RPC

Why we need Remote Procedures?

The client needs an easy way to call the procedures of the server to get some services.
RPC enables clients to communicate with servers by calling procedures in a similar way to the conventional use of procedure calls in high-level languages.
RPC is modeled on the local procedure call, but the called procedure is executed in a different process and usually a different computer.

Adaption of the send() & receive() IPC paradigm.
Allows programs in distributed systems to interact with each other via function call/return semantics.
Abstracts the network to the user and makes it look like the programs are on the same machine.
Allows to specify a clearly documented interface for the remote server.
- This also allows to automatically generate the code.
Due to the clearly documented interface, it is easy to write applications that run on multiple operating systems.

When a process on machine A calls a procedure on machine B, the calling process on A is suspended, and the execution of the called procedure takes place on B.
Information can be transported from the caller to the callee in the parameters and can come back in the procedure result.
No message passing or I/O at all is visible to the programmer.

How does the client transfer its call request (the procedure name) and the arguments to the server via network?
Marshalling and communication with server:
- For each remote procedure call, a (client) stub procedure is generated and attached to the (client) program.
- Replace the remote procedure call to a (local) call to the stub procedure.
- The (codes in the) stub procedure marshals (the input) arguments and places them into a message together with the procedure identifier (of the remote procedure).

Most RPC systems use call by value as it is easy to realize.
To implement call by reference a lot has to be done – might not be worth the effort.
If both systems use the same language parameter, representation is easy!
- If not, there exist special RPC facilities that handle the problem of data representation.
RPC is more vulnerable to failure (since it involves a communication system, another machine, and another process).
- The programmer should be aware of the call semantics, i.e., programs that make use of RPC must have the capability of handling errors that cannot occur in local procedure calls.