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ECSE 427/COMP 310 – Operating Systems Assignment 01 – Simple RPC Service
Fall 2020 (version 2) Page 1 9/21/2020
Simple Remote Procedure Call Service
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High-Level Description
Consider a simple calculator. It has a read-eval-print-loop (REPL), where you enter something like
the following.
The calculator is structured as two programs: frontend and backend. The frontend does not do the
actual calculations. It just prints the prompt, get the input from the user, parses the input into a
message. The frontend passes the message to the backend that does the actual calculations and
passes the result back to the frontend, where it is printed to the user. We write two programs
backend.c and frontend.c. The backend has some functions it wants to expose for the frontend.
The frontend needs to call those functions that are exposed by the backend. The remote procedure
call service (RPCServ) is responsible for linking the two. You are expected to develop such a
RPCServ as part of this assignment. We start with a very simple implementation. The frontend
issues an execution command and backend runs it and returns the result. This simple structure
would work even with multiple frontends if the commands issued by the frontends can be executed
very quickly. If some commands can hold the backend for a long time (like seconds), we have a
problem. When a frontend is running a long command, other frontends will find the backend
unavailable. That is, you run sleep 5 in the frontend and the backend is held by that frontend for 5
seconds. If other frontends try to run a calculation, they will not get any results. To solve this
problem, you will use multi-processing. That is the backend will create a serving process for each
frontend. In this configuration, as soon as the frontend connects to the backend, we create a new
serving process that is dedicated to the frontend and let it serve the frontend’s requests. Even if the
frontend does a sleep 5, it would not cause problems for other frontends. The backend is still
available for requests from other frontends. To keep things simple, we limit the number of
concurrent frontends to 5.
Frontend Requirements
The pseudo code shown below for the frontend is not complete. It shows the bare essentials. You
need to add the missing functions to make the frontend meet all the requirements and make it work
with RPCServ and the backend.
backend = RPC_Connect(backendIP, backendPort)
while(no_exit) {
print_prompt()
line = read_line()
ECSE 427/COMP 310 – Operating Systems Assignment 01 – Simple RPC Service
Fall 2020 (version 2) Page 2 9/21/2020
cmd = parse_line(line)
RPC_Call(backend, cmd.name, cmd.args)
}
RPC_Close(backend)
The frontend does not implement any of the commands the user enters into the shell. It simply
relays them to the backend. You will notice that the command entered by the user looks like the
following: command (string) and parameters. We will restrict the parameters to 2 or less. You can
have commands with no parameters. The parameters can be integers or floating-point numbers.
You need to have an RPCServ interface to send the command and parameters to the backend. In
the pseudo code, we show such an interface – RPC_Call(). The frontend will check if the user
has entered the exit command. If that is the case, the frontend will stop reading the next command
and terminate the association with the backend. The backend is a separate process, so it keeps
running even after the frontend has stopped running. The user can enter the shutdown command
in the frontend to terminate the backend. With multiple frontends connecting to the backend, the
shutdown can be tricky. More on this in the backend requirements. Some commands entered by
the user in the frontend may not be recognized by the backend, in that case the backend will send
the NOT_FOUND error message. The front needs to display this to the user. We can also have
error message for certain operations such as division by zero errors. These error messages need to
be displayed as well.
Backend Requirements
The pseudo code shown below for the backend is not complete. It shows the bare essentials. You
need to add the missing functions to make the backend meet all the requirements and make it work
with RPCServ and the frontend.
serv = RPC_Init(myIP, myPort)
for_all_functions(name)
RPC_Register(serv, name, function)
while(no_shutdown) {
client = accept_on_server_socket(serv)
serv_client(client)
}
The backend sets up a server at myPort in the current machine (you can use “127.0.0.1” to point
to the current machine). The server should be set up, so it is bound to the given port and is listening
for incoming connections from the frontend. Before you start accepting the connections, you need
to register all the functions that the RPCServ is willing to offer as a service. For example, if you
have the following function to add two integers that you want to expose, you need to register it
with the RPCServ as shown below.
int addInts(int x, int y) {
return x + y;
}
ECSE 427/COMP 310 – Operating Systems Assignment 01 – Simple RPC Service
Fall 2020 (version 2) Page 3 9/21/2020
RPC_Register(“add”, addInts)
The RPCServ is responsible for invoking (that is calling) the addInts function with the
appropriate parameters when a request comes from the frontend.
The RPCServ keeps running until a shutdown command is issued by the frontend. With a single
frontend, this is quite simple. You simply exit the program after closing the sockets in an orderly
manner. With multiple frontends, things can get little bit tricky.
With multiple frontends, the pseudo code shown above needs some revision. Soon after accepting
a connection, you need to create a child process and let that child process handle the new
connection. That is the socket connection (client) is passed to the child process and it will be doing
the calculations and sending the results or error back to the client. The server is free to loop back
and accept another connection without waiting for the previous frontend’s service to complete.
With multiple frontends, let’s consider the situation where the backend received the shutdown
command. The command is received in a child process. For the backend to terminate, we must
terminate the parent process that started running the backend. The child that received the shutdown
must notify the parent about the reception of the shutdown. For this purpose, the child can use the
return value in an exit() system call. The parents gets the return value of the child using the
waitpid() system call. The example code below shows how you can return a value from a child
to parent.
#include
#include
#include
#include
int main() {
int pid;
int rval;
if ((pid = fork()) == 0) {
sleep(10);
return 10;
}
while (1) {
sleep(1);
int res = waitpid(pid, &rval, WNOHANG);
printf("Returned value %d\n", WEXITSTATUS(rval));
}
}
On the server socket, the parent is going to block. It is waiting for new connections from the
frontends. Once a connection comes in, the parent is going to unblock and proceed to create a child
process to handle the frontend. To handle the shutdown properly, we need to check whether any
child processes have already issued a shutdown. If so, we close the client connection that just
arrived and stop accepting any more connections. When all the child processes that are running
have completed their execution, the parent terminates. Because the parent is blocking on the server
socket, it would not be able to shutdown or even know about the shutdown when it arrives. The
ECSE 427/COMP 310 – Operating Systems Assignment 01 – Simple RPC Service
Fall 2020 (version 2) Page 4 9/21/2020
parent would only detect the shutdown at the arrival of the next frontend processing request.
Therefore, the backend would keep going even after shutdown has been sent until the next frontend
request.
NOTE: There is a better way of doing the same activity as the above that will use epoll() or select().
If you are following the advanced tutorials and are already familiar with C/Linux socket
programming, you are strongly encouraged to use those system calls. The design is left to you, but
your design needs to meet or exceed the above functionality. For example, with epoll() or select()
you could terminate the backend without waiting until the next frontend request.
Backend Functions: You need to provide the following functions in the backend implementation.
1. int addInts(int a, int b);
// add two integers
2. int multiplyInts(int a, int b);
// multiple two integers
3. float divideFloats(float a, float b);
// divide float numbers (report divide by zero error)
4. int sleep(int x);
// make the calculator sleep for x seconds – this is blocking
5. uint64_t factorial(int x);
// return factorial x
RPCServ Requirements
We are not going to test your RPCServ implementation with applications other than the calculator.
Therefore, we are not standardizing on the RPCServ interface. However, you are strongly
encouraged to provide at least the following.
rpc_t *RPC_Init(char *host, int port)
// rpc_t is a type defined by you and it holds
// all necessary state, config about the RPC connection
RPC_Register(rpc_t *r, char *name, callback_t fn)
// callback_t is type defined by you
rpc_t *RPC_Connect(char *name, int port)
RPC_Close(rpc_t *r)
RPC_Call(rpc_t *r, char *name, args..)
// You can have different variations to
// handle different number of parameters and types
This is a guide for you to organize the RPCServ implementation. You can change the function
signatures and have more functions in your RPCServ implementation.
How the Assignment will be Graded?
ECSE 427/COMP 310 – Operating Systems Assignment 01 – Simple RPC Service
Fall 2020 (version 2) Page 5 9/21/2020
We will use a shell script to grade your assignment. The shell script will start the backend and start
the frontend and inject different inputs. The script checks the output from your frontend against an
expected value and reports an error.
Grade distribution will be notified very soon by the TAs.
What You Need to Handin?
What needs to be handed in?
Source files, Makefile or CMakeLists.txt?
Can I Collaborate with My Friends?
This is an individual assignment. You can brainstorm with your friends in developing RPCServ
and other components. The final implementation must be yours only. You cannot do group coding.

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