The callable RPC server interface enables you to write your own RPC Server. This offers the possibility to integrate RPC servers into third-party systems, as well as to call target servers in programming languages other than C by wrapping them. The programming language for writing a callable RPC Server is C. This document covers the following topics:
The callable RPC server consists of a function containing a loop for recurring RPC execution using a callback technique for user interaction. The function performs all of the necessary communication with the broker such as logon and logoff, service registration, receive and send. The behavior of the callable RPC server can be configured with a server configuration file.
The main part of the callable RPC server is the C Wrapper
runtime function ERXServingCallback. This function
manages Broker communication as well as the marshalling and unmarshalling of
RPC requests using callbacks. The function consists of a loop for RPC execution
using callback technique for user interaction. The behavior of
ERXServingCallback is driven by a configuration file where you set the
necessary broker parameters etc. See Configuring the RPC Server under Administering the RPC Server for C. In the
example below, the name of the configuration file is passed as a parameter to
the callable RPC server and given to the ERXServingCallback function.
The callback function ERX_Callback_SERVER_CALL is
called when an RPC request has to be executed. Implement a call to your target
server within this callback event. See Writing the Callback. The interface of the unmarshalled data given to
the callback is compatible with the generated server interface objects for the
programming language C. See Software AG IDL to C Mapping. Upon return from the callback function,
the same applies to parameters replied to the client.
ERXServingCallback requires registration of the events before getting
control of them. Use ERXRegisterEvent with the Event ID ERX_EVENT_SERVER_CALL
to register the callback function ERX_Callback_SERVER_CALL for this
purpose.
The criteria to break/stop the RPC execution loop and give control back
to the caller can be configured with the configuration file parameter
endworkers. See Configuring the RPC Server under Administering the RPC Server for C. The
example below implements a single RPC worker thread within the main function
which is ended by a shutdown from outside. Hence use
endworkers=never as the setting for the configuration file
parameter endworkers for the example below. This ensures
the RPC execution loop is not stopped by broker timeouts or after an RPC
request is executed, etc. Use one of the usual ways to stop RPC servers on your
platform to stop the callable RPC server. See Break/Stop the RPC Execution Loop for more information.
As a general rule before using the RPC C runtime at all, every worker
thread must be registered with it using the ERXRegister function.
ERXRegister is
therefore the first call to the RPC C runtime and
ERXUnregister the last. See Using the RPC Runtime.
void main( int argc, char *argv[ ])
{
int bRuntimeRegistered = 0;
char myConfigurationFile[512] = "..\\server\\server.cfg";
void * myParms = NULL;
ERXeReturnCode rc = ERX_S_SUCCESS;
ERX_ERROR_INFO ErrorInfo;
memset(&ErrorInfo,'\0',sizeof(ErrorInfo));
/* Treat the input Parameter */
if( argc == 2)
{
strncpy( myConfigurationFile, argv[ 1 ], 512 );
}
printf("\nEntireX callable RPC server is running:\n"
" Configuration File: %s\n",
myConfigurationFile);
/* Register to EntireX RPC Runtime */
rc = ERXRegister( ERX_CURRENT_VERSION );
if ( ERX_FAILED(rc ))
{
PrintReturnCode(rc,&ErrorInfo);
goto done; /* ===> */
}
bRuntimeRegistered = 1;
/* Register the Callback Event */
rc = ERXRegisterEvent(ERX_EVENT_SERVER_CALL,
myERX_Callback_SERVER_CALL);
if ( ERX_FAILED(rc))
{
PrintReturnCode(rc,&ErrorInfo);
goto done; /* ===> */
}
/* Execute the Callable RPC Server */
rc = ERXServingCallback( myConfigurationFile,
myParms,
(ERX_CF_NOTHING)
);
PrintReturnCode(rc,&ErrorInfo);
done:
if (bRuntimeRegistered == 1)
{
ERXUnregister();
}
return;
}
This very simple example of a callback implementation uses the
szLibraryName and szProgramName
from the ERX_CALL_INFORMATION_BLOCK to
select requests for the library named EXAMPLE and the programs named CALC and
SQUARE in a hard-wired fashion. Other RPC requests are rejected with
appropriate error messages. The library and program names correspond to the
names given in the IDL file of the calling client.
The focus here is not to show how functions can be called dynamically. Dynamic calling depends on the possibilities of your implementation platform and support by the programming languages in use.
The interface of the unmarshalled data given to the callback is compatible with the generated server interface objects for the programming language C. See Software AG IDL to C Mapping. Upon return from the callback function, the same applies to parameters expected by and replied to the client.
For the IDL data type A, null terminated strings are supplied and
expected (corresponding to DATA_CONV_A=1).
For the IDL data types N and P, double is supplied and expected
(corresponding to DATA_CONV_NP=1).
With the structure ERX_ERROR_INFORMATION either
success or failure must always be returned.
The callable RPC server supports a concept of user-specific data. With
this feature it is possible to pass a pointer through the ERXServingCallback
function directly into the callback. The pointer myParms, second parameter of
the ERXServingCallback in the example above, is available "as is"
in the callback here in the first parameter as pointer pUserInfo. It can be
used, for example, to provide a pointer to a memory location with user-specific
data.
void myERX_Callback_SERVER_CALL
(
void * pUserInfo,
ERX_CLIENT_IDENTIFICATION * pClientInformation,
ERX_CALL_INFORMATION_BLOCK * pCallInformation,
void * pParameterArea,
ERX_ERROR_INFO * pReturnInfo
)
{
ERXeReturnCode rc = ERX_S_SUCCESS;
printf("\nThis is Callback_SERVER_CALL, serving for %s,%s \n",
pCallInformation->Callee.szLibraryName,
pCallInformation->Callee.szProgramName);
if (strcmp(pCallInformation->Callee.szLibraryName,"EXAMPLE") == 0)
{
if (strcmp(pCallInformation->Callee.szProgramName,"CALC") == 0)
{
S_CALC *pParm = (S_CALC *) pParameterArea;
/* Execute Function */
pParm->function_result = CALC( pParm->operation,
pParm->operand_1,
pParm->operand_2 );
}
else if (strcmp(pCallInformation->Callee.szProgramName,"SQUARE")== 0)
{
S_SQUARE *pParm = (S_SQUARE *) pParameterArea;
/* Execute Function */
SQUARE( pParm->operand, &(pParm->result) );
}
else
{
rc = ERX_E_RPC_CALLEE_NOT_FOUND;
}
}
else
{
rc = ERX_E_RPC_LIBRARY_NOT_FOUND;
}
pReturnInfo->rc = rc;
return;
}
The RPC execution loop should normally run continuously until the RPC
server is shut down from outside. With the setting of the configuration file
parameter endworkers you can configure when the RPC
execution loop is stopped and control is given back to the caller. See Configuring the RPC Server under Administering the RPC Server for C.
The following table explains the endworkers
parameter.
| Value | Explanation |
|---|---|
| N |
Never The callable RPC server's
function ERXServingCallback breaks/stops the RPC execution loop only if a
normal shut down of the RPC server takes place. This setting makes sense with a
simple callable RPC server (see Writing a Callable RPC Server).
|
| T |
Timeout The callable RPC server's
function ERXServingCallback breaks/stops the RPC execution loop if
|
| I |
Immediately The callable RPC server's
function ERXServingCallback breaks/stops the RPC execution loop if
ERXServingCallback
function.
|
This section provides some hints on how to implement a callable RPC server with a scalable number of worker threads. This is a more complex server with the ability to clone worker threads to satisfy a high load of client requests.
Implement a main function registering as an attach server by the
broker using REGISTER, OPTION=ATTACH. When this server receives attach service
requests for clients waiting to be served, start a suitable number of worker
threads. See Implementing an Attach Server.
Implement a callable RPC server and its callback to be attached in a thread as described under Writing a Callable RPC Server and Writing the Callback.
Use endworkers=timeout for the configuration file
parameter endworkers, if you wish to
implement a server that does not exit after the first conversation
reduce the number of servers when they are no longer needed
Use endworkers=immed if you wish to
implement a server that handles only one client for one conversation
See Implementing Servers Started by an Attach Server for more details.