Introduction

Searpc is a simple C language RPC framework based on GObject system. Searpc handles the serialization/deserialization part of RPC, the transport part is left to users.

The serialization/deserialization uses JSON format via json-glib library. A serialized json object is returned from server to client after executing the RPC function. Each RPC function defined in the server side should take an extra GError argument to report error. The returned json object contains three fields:

• ret: the return value of the RPC function
• err_code: error code. This field is only set if the RPC function reports an error.
• err_msg: error message. This field is only set if the RPC function reports an error.

Compile

Just

./autogen.sh; ./configure; make; make install

To enable profile, Use

CFLAGS="-DPROFILE" ./configure

When profile is enabled, the time spend in each rpc call will be printed.

Example

Client

In the client side, you need to:

• Create a rpc_client and supply the transport function.
• write code to send the request to server and get the resonpse from it.

Create rpc_client

The client needs to create a SearpcClient object and supply a transport function. For example:

/* create an rpc_client and supply the transport function. */
SearpcClient *rpc_client;
rpc_client = searpc_client_new();
rpc_client->transport = transport_callback;
rpc_client->arg = &sockfd;

Suppose we have a get_substring function defined in server as follows:

gchar *get_substring (const gchar *orig_str, int sub_len, GError **error)

To call this function, we type:

gchar* result;
GError *error = NULL;
result = searpc_client_call__string (client, "get_substring", &error,
                                     2, "string", "hello", "int", 2);

string in searpc_client_call__string specify the return type. "get_substring" is the function name. The remain parameters specify the number of parameters the rpc function took and the type of each parameter and its value. So

2, "string", "hello", "int", 2

means "get_substring" takes 2 parameters, the first is of type "string", the value is "hello", the second is of type "int", the value is 2.

Transport function

When the client-side function is called, Searpc does the following work:

• Pack the function name and the params into JSON data format.
• Call your transport function to send the JSON data to the server, and get the returned data from the server.
• Unpack the returned JSON data and return the value as the return value of the client-side function.

Your transport function is supposed to:

• Send the request data to the server.
• Receive the returned data from the server.

The prototype of the transport function is:

/* 
 * arg: rpc_client->arg. Normally a socket number.
 * fcall_str: the JSON data stream generated by Searpc.
 * fcall_len: the length of `fcall_str`.
 * ret_len: place to get the length of the returned json data stream.
 * Returns: A newly allocated string stores the JSON data stream.
 */
static char *transport_callback (void *arg, const char *fcall_str, size_t fcall_len, size_t *ret_len);

Server

In the server side, you need to:

• Init searpc server
• Create services and register your functions
• write code to receive the request and send the result

And Searpc handles the others for you.

Concepts

• Marshal: The process of unpacking the function arguments from JSON data, call the RPC function and packing the result into JSON data format is called marshalling. The function used to pack the result is called a marshal.

• Signature: Every function has a signature determined by its return type and parameter types. Knowning a function's signature enable us to use a corresponding marshal to call it and convert the result into json string.

Init Searpc Server

First write rpc_table.py to contain the rpc function signatures as follows:

# [ <ret-type>, [<arg_types>] ]
func_table = [
     [ "int", ["string"] ],
     [ "string", ["int", "string"] ],
]

Add makefile rule:

searpc-signature.h searpc-marshal.h: rpc_table.py
    python searpc-codegen.py rpc_table.py

searpc-signature.h and searpc-marshal.h will be created containing the function signatures and corresponding marshals. searpc-marshal.h also contains a function called register_marshals.

Then we init the server as follows:

#include "searpc-signature.h"
#include "searpc-marshal.h"

static void
init_rpc_service(void)
{
    /* register_marshals is defined in searpc-marshal.h */
    searpc_server_init(register_marshals);
}

Register Functions

To register a function we first need to create a service. A service is a set of functions.

Suppose we want to make searpc_strlen callable from some network clients, we can do this by putting the following code somewhere:

static int
searpc_strlen(const char *str)
{
    if (str == NULL)
        return -1;
    else
        return strlen(str);
}

static void
register_functions()
{

    searpc_create_service("searpc-demo");

    /* The first parameter is the implementation function.
     * The second parameter is the name of the rpc function the 
     * client would call.
     * The third parameter is the signature.
     */
    searpc_server_register_function("searpc-demo",
                                    searpc_strlen,
                                    "searpc_strlen",
                                    searpc_signature_int__string());
 }

The seaprc_server_register_function routine registers a function as a RPC function. The prototype of this function is:

/*
 * service:     the name of the service
 * func:        pointer to the function you want to register
 * fname:       the name of the function. It would be the key of your 
 *              function in the fucntion hash table.
 * signature:   the identifier used to get the corresponding marshal.
 * Returns:     a gboolean value indicating success or failure
 */
 gboolean searpc_server_register_function (const char *service,
                                           void* func,
                                           const gchar *fname,
                                           gchar *signature);

Call the RPC fucntion

After the registration, you should listen to the socket and wait for the incoming request data stream. Once you get a valid request, call the searpc_server_call_function() routine, which will automatically do the following work for you:

• Parse the JSON data stream to resolve the function name and the data.
• Lookup the function in internal function table according to the funcname.
• If a proper function is found, call the function with the given params.
• Packing the result into a JSON data string.

The prototype of searpc_server_call_function is:

/*
 * service: Service name.
 * data:    The incoming JSON data stream.
 * len:     The length of **`data`**.
 * ret_len: Place to hold the length of the JSON data stream to be returned
 * Returns: The JSON data containing the result of the RPC
 */
gchar* searpc_server_call_function (const char *service,
                                    gchar *data, gsize len, gsize *ret_len)

The value returned by searpc_server_call_function() is the JSON data ready to send back to the client.

Note, the JSON data stream from client does not contain the service name, it's left to the transport layer to solve the problem. There are several ways, for example:

1. You may listen on different sockets and determine the service by the incoming socket.
2. The client transport function prepend the service name into the request before the json data, and the server transport function first read the service name and read the json data.

Pysearpc

Pysearpc is the Python binding of Searpc. Only the client side function is supported. To use it, simply define a class which inherits SearpcClient, and provide a call_remote_func_sync method, which is equivalent to the transport_callback.

To define your RPC funtion, use the @searpc_func decorator. It is equivalent to SEARPC_CLIENT_DEFUN_XXX__YYY macro. To define a RPC function which accepts multiple params, here is an example:

class SampeSearpcClient(SearpcClient):
    def call_remote_func_sync(self, fcall_str):
        # your transport code here
        ...
    
    @searpc_func("int", ["string", "string"])
    def searpc_demo_func(self):
        # this is enough for the client side
        pass

See the demo program for a more detailed example.

Demos

There are well-commented demos in both C and Python.

• searpc-demo-server.c: The server side demo program
• searpc-demo-client.c: The client side demo in C
• pysearpc-demo-client.py: The client side demo in Python

To run the demo, run the server demo in a shell, and run the client demo in another. To run the python demo, you should first install the package and setup the PYTHONPATH appropriately.

Dependency

The following packages are required to build libsearpc:

• glib-2.0 >= 2.26.0
• gobject-2.0 >= 2.26.0
• jansson >= 2.2.1
• python simplejson (for pysearpc)