Provided by: erlang-manpages_25.2.3+dfsg-1_all 
NAME
gen_server - Generic server behavior.
DESCRIPTION
This behavior module provides the server of a client-server relation. A generic server
process (gen_server) implemented using this module has a standard set of interface
functions and includes functionality for tracing and error reporting. It also fits into an
OTP supervision tree. For more information, see section gen_server Behaviour in OTP
Design Principles.
A gen_server process assumes all specific parts to be located in a callback module
exporting a predefined set of functions. The relationship between the behavior functions
and the callback functions is as follows:
gen_server module Callback module
----------------- ---------------
gen_server:start
gen_server:start_monitor
gen_server:start_link -----> Module:init/1
gen_server:stop -----> Module:terminate/2
gen_server:call
gen_server:send_request
gen_server:multi_call -----> Module:handle_call/3
gen_server:cast
gen_server:abcast -----> Module:handle_cast/2
- -----> Module:handle_info/2
- -----> Module:handle_continue/2
- -----> Module:terminate/2
- -----> Module:code_change/3
If a callback function fails or returns a bad value, the gen_server process terminates.
A gen_server process handles system messages as described in sys(3erl). The sys module can
be used for debugging a gen_server process.
Notice that a gen_server process does not trap exit signals automatically, this must be
explicitly initiated in the callback module.
Unless otherwise stated, all functions in this module fail if the specified gen_server
process does not exist or if bad arguments are specified.
The gen_server process can go into hibernation (see erlang:hibernate/3) if a callback
function specifies 'hibernate' instead of a time-out value. This can be useful if the
server is expected to be idle for a long time. However, use this feature with care, as
hibernation implies at least two garbage collections (when hibernating and shortly after
waking up) and is not something you want to do between each call to a busy server.
If the gen_server process needs to perform an action immediately after initialization or
to break the execution of a callback into multiple steps, it can return
{continue,Continue} in place of the time-out or hibernation value, which will immediately
invoke the handle_continue/2 callback.
If the gen_server process terminates, e.g. as a result of a function in the callback
module returning {stop,Reason,NewState}, an exit signal with this Reason is sent to linked
processes and ports. See Processes in the Reference Manual for details regarding error
handling using exit signals.
DATA TYPES
server_name() =
{local, LocalName :: atom()} |
{global, GlobalName :: term()} |
{via, RegMod :: module(), ViaName :: term()}
Name specification to use when starting a gen_server. See functions start/3,4,
start_link/3,4, start_monitor/3,4, enter_loop/3,4,5, and the type server_ref()
below.
{local,LocalName}:
Register the gen_server locally as LocalName using register/2.
{global,GlobalName}:
Register the gen_server process id globally as GlobalName using
global:register_name/2.
{via,RegMod,ViaName}:
Register the gen_server process with the registry represented by RegMod. The
RegMod callback is to export the functions register_name/2, unregister_name/1,
whereis_name/1, and send/2, which are to behave like the corresponding
functions in global. Thus, {via,global,GlobalName} is a valid reference
equivalent to {global,GlobalName}.
server_ref() =
pid() |
(LocalName :: atom()) |
{Name :: atom(), Node :: atom()} |
{global, GlobalName :: term()} |
{via, RegMod :: module(), ViaName :: term()}
Server specification to use when addressing a gen_server. See call/2,3, cast/2,
send_request/2, check_response/2, wait_response/2, stop/2,3 and the type
server_name() above.
It can be:
pid():
The gen_server's process identifier.
LocalName:
The gen_server is locally registered as LocalName with register/2.
{Name,Node}:
The gen_server is locally registered on another node.
{global,GlobalName}:
The gen_server is globally registered in global.
{via,RegMod,ViaName}:
The gen_server is registered in an alternative process registry. The registry
callback module RegMod is to export functions register_name/2,
unregister_name/1, whereis_name/1, and send/2, which are to behave like the
corresponding functions in global. Thus, {via,global,GlobalName} is the same as
{global,GlobalName}.
start_opt() =
{timeout, Timeout :: timeout()} |
{spawn_opt, SpawnOptions :: [proc_lib:spawn_option()]} |
enter_loop_opt()
Options that can be used when starting a gen_server server through, for example,
start_link/3,4.
{timeout,Timeout}:
How many milliseconds the gen_server process is allowed to spend initializing
or it is terminated and the start function returns {error,timeout}.
{spawn_opt,SpawnOptions}:
The SpawnOptions option list is passed to the function used to spawn the
gen_server; see spawn_opt/2.
Note:
Using spawn option monitor is not allowed, it causes a badarg failure.
enter_loop_opt():
See the type enter_loop_opt() below for more start options that are also
allowed by enter_loop/3,4,5.
enter_loop_opt() =
{hibernate_after, HibernateAfterTimeout :: timeout()} |
{debug, Dbgs :: [sys:debug_option()]}
Options that can be used when starting a gen_server server through enter_loop/3-5
or the start functions such as start_link/3,4.
{hibernate_after,HibernateAfterTimeout}:
Specifies that the gen_server process awaits any message for
HibernateAfterTimeout milliseconds and if no message is received, the process
goes into hibernation automatically (by calling proc_lib:hibernate/3).
{debug,Dbgs}:
For every entry in Dbgs, the corresponding function in sys(3erl) is called.
start_ret() =
{ok, Pid :: pid()} | ignore | {error, Reason :: term()}
Return value from the start/3,4 and start_link/3,4 functions.
{ok,Pid}:
The gen_server process was succesfully created and initialized, with the
process identifier Pid.
{error,{already_started,OtherPid}}:
A process with the specified ServerName exists already with the process
identifier OtherPid. This gen_server was not started.
ignore:
Module:init/1 returned ignore, the gen_server process is terminated. An exit
signal normal is sent to linked processes and ports.
{error,Reason}:
Module:init/1 returned {stop,Reason} or failed with Reason. The gen_server
process is terminated and an exit signal with the same Reason is sent to linked
processes and ports.
start_mon_ret() =
{ok, {Pid :: pid(), MonRef :: reference()}} |
ignore |
{error, Reason :: term()}
Return value from the start_monitor/3,4 functions. The same as type start_ret()
except that for a succesful start it returns both the process identifier Pid and a
monitor/2,3 reference() MonRef.
from() = {Client :: pid(), Tag :: reply_tag()}
Destination, given to the gen_server as the first argument to the callback function
Module:handle_call/3, to be used by the when replying through reply/2 (instead of
through the callback function's return value) to the process Client that has called
the gen_server using call/2,3. Tag is a term that is unique for this call/request
instance.
reply_tag()
A handle that associates a reply to the corresponding request.
request_id()
An opaque request identifier. See send_request/2 for details.
request_id_collection()
An opaque collection of request identifiers (request_id()) where each request
identifier can be associated with a label chosen by the user. For more information
see reqids_new/0.
response_timeout() = timeout() | {abs, integer()}
Used to set a time limit on how long to wait for a response using either
receive_response/2, receive_response/3, wait_response/2, or wait_response/3. The
time unit used is millisecond. Currently valid values:
0..4294967295:
Timeout relative to current time in milliseconds.
infinity:
Infinite timeout. That is, the operation will never time out.
{abs, Timeout}:
An absolute Erlang monotonic time timeout in milliseconds. That is, the
operation will time out when erlang:monotonic_time(millisecond) returns a value
larger than or equal to Timeout. Timeout is not allowed to identify a time
further into the future than 4294967295 milliseconds. Identifying the timeout
using an absolute timeout value is especially handy when you have a deadline
for responses corresponding to a complete collection of requests
(request_id_collection()) , since you do not have to recalculate the relative
time until the deadline over and over again.
format_status() =
#{state => term(),
message => term(),
reason => term(),
log => [sys:system_event()]}
A map that describes the gen_server status. The keys are:
state:
The internal state of the gen_server process.
message:
The message that caused the server to terminate.
reason:
The reason that caused the server to terminate.
log:
The sys log of the server.
New associations may be added to the status map without prior notice.
EXPORTS
abcast(Name :: atom(), Request :: term()) -> abcast
abcast(Nodes :: [node()], Name :: atom(), Request :: term()) ->
abcast
Sends an asynchronous request to the gen_server processes locally registered as
Name at the specified nodes. The function returns immediately and ignores nodes
that do not exist, or where the gen_server Name does not exist. The gen_server
processes call Module:handle_cast/2 to handle the request.
For a description of the arguments, see multi_call/2,3,4.
call(ServerRef :: server_ref(), Request :: term()) ->
Reply :: term()
call(ServerRef :: server_ref(),
Request :: term(),
Timeout :: timeout()) ->
Reply :: term()
Makes a synchronous call to the ServerRef of the gen_server process by sending a
request and waiting until a reply arrives or a time-out occurs. The gen_server
process calls Module:handle_call/3 to handle the request.
See also ServerRef's type server_ref().
Request is any term that is passed as the first argument to Module:handle_call/3.
Timeout is an integer that specifies how many milliseconds to wait for a reply, or
the atom infinity to wait indefinitely. Defaults to 5000. If no reply is received
within the specified time, this function exits the calling process with an exit
term containing Reason = timeout as described below.
Note:
Before OTP 24, if the caller uses (try...)catch to avoid process exit, and the
server happens to just be late with the reply, it may arrive to the process message
queue any time later. The calling process must therefore after catching a time-out
exit be prepared to receive garbage message(s) on the form {reference(), _} and
deal with them appropriately (discard them) so they do not clog the process message
queue or gets mistaken for other messages.
Starting with OTP 24, gen_server:call uses process aliases, so late replies will
not be received.
The return value Reply is passed from the return value of Module:handle_call/3.
This call may exit the calling process with an exit term on the form {Reason,
Location} where Location = {gen_server,call,ArgList} and Reason can be (at least)
one of:
timeout:
The call was aborted after waiting Timeout milliseconds for a reply, as
described above.
noproc:
The ServerRef refers to a server by name (it is not a pid()) and looking up the
server process failed, or the pid() was already terminated.
{nodedown,Node}:
The ServerRef refers to a server on the remote node Node and the connection to
that node failed.
calling_self:
A call to self() would hang indefinitely.
shutdown
: The server was stopped during the call by its supervisor. See also stop/3.
normal
{shutdown,Term}
: The server stopped during the call by returning {stop,Reason,_} from its
Module:handle_call/3 callback, without replying. See also stop/3.
_OtherTerm:
The server process exited during the call, with reason Reason. Either by
returning {stop,Reason,_} from its Module:handle_call/3 callback (without
replying), by raising an exception, or due to getting an exit signal it did not
trap.
cast(ServerRef :: server_ref(), Request :: term()) -> ok
Sends an asynchronous request to the ServerRef of the gen_server process and
returns ok immediately, ignoring if the destination node or gen_server process does
not exist. The gen_server process calls Module:handle_cast/2 to handle the request.
See also ServerRef's type server_ref().
Request is any term that is passed as the first argument to Module:handle_cast/2.
check_response(Msg, ReqId) -> Result
Types:
Msg = term()
ReqId = request_id()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result = Response | no_reply
Check if Msg is a response corresponding to the request identifier ReqId. The
request must have been made by send_request/2, and it must have been made by the
same process calling this function.
If Msg is a response corresponding to ReqId the response is returned; otherwise,
no_reply is returned and no cleanup is done, and thus the function must be invoked
repeatedly until a response is returned.
The return value Reply is passed from the return value of Module:handle_call/3.
The function returns an error if the gen_server died before a reply was sent.
check_response(Msg, ReqIdCollection, Delete) -> Result
Types:
Msg = term()
ReqIdCollection = request_id_collection()
Delete = boolean()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result =
{Response,
Label :: term(),
NewReqIdCollection :: request_id_collection()} |
no_request | no_reply
Check if Msg is a response corresponding to a request identifier saved in
ReqIdCollection. All request identifiers of ReqIdCollection must correspond to
requests that have been made using send_request/2 or send_request/4, and all
request must have been made by the process calling this function.
The Label in the response equals the Label associated with the request identifier
that the response corresponds to. The Label of a request identifier is associated
when saving the request id in a request identifier collection, or when sending the
request using send_request/4.
Compared to check_response/2, the returned result associated with a specific
request identifier or an exception associated with a specific request identifier
will be wrapped in a 3-tuple. The first element of this tuple equals the value that
would have been produced by check_response/2, the second element equals the Label
associated with the specific request identifier, and the third element
NewReqIdCollection is a possibly modified request identifier collection.
If ReqIdCollection is empty, the atom no_request will be returned. If Msg does not
correspond to any of the request identifiers in ReqIdCollection, the atom no_reply
is returned.
If Delete equals true, the association with Label will have been deleted from
ReqIdCollection in the resulting NewReqIdCollection. If Delete equals false,
NewReqIdCollection will equal ReqIdCollection. Note that deleting an association is
not for free and that a collection containing already handled requests can still be
used by subsequent calls to check_response/3, receive_response/3, and
wait_response/3. However, without deleting handled associations, the above calls
will not be able to detect when there are no more outstanding requests to handle,
so you will have to keep track of this some other way than relying on a no_request
return. Note that if you pass a collection only containing associations of already
handled or abandoned requests to check_response/3, it will always return no_reply.
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term()) ->
no_return()
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
ServerName :: server_name() | pid()) ->
no_return()
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
Timeout :: timeout()) ->
no_return()
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
Hibernate :: hibernate) ->
no_return()
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
Cont :: {continue, term()}) ->
no_return()
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
ServerName :: server_name() | pid(),
Timeout :: timeout()) ->
no_return()
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
ServerName :: server_name() | pid(),
Hibernate :: hibernate) ->
no_return()
enter_loop(Module :: module(),
Options :: [enter_loop_opt()],
State :: term(),
ServerName :: server_name() | pid(),
Cont :: {continue, term()}) ->
no_return()
Makes an existing process a gen_server process. Does not return, instead the
calling process enters the gen_server process receive loop and becomes a gen_server
process. The process must have been started using one of the start functions in
proc_lib(3erl). The user is responsible for any initialization of the process,
including registering a name for it.
This function is useful when a more complex initialization procedure is needed than
the gen_server process behavior provides.
Module, Options, and ServerName have the same meanings as when calling
start[_link|_monitor]/3,4 or it can be self() for an anonymous server, which is the
same as calling an enter_loop/3,4 function without a ServerName argument. However,
if ServerName is specified (and not as self()), the process must have been
registered accordingly before this function is called.
State, Timeout, Hibernate and Cont have the same meanings as in the return value of
Module:init/1, which is not called when enter_loop/3,4,5 is used. Note that to
adhere to the gen_server Behaviour such a callback function needs to be defined,
and it might as well be the one used when starting the gen_server process through
proc_lib, and then be the one that calls enter_loop/3,4,5. But if such a
Module:init/1 function in for example error cases cannot call enter_loop/3,4,5, it
should return a value that follows the type specification for Module:init/1 such as
ignore, although that value will be lost when returning to the spawning function.
This function fails if the calling process was not started by a proc_lib start
function, or if it is not registered according to ServerName.
multi_call(Name :: atom(), Request :: term()) ->
{Replies :: [{Node :: node(), Reply :: term()}],
BadNodes :: [node()]}
multi_call(Nodes :: [node()], Name :: atom(), Request :: term()) ->
{Replies :: [{Node :: node(), Reply :: term()}],
BadNodes :: [node()]}
multi_call(Nodes :: [node()],
Name :: atom(),
Request :: term(),
Timeout :: timeout()) ->
{Replies :: [{Node :: node(), Reply :: term()}],
BadNodes :: [node()]}
Makes a synchronous call to all gen_server processes locally registered as Name at
the specified nodes, by first sending the request to the nodes, and then waiting
for the replies. The gen_server processes on the nodes call Module:handle_call/3 to
handle the request.
The function returns a tuple {Replies,BadNodes}, where Replies is a list of
{Node,Reply} tuples, and BadNodes is a list of nodes that either did not exist,
where Name was not a registered gen_server, or where it did not reply.
Nodes is a list of node names to which the request is to be sent. Default value is
the list of all known nodes [node()|nodes()].
Name is the locally registered name for each gen_server process.
Request is any term that is passed as the first argument to Module:handle_call/3.
Timeout is an integer that specifies how many milliseconds to wait for all replies,
or the atom infinity to wait indefinitely, which is the default. If no reply is
received from a node within the specified time, the node is added to BadNodes.
When a reply Reply is received from the gen_server process at a node Node,
{Node,Reply} is added to Replies. Reply is passed from the return value of
Module:handle_call/3.
Warning:
If one of the nodes cannot process monitors, for example, C or Java nodes, and the
gen_server process is not started when the requests are sent, but starts within 2
seconds, this function waits the whole Timeout, which may be infinity.
This problem does not exist if all nodes are Erlang nodes.
To prevent late answers (after the time-out) from polluting the message queue of
the caller, a middleman process is used to do the calls. Late answers are then
discarded when they arrive to a terminated process.
receive_response(ReqId, Timeout) -> Result
Types:
ReqId = request_id()
Timeout = response_timeout()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result = Response | timeout
Receive a response corresponding to the request identifier ReqId. The request must
have been made by send_request/2, and it must have been made by the same process
calling this function.
Timeout specifies how long to wait for a response. If no response is received
within the specified time, the function returns timeout. Assuming that the server
executes on a node supporting aliases (introduced in OTP 24) the request will also
be abandoned. That is, no response will be received after a timeout. Otherwise, a
stray response might be received at a later time.
The return value Reply is passed from the return value of Module:handle_call/3.
The function returns an error if the gen_server died before a reply was sent.
The difference between receive_response/2 and wait_response/2 is that
receive_response/2 abandons the request at timeout so that a potential future
response is ignored, while wait_response/2 does not.
receive_response(ReqIdCollection, Timeout, Delete) -> Result
Types:
ReqIdCollection = request_id_collection()
Timeout = response_timeout()
Delete = boolean()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result =
{Response,
Label :: term(),
NewReqIdCollection :: request_id_collection()} |
no_request | timeout
Receive a response corresponding to a request identifier saved in ReqIdCollection.
All request identifiers of ReqIdCollection must correspond to requests that have
been made using send_request/2 or send_request/4, and all request must have been
made by the process calling this function.
The Label in the response equals the Label associated with the request identifier
that the response corresponds to. The Label of a request identifier is associated
when adding the request id in a request identifier collection, or when sending the
request using send_request/4.
Compared to receive_response/2, the returned result associated with a specific
request identifier will be wrapped in a 3-tuple. The first element of this tuple
equals the value that would have been produced by receive_response/2, the second
element equals the Label associated with the specific request identifier, and the
third element NewReqIdCollection is a possibly modified request identifier
collection.
If ReqIdCollection is empty, the atom no_request will be returned.
Timeout specifies how long to wait for a response. If no response is received
within the specified time, the function returns timeout. Assuming that the server
executes on a node supporting aliases (introduced in OTP 24) all requests
identified by ReqIdCollection will also be abandoned. That is, no responses will be
received after a timeout. Otherwise, stray responses might be received at a later
time.
The difference between receive_response/3 and wait_response/3 is that
receive_response/3 abandons the requests at timeout so that potential future
responses are ignored, while wait_response/3 does not.
If Delete equals true, the association with Label will have been deleted from
ReqIdCollection in the resulting NewReqIdCollection. If Delete equals false,
NewReqIdCollection will equal ReqIdCollection. Note that deleting an association is
not for free and that a collection containing already handled requests can still be
used by subsequent calls to receive_response/3, check_response/3, and
wait_response/3. However, without deleting handled associations, the above calls
will not be able to detect when there are no more outstanding requests to handle,
so you will have to keep track of this some other way than relying on a no_request
return. Note that if you pass a collection only containing associations of already
handled or abandoned requests to receive_response/3, it will always block until a
timeout determined by Timeout is triggered.
reply(Client :: from(), Reply :: term()) -> ok
This function can be used by a gen_server process to explicitly send a reply to a
client that called call/2,3 or multi_call/2,3,4, when the reply cannot be passed in
the return value of Module:handle_call/3.
Client must be the From argument provided to the handle_call callback function.
Reply is any term passed back to the client as the return value of call/2,3 or
multi_call/2,3,4.
reqids_add(ReqId :: request_id(),
Label :: term(),
ReqIdCollection :: request_id_collection()) ->
NewReqIdCollection :: request_id_collection()
Saves ReqId and associates a Label with the request identifier by adding this
information to ReqIdCollection and returning the resulting request identifier
collection.
reqids_new() -> NewReqIdCollection :: request_id_collection()
Returns a new empty request identifier collection. A request identifier collection
can be utilized in order the handle multiple outstanding requests.
Request identifiers of requests made by send_request/2 can be saved in a request
identifier collection using reqids_add/3. Such a collection of request identifiers
can later be used in order to get one response corresponding to a request in the
collection by passing the collection as argument to receive_response/3,
wait_response/3, or, check_response/3.
reqids_size/1 can be used to determine the amount of request identifiers in a
request identifier collection.
reqids_size(ReqIdCollection :: request_id_collection()) ->
integer() >= 0
Returns the amount of request identifiers saved in ReqIdCollection.
reqids_to_list(ReqIdCollection :: request_id_collection()) ->
[{ReqId :: request_id(), Label :: term()}]
Returns a list of {ReqId, Label} tuples which corresponds to all request
identifiers with their associated labels present in the ReqIdCollection collection.
send_request(ServerRef :: server_ref(), Request :: term()) ->
ReqId :: request_id()
Sends an asynchronous call request Request to the gen_server process identified by
ServerRef and returns a request identifier ReqId. The return value ReqId shall
later be used with receive_response/2, wait_response/2, or check_response/2 to
fetch the actual result of the request. Besides passing the request identifier
directly to these functions, it can also be saved in a request identifier
collection using reqids_add/3. Such a collection of request identifiers can later
be used in order to get one response corresponding to a request in the collection
by passing the collection as argument to receive_response/3, wait_response/3, or
check_response/3. If you are about to save the request identifier in a request
identifier collection, you may want to consider using send_request/4 instead.
The call gen_server:receive_response(gen_server:send_request(ServerRef, Request),
Timeout) can be seen as equivalent to gen_server:call(ServerRef, Request, Timeout),
ignoring the error handling.
The gen_server process calls Module:handle_call/3 to handle the request.
See the type server_ref() for the possible values for ServerRef.
Request is any term that is passed as the first argument to Module:handle_call/3.
send_request(ServerRef :: server_ref(),
Request :: term(),
Label :: term(),
ReqIdCollection :: request_id_collection()) ->
NewReqIdCollection :: request_id_collection()
Sends an asynchronous call request Request to the gen_server process identified by
ServerRef. The Label will be associated with the request identifier of the
operation and added to the returned request identifier collection
NewReqIdCollection. The collection can later be used in order to get one response
corresponding to a request in the collection by passing the collection as argument
to receive_response/3, wait_response/3, or, check_response/3.
The same as calling gen_server:reqids_add(gen_server:send_request(ServerRef,
Request), Label, ReqIdCollection), but calling send_request/4 is slightly more
efficient.
start(Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_ret()
start(ServerName :: server_name(),
Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_ret()
Creates a standalone gen_server process, that is, a gen_server process that is not
part of a supervision tree and thus has no supervisor.
Other than that see start_link/3,4.
start_link(Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_ret()
start_link(ServerName :: server_name(),
Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_ret()
Creates a gen_server process as part of a supervision tree. This function is to be
called, directly or indirectly, by the supervisor. For example, it ensures that the
gen_server process is linked to the supervisor.
The gen_server process calls Module:init/1 to initialize. To ensure a synchronized
startup procedure, start_link/3,4 does not return until Module:init/1 has returned.
Using the argument ServerName creates a gen_server with a registered name. See type
server_name() for different name registrations. If no ServerName is provided, the
gen_server process is not registered.
Module is the name of the callback module.
Args is any term that is passed as the argument to Module:init/1.
See type start_opt() for Options when starting the gen_server process.
See type start_ret() for a description this function's return values.
start_monitor(Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_mon_ret()
start_monitor(ServerName :: server_name(),
Module :: module(),
Args :: term(),
Options :: [start_opt()]) ->
start_mon_ret()
Creates a standalone gen_server process, that is, a gen_server process that is not
part of a supervision tree (and thus has no supervisor) and atomically sets up a
monitor to the newly created server.
Other than that see start_link/3,4. Note that the return value for a successful
start differs in that it returns a monitor reference. See type start_mon_ret().
If the start is not successful, the caller will be blocked until the monitor's
'DOWN' message has been received and removed from the message queue.
stop(ServerRef :: server_ref()) -> ok
stop(ServerRef :: server_ref(),
Reason :: term(),
Timeout :: timeout()) ->
ok
Orders the generic server specified by ServerRef to exit with the specified Reason,
default 'normal', and waits for it to terminate. The gen_server process calls
Module:terminate/2 before exiting.
The function returns ok if the server terminates with the expected reason. Any
other reason than normal, shutdown, or {shutdown,Term} causes an error report to be
issued using logger(3erl). An exit signal with the same reason is sent to linked
processes and ports.
Timeout is an integer that specifies how many milliseconds to wait for the server
to terminate, or the atom infinity to wait indefinitely, which is the default. If
the server has not terminated within the specified time, the call exits the calling
process with reason timeout.
If the process does not exist, the call exits the calling process with reason
noproc, and with reason {nodedown,Node} if the connection fails to the remote Node
where the server runs.
wait_response(ReqId, WaitTime) -> Result
Types:
ReqId = request_id()
WaitTime = response_timeout()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result = Response | timeout
Wait for a response corresponding to the request identifier ReqId. The request must
have been made by send_request/2, and it must have been made by the same process
calling this function.
WaitTime specifies how long to wait for a reply. If no reply is received within the
specified time, the function returns timeout and no cleanup is done, and thus the
function can be invoked repeatedly until a reply is returned.
The return value Reply is passed from the return value of Module:handle_call/3.
The function returns an error if the gen_server died before a reply was sent.
The difference between receive_response/2 and wait_response/2 is that
receive_response/2 abandons the request at time-out so that a potential future
response is ignored, while wait_response/2 does not.
wait_response(ReqIdCollection, WaitTime, Delete) -> Result
Types:
ReqIdCollection = request_id_collection()
WaitTime = response_timeout()
Delete = boolean()
Response =
{reply, Reply :: term()} |
{error, {Reason :: term(), server_ref()}}
Result =
{Response,
Label :: term(),
NewReqIdCollection :: request_id_collection()} |
no_request | timeout
Wait for a response corresponding to a request identifier saved in ReqIdCollection.
All request identifiers of ReqIdCollection must correspond to requests that have
been made using send_request/2 or send_request/4, and all request must have been
made by the process calling this function.
The Label in the response equals the Label associated with the request identifier
that the response corresponds to. The Label of a request identifier is associated
when saving the request id in a request identifier collection, or when sending the
request using send_request/4.
Compared to wait_response/2, the returned result associated with a specific request
identifier or an exception associated with a specific request identifier will be
wrapped in a 3-tuple. The first element of this tuple equals the value that would
have been produced by wait_response/2, the second element equals the Label
associated with the specific request identifier, and the third element
NewReqIdCollection is a possibly modified request identifier collection.
If ReqIdCollection is empty, no_request will be returned. If no response is
received before the WaitTime timeout has triggered, the atom timeout is returned.
It is valid to continue waiting for a response as many times as needed up until a
response has been received and completed by check_response(), receive_response(),
or wait_response().
The difference between receive_response/3 and wait_response/3 is that
receive_response/3 abandons requests at timeout so that a potential future
responses are ignored, while wait_response/3 does not.
If Delete equals true, the association with Label will have been deleted from
ReqIdCollection in the resulting NewReqIdCollection. If Delete equals false,
NewReqIdCollection will equal ReqIdCollection. Note that deleting an association is
not for free and that a collection containing already handled requests can still be
used by subsequent calls to wait_response/3, check_response/3, and
receive_response/3. However, without deleting handled associations, the above calls
will not be able to detect when there are no more outstanding requests to handle,
so you will have to keep track of this some other way than relying on a no_request
return. Note that if you pass a collection only containing associations of already
handled or abandoned requests to wait_response/3, it will always block until a
timeout determined by WaitTime is triggered and then return no_reply.
CALLBACK FUNCTIONS
The following functions are to be exported from a gen_server callback module.
EXPORTS
Module:code_change(OldVsn, State, Extra) -> {ok, NewState} | {error, Reason}
Types:
OldVsn = Vsn | {down, Vsn}
Vsn = term()
State = NewState = term()
Extra = term()
Reason = term()
Note:
This callback is optional, so callback modules need not export it. If a release
upgrade/downgrade with Change={advanced,Extra} specified in the appup file is made
when code_change/3 isn't implemented the process will crash with an undef exit
reason.
This function is called by a gen_server process when it is to update its internal
state during a release upgrade/downgrade, that is, when the instruction
{update,Module,Change,...}, where Change={advanced,Extra}, is specifed in the appup
file. For more information, see section Release Handling Instructions in OTP
Design Principles.
For an upgrade, OldVsn is Vsn, and for a downgrade, OldVsn is {down,Vsn}. Vsn is
defined by the vsn attribute(s) of the old version of the callback module Module.
If no such attribute is defined, the version is the checksum of the Beam file.
State is the internal state of the gen_server process.
Extra is passed "as is" from the {advanced,Extra} part of the update instruction.
If successful, the function must return the updated internal state.
If the function returns {error,Reason}, the ongoing upgrade fails and rolls back to
the old release.
Module:format_status(Status) -> NewStatus
Types:
Status = format_status()
NewStatus = format_status()
Note:
This callback is optional, so callback modules need not export it. The gen_server
module provides a default implementation of this function that returns the callback
module state.
If this callback is exported but fails, to hide possibly sensitive data, the
default function will instead return the fact that format_status/1 has crashed.
This function is called by a gen_server process in the following situations:
* sys:get_status/1,2 is invoked to get the gen_server status.
* The gen_server process terminates abnormally and logs an error.
This callback is used to limit the status of the process returned by
sys:get_status/1,2 or sent to logger.
The callback gets a map Status describing the current status and shall return a map
NewStatus with the same keys, but it may transform some values.
Two possible use cases for this callback is to remove sensitive information from
the state to prevent it from being printed in log files, or to compact large
irrelevant status items that would only clutter the logs.
Example:
format_status(Status) ->
maps:map(
fun(state,State) ->
maps:remove(private_key, State);
(message,{password, _Pass}) ->
{password, removed};
(_,Value) ->
Value
end, Status).
Module:format_status(Opt, [PDict, State]) -> Status
Types:
Opt = normal | terminate
PDict = [{Key, Value}]
State = term()
Status = term()
Warning:
This callback is deprecated, in new code use format_status/1. If a format_status/1
callback exists, then this function will never be called.
Note:
This callback is optional, so callback modules need not export it. The gen_server
module provides a default implementation of this function that returns the callback
module state.
This function is called by a gen_server process in the following situations:
* One of sys:get_status/1,2 is invoked to get the gen_server status. Opt is set
to the atom normal.
* The gen_server process terminates abnormally and logs an error. Opt is set to
the atom terminate.
This function is useful for changing the form and appearance of the gen_server
status for these cases. A callback module wishing to change the sys:get_status/1,2
return value, as well as how its status appears in termination error logs, exports
an instance of format_status/2 that returns a term describing the current status of
the gen_server process.
PDict is the current value of the process dictionary of the gen_server process..
State is the internal state of the gen_server process.
The function is to return Status, a term that changes the details of the current
state and status of the gen_server process. There are no restrictions on the form
Status can take, but for the sys:get_status/1,2 case (when Opt is normal), the
recommended form for the Status value is [{data, [{"State", Term}]}], where Term
provides relevant details of the gen_server state. Following this recommendation is
not required, but it makes the callback module status consistent with the rest of
the sys:get_status/1,2 return value.
One use for this function is to return compact alternative state representations to
avoid that large state terms are printed in log files.
Module:handle_call(Request, From, State) -> Result
Types:
Request = term()
From = from()
State = term()
Result = {reply,Reply,NewState}
| {reply,Reply,NewState,Timeout}
| {reply,Reply,NewState,hibernate}
| {reply,Reply,NewState,{continue,Continue}}
| {noreply,NewState}
| {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {noreply,NewState,{continue,Continue}}
| {stop,Reason,Reply,NewState}
| {stop,Reason,NewState}
Reply = term()
NewState = term()
Timeout = timeout()
Continue = term()
Reason = term()
Whenever a gen_server process receives a request sent using call/2,3 or
multi_call/2,3,4, this function is called to handle the request.
State is the internal state of the gen_server process, and NewState a possibly
updated one.
Request is passed from the same argument provided to call or multi_call.
The return value Result is interpreted as follows:
{reply,Reply,NewState}
{reply,Reply,NewState,_}: The Reply value is sent back to the client request
and there becomes its return value.
The gen_server process continues executing with the possibly updated internal
state NewState.
{noreply,NewState}
{noreply,NewState,_}: The gen_server process continues executing with the
possibly updated internal state NewState.
A reply to the client request has to be created by calling reply(From, Reply),
either in this or in a later callback.
{reply,_,_,Timeout}
{noreply,_,Timeout}: If an integer Timeout is provided, a time-out occurs
unless a request or a message is received within that many milliseconds. A
time-out is represented by the atom timeout to be handled by the
Module:handle_info/2 callback function. Timeout =:= infinity can be used to
wait indefinitely, which is the same as returning a value without a Timeout
member.
{reply,_,_,hibernate}
{noreply,_,hibernate}: The process goes into hibernation waiting for the next
message to arrive (by calling proc_lib:hibernate/3).
{reply,_,_,{continue,Continue}}
{noreply,_,{continue,Continue}}: The process will execute the
Module:handle_continue/2 callback function, with Continue as the first
argument.
{stop,Reason,NewState}
{stop,Reason,Reply,NewState}: The gen_server process will call
Module:terminate(Reason,NewState) and then terminate.
{stop,_,Reply,_} will create a reply to the client request just as
{reply,Reply,...} while {stop,_,_} will not, so just as for
{noreply,NewState,...} a reply has to be created by calling reply(From, Reply)
before returning {stop,_,_}.
Module:handle_cast(Request, State) -> Result
Types:
Request = term()
State = term()
Result = {noreply,NewState}
| {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {noreply,NewState,{continue,Continue}}
| {stop,Reason,NewState}
NewState = term()
Timeout = timeout()
Continue = term()
Reason = term()
Whenever a gen_server process receives a request sent using cast/2 or abcast/2,3,
this function is called to handle the request.
For a description of the arguments and possible return values, see
Module:handle_call/3.
Module:handle_continue(Continue, State) -> Result
Types:
Continue = term()
State = term()
Result = {noreply,NewState}
| {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {noreply,NewState,{continue,Continue}}
| {stop,Reason,NewState}
NewState = term()
Timeout = timeout()
Continue = term()
Reason = normal | term()
Note:
This callback is optional, so callback modules need to export it only if they
return one of the tuples containing {continue,Continue} from another callback. If
such a {continue,_} tuple is used and the callback is not implemented, the process
will exit with undef error.
This function is called by a gen_server process whenever a previous callback
returns one of the tuples containing {continue, Continue}. handle_continue/2 is
invoked immediately after the previous callback, which makes it useful for
performing work after initialization or for splitting the work in a callback in
multiple steps, updating the process state along the way.
For a description of the other arguments and possible return values, see
Module:handle_call/3.
Module:handle_info(Info, State) -> Result
Types:
Info = timeout | term()
State = term()
Result = {noreply,NewState}
| {noreply,NewState,Timeout}
| {noreply,NewState,hibernate}
| {noreply,NewState,{continue,Continue}}
| {stop,Reason,NewState}
NewState = term()
Timeout = timeout()
Reason = normal | term()
Note:
This callback is optional, so callback modules need not export it. The gen_server
module provides a default implementation of this function that logs about the
unexpected Info message, drops it and returns {noreply, State}.
This function is called by a gen_server process when a time-out occurs or when it
receives any other message than a synchronous or asynchronous request (or a system
message).
Info is either the atom timeout, if a time-out has occurred, or the received
message.
For a description of the other arguments and possible return values, see
Module:handle_call/3.
Module:init(Args) -> Result
Types:
Args = term()
Result = {ok,State}
| {ok,State,Timeout}
| {ok,State,hibernate}
| {ok,State,{continue,Continue}}
| {stop,Reason}
| ignore
State = term()
Timeout = timeout()
Reason = term()
Whenever a gen_server process is started using start/3,4, start_monitor/3,4, or
start_link/3,4, this function is called by the new process to initialize.
Args is the Args argument provided to the start function.
The return value Result is interpreted as follows:
{ok,State}
{ok,State,_}: Initialization was succesful and State is the internal state of
the gen_server process.
{ok,_,Timeout}
{ok,_,hibernate}
{ok,_,{continue,Continue}}: See the corresponding return values from
Module:handle_call/3 for a description of this tuple member.
{stop,Reason}
ignore: Initialization failed. An exit signal with this Reason (or with reason
normal if ignore is returned) is sent to linked processes and ports, notably to
the process starting the gen_server when start_link/3,4 is used.
Module:terminate(Reason, State)
Types:
Reason = normal | shutdown | {shutdown,term()} | term()
State = term()
Note:
This callback is optional, so callback modules need not export it. The gen_server
module provides a default implementation without cleanup.
This function is called by a gen_server process when it is about to terminate. It
is to be the opposite of Module:init/1 and do any necessary cleaning up. When it
returns, the gen_server process terminates with Reason. The return value is
ignored.
Reason is a term denoting the stop reason and State is the internal state of the
gen_server process.
Reason depends on why the gen_server process is terminating. If it is because
another callback function has returned a stop tuple {stop,..}, Reason has the value
specified in that tuple. If it is because of a failure, Reason is the error reason.
If the gen_server process is part of a supervision tree and is ordered by its
supervisor to terminate, this function is called with Reason=shutdown if the
following conditions apply:
* The gen_server process has been set to trap exit signals.
* The shutdown strategy as defined in the child specification of the supervisor
is an integer time-out value, not brutal_kill.
Even if the gen_server process is not part of a supervision tree, this function is
called if it receives an 'EXIT' message from its parent. Reason is the same as in
the 'EXIT' message.
Otherwise, the gen_server process terminates immediately.
Notice that for any other reason than normal, shutdown, or {shutdown,Term}, see
stop/3, the gen_server process is assumed to terminate because of an error, and an
error report is issued using logger(3erl).
When the gen_server process exits, an exit signal with the same reason is sent to
linked processes and ports.
SEE ALSO
gen_event(3erl), gen_statem(3erl), proc_lib(3erl), supervisor(3erl), sys(3erl)