Remote Procedure Calls

Example : GetFile App

• Client Server
• Create and init sockets
• Allocate and populate buffers
• Include 'protocol' info
  • GetFile, size
• Copy data into buffers
  • filename, file
• common steps related to remote IPC

Remote Procedure Calls (RPC)

• Intended to simplify the development of cross address space and cross machine interactions

+ Higher-level interface for data movement and communication
+ Error handling
+ Hiding complexities of cross machine interactions

RPC requirements

1. Client/Server interactions
2. Procedure Call Interface => RPC
   • sync call semantics
3. Type checking
   • error handling
   • packet bytes interpretation
4. Cross machine conversion
   • e.g. big/little endian
5. Higher level protocol
   • access control, fault tolerance, different transport protocols

Structure of RPC

RPC Steps:

(-1.) register : server registers procedure, arg types, location
(0.) bind : client finds and binds to desired server

1. call : client make RPC call; control passed to stub, client code blocks
2. marshal : client stub "marshals" args (serialize args into buffer)
3. send : client sends message to server
4. receive : server receives message; passes message to server stub; access control
5. unmarshal : server stub "unmarshals" args (extract args from buffer)
6. actual call : server stub calls local procedure implementation
7. result : server performs operation and computes result of RPC operation

(same on return <=)

Interface definition Language (IDL)

• Used to describe the interface the server expects
  • procedure name, args, 2 result types
  • version number

RPC can use IDL that is

1. Language agnostic
   • XDR in SunRPC
2. Language specific
   • Java in JavaRMI

Marshalling

Unmarshalling

Marshalling/Unmarshalling routines are provided by RPC system compiler.

Binding and Registry

• Client determines
  • which server to connect to?
    • service name. version number
  • how to connect to that server?
    • IP address, network protocol
• Registry : database of available services
  • search for service name to find server(which) and contact details(how)
  • distributed
    • any RPC service can register
  • machine-specific
    • for services running on same machine
    • clients must know machine addresses
    • registry provides port number needed for connection
• Who can provide a service?
  • lookup registry for image processing
• What services do they provide?
  • compress/filter.. version number => IDL
• How will they ship package?
  • TCP / UDP -> registry

Pointers

• Procedure interface : foo(int,int)
• in Local Calls : foo(x,y) => okay
• in Remote Calls : foo(x,y) => ?

here, y points to location in caller address space

• Solutions:
  • No pointers
  • Serialize pointers; copy referenced ("points to") data structure to send buffer

Handling Partial Failures

• Special RPC error notification (signal, exception..)
  • Catch all possible ways in which RPC can (partially) fail

RPC Design choice

• Binding => How to find the server
• IDL => How to talk to server; how to package data
• Pointers as args => Disallow or serialize pointer data
• Partial failures => Special error notifications

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