-spec abcast(Name :: atom(), Request :: term()) -> abcast.

Equivalent to abcast(Nodes, Name, Request) where Nodes is all nodes connected to the calling node, including the calling node.

-spec 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.

-spec call(ServerRef :: server_ref(), Request :: term()) -> Reply :: term().

Equivalent to call(ServerRef, Request, 5000)

-spec 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/0.

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. 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/0) and looking up the server process failed, or the pid/0 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/0 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 one of its callbacks without replying to this call. See also stop/3.

• _OtherTerm - The server process exited during the call, with reason Reason. Either by returning {stop,Reason,_} from one of its callbacks (without replying to this call), by raising an exception, or due to getting an exit signal it did not trap.

-spec 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/0.

Request is any term that is passed as the first argument to Module:handle_cast/2.

-spec check_response(Msg, ReqId) -> Result
                  when
                      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.

-spec check_response(Msg, ReqIdCollection, Delete) -> Result
                  when
                      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 requests 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.

-spec enter_loop(Module :: module(), Options :: [enter_loop_opt()], State :: term()) -> no_return().

Equivalent to enter_loop(Mod, Options, State, self())

-spec enter_loop(Module :: module(),
           Options :: [enter_loop_opt()],
           State :: term(),
           ServerName :: server_name() | pid()) ->
              no_return();
          (Module :: module(),
           Options :: [enter_loop_opt()],
           State :: term(),
           How :: timeout() | hibernate | {continue, term()}) ->
              no_return().

Makes an existing process a gen_server process.

Equivalent to enter_loop(Module, Options, State, ServerName, infinity) if called as enter_loop(Module, Options, State, ServerName).

Equivalent to enter_loop(Module, Options, State, self(), How) if called as enter_loop(Module, Options, State, How).

-spec enter_loop(Module :: module(),
           Options :: [enter_loop_opt()],
           State :: term(),
           ServerName :: server_name() | pid(),
           Timeout :: timeout()) ->
              no_return();
          (Module :: module(),
           Options :: [enter_loop_opt()],
           State :: term(),
           ServerName :: server_name() | pid(),
           Hibernate :: hibernate) ->
              no_return();
          (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. 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/0 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/0), 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.

-spec multi_call(Name :: atom(), Request :: term()) ->
              {Replies :: [{Node :: node(), Reply :: term()}], BadNodes :: [node()]}.

Equivalent to multi_call(Nodes, Name, Request) where Nodes is all nodes connected to the calling node, including the calling node.

-spec multi_call(Nodes :: [node()], Name :: atom(), Request :: term()) ->
              {Replies :: [{Node :: node(), Reply :: term()}], BadNodes :: [node()]}.

Equivalent to multi_call(Nodes, Name, Request, infinity)

-spec 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.

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. 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.

-spec receive_response(ReqId, Timeout) -> Result
                    when
                        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.

-spec receive_response(ReqIdCollection, Timeout, Delete) -> Result
                    when
                        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 requests 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.

-spec 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.

-spec 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.

-spec 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.

-spec reqids_size(ReqIdCollection :: request_id_collection()) -> non_neg_integer().

Returns the amount of request identifiers saved in ReqIdCollection.

-spec 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.

-spec 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/0 for the possible values for ServerRef.

Request is any term that is passed as the first argument to Module:handle_call/3.

-spec 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.

-spec start(Module :: module(), Args :: term(), Options :: [start_opt()]) -> start_ret().

Equivalent to start/4 except that the gen_server process is not registered with any name service.

-spec 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/4.

-spec start_link(Module :: module(), Args :: term(), Options :: [start_opt()]) -> start_ret().

Equivalent to start_link/4 except that the gen_server process is not registered with any name service.

-spec 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 spawned as linked to the caller (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 or failed.

Using the argument ServerName creates a gen_server with a registered name. See type server_name/0 for different name registrations.

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/0 for Options when starting the gen_server process.

See type start_ret/0 for a description this function's return values.

If start_link/3,4 returns ignore or {error,_}, the started gen_server process has terminated. If an 'EXIT' message was delivered to the calling process (due to the process link), that message has been consumed.

Warning

Before OTP 26.0, if the started gen_server process returned e.g. {stop,Reason} from Module:init/1, this function could return {error,Reason} before the started gen_statem process had terminated so starting again might fail because VM resources such as the registered name was not yet unregistered. An 'EXIT' message could arrive later to the process calling this function.

But if the started gen_server process instead failed during Module:init/1, a process link {'EXIT',Pid,Reason} message caused this function to return {error,Reason} so the 'EXIT' message had been consumed and the started gen_statem process had terminated.

Since it was impossible to tell the difference between these two cases from start_link/3,4's return value, this inconsistency was cleaned up in OTP 26.0.

The difference between returning {stop,_} and {error,_} from Module:init/1, is that {error,_} results in a graceful ("silent") termination since the gen_server process exits with reason normal.

-spec start_monitor(Module :: module(), Args :: term(), Options :: [start_opt()]) -> start_mon_ret().

Equivalent to start_monitor/4 except that the gen_server process is not registered with any name service.

-spec 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/0.

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.

-spec stop(ServerRef :: server_ref()) -> ok.

Equivalent to stop(ServerRef, normal, infinity)

-spec stop(ServerRef :: server_ref(), Reason :: term(), Timeout :: timeout()) -> ok.

Orders the generic server specified by ServerRef to exit with the specified Reason 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. 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. 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.

-spec wait_response(ReqId, WaitTime) -> Result
                 when
                     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.

-spec wait_response(ReqIdCollection, WaitTime, Delete) -> Result
                 when
                     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 requests 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 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.