Adding RPC Functions With Margo Library

In this section, we describe how to add an RPC function using the Margo library API.


The following documentation uses unifyfs_mount_rpc() as an example client-server RPC function to demonstrate the required code modifications.


  1. Define structs for the input and output parameters of your RPC handler.

    The struct definition macro MERCURY_GEN_PROC() is used to define both input and output parameters. For client-server RPCs, the definitions should be placed in common/src/unifyfs_client_rpcs.h, while server-server RPC structs are defined in common/src/unifyfs_server_rpcs.h.

    The input parameters struct should contain all values the client needs to pass to the server handler function. The output parameters struct should contain all values the server needs to pass back to the client upon completion of the handler function. The following shows the input and output structs used by unifyfs_mount_rpc().



Passing some types can be an issue. Refer to the Mercury documentation for supported types: (look under Predefined Types). If your type is not predefined, you will need to either convert it to a supported type or write code to serialize/deserialize the input/output parameters. Phil Carns said he has starter code for this, since much of the code is similar.


  1. Implement the RPC handler function for the server.

    This is the function that will be invoked on the client and executed on the server. Client-server RPC handler functions are implemented in server/src/unifyfs_client_rpc.c, while server-server RPC handlers go in server/src/unifyfs_p2p_rpc.c or server/src/unifyfs_group_rpc.c.

    All the RPC handler functions follow the same protoype, which is passed a Mercury handle as the only argument. The handler function should use margo_get_input() to retrieve the input parameters struct provided by the client. After the RPC handler finishes its intended action, it replies using margo_respond(), which takes the handle and output parameters struct as arguments. Finally, the handler function should release the input struct using margo_free_input(), and the handle using margo_destroy(). See the existing RPC handler functions for more info.

    After implementing the handler function, place the Margo RPC handler definition macro immediately following the function.

    static void unifyfs_mount_rpc(hg_handle_t handle)
  2. Register the server RPC handler with margo.

    In server/src/margo_server.c, update the client-server RPC registration function register_client_server_rpcs() to include a registration macro for the new RPC handler. As shown below, the last argument to MARGO_REGISTER() is the handler function address. The prior two arguments are the input and output parameters structs.



  1. Add a Mercury id for the RPC handler to the client RPC context.

    In client/src/margo_client.h, update the ClientRpcIds structure to add a new hg_id_t <name>_id variable to hold the RPC handler id.

    typedef struct ClientRpcIds {
        hg_id_t mount_id;
  2. Register the RPC handler with Margo.

    In client/src/margo_client.c, update register_client_rpcs() to register the new RPC handler by its name using CLIENT_REGISTER_RPC(<name>), which will store its Mercury id in the <name>_id structure variable defined in the first step. For example:

  3. Define and implement an invocation function that will execute the RPC.

    The declaration should be placed in client/src/margo_client.h, and the definition should go in client/src/margo_client.c.

    int invoke_client_mount_rpc(unifyfs_client* client, ...);

    A handle for the RPC is obtained using create_handle(), which takes the the id of the RPC as its only parameter. The RPC is actually initiated using forward_to_server(), where the RPC handle, input struct address, and RPC timeout are given as parameters. Use margo_get_output() to obtain the returned output parameters struct, and release it with margo_free_output(). Finally, margo_destroy() is used to release the RPC handle. See the existing invocation functions for more info.


The general workflow for creating new RPC functions is the same if you want to invoke an RPC on the server, and execute it on the client. One difference is that you will have to pass NULL to the last parameter of MARGO_REGISTER() on the server, and on the client the last parameter to MARGO_REGISTER() will be the name of the RPC handler function. To execute RPCs on the client it needs to be started in Margo as a SERVER, and the server needs to know the address of the client where the RPC will be executed. The client has already been configured to do those two things, so the only change going forward is how MARGO_REGISTER() is called depending on where the RPC is being executed (client or server).