started later with Supervisor.restart_child/2
as the child
specification is saved in the parent supervisor. The main use cases for
this are:
- The
GenServer
is disabled by configuration but might be enabled later. - An error occurred and it will be handled by a different mechanism than the
Supervisor
. Likely this approach involves callingSupervisor.restart_child/2
after a delay to attempt a restart.
Returning {:stop, reason}
will cause start_link/3
to return
{:error, reason}
and the process to exit with reason reason
without
entering the loop or calling terminate/2
.
@callback terminate(reason, state :: term()) :: term() when reason: :normal | :shutdown | {:shutdown, term()} | term()
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.
terminate/2
is called if the GenServer
traps exits (using Process.flag/2
)
and the parent process sends an exit signal, or a callback (except init/1
)
does one of the following:
If part of a supervision tree, a GenServer
will receive an exit
signal 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
: theGenServer
is killed and soterminate/2
is not called.a timeout value, where the supervisor will send the exit signal
:shutdown
and theGenServer
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 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.
Therefore it is not guaranteed that terminate/2
is called when a GenServer
exits. For such reasons, we usually recommend important clean-up rules to
happen in separated processes either by use of monitoring or by links
themselves. There is no cleanup needed when the GenServer
controls a port
(for example,
:gen_tcp.socket
) or 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 terminate/2
.
If reason
is neither :normal
, :shutdown
, nor {:shutdown, term}
an error is
logged.
This callback is optional.
Link to this section Functions
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.
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. 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.
Sends an asynchronous request to the server
.
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 message.
server
can be any of the values described in the "Name registration"
section of the documentation for this module.
multi_call(nodes \\ [node() | Node.list()], name, request, timeout \\ :infinity)
View Source@spec multi_call([node()], name :: atom(), term(), timeout()) :: {replies :: [{node(), term()}], bad_nodes :: [node()]}
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 wherenode
is the node that replied andreply
is its replybad_nodes
- is a list of nodes that either did not exist or where a server with the givenname
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).
To avoid that late answers (after the timeout) pollute the caller's message queue, a middleman process is used to do the actual calls. Late answers will then be discarded when they arrive to a terminated process.
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}], []}
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 handle_call/3
.
client
must be the from
argument (the second argument) accepted by
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
Starts a GenServer
process without links (outside of a supervision tree).
See start_link/3
for more information.
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 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 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 init/1
callback.
Options
:name
- used for name registration as described in the "Name registration" section in the documentation forGenServer
: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 is invoked:spawn_opt
- if present, its value is passed as options to the underlying process as inProcess.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 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.
Synchronously stops the server with the given reason
.
The 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.
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)