 Remote Procedure Call (RPC) is a high-level model for client-sever communication.  It provides the programmers with a familiar mechanism for building distributed systems.  Examples: File service, Authentication service.

Why we need Remote Procedure Call (RPC)? Why we need Remote Procedure Call (RPC)? –The client needs a 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.

 How to operate RPC? ◦ 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.

The RPC model Blocking state client server request reply Executing state Call procedure and wait for reply Receive request and start process execution Send reply and wait for next execution Resume execution

 The called procedure is in another process which may reside in another machine.  The processes do not share address space. ◦ Passing of parameters by reference and passing pointer values are not allowed. ◦ Parameters are passed by values.  The called remote procedure executes within the environment of the server process. ◦ The called procedure does not have access to the calling procedure's environment.

 Simple call syntax  Familiar semantics  Well defined interface  Ease of use  Efficient  Can communicate between processes on the same machine or different machines

 Exception handling ◦ Necessary because of possibility of network and nodes failures; ◦ RPC uses return value to indicate errors;  Transparency ◦ Syntactic  achievable, exactly the same syntax as a local procedure call; ◦ Semantic  impossible because of RPC limitation: failure (similar but not exactly the same);

 Based on concepts of stubs  Stub is a code used for converting parameter used in procedure call  RPC involves Client & Server Process  Mechanism involves following five elements: ◦ The Client ◦ The Client Stub ◦ The RPCRuntime ◦ The Server Stub ◦ The Server

Return Call Call Return Unpack Pack Receive Send Client Stub RPC Runtime Server Stub Client Server Call Packet Result Packet Client Machine Server Machine

1. Client procedure calls client stub in normal way 2. Client stub builds message, calls local OS 3. Client's OS sends message to remote OS 4. Remote OS gives message to server stub 5. Server stub unpacks parameters, calls server 6. Server does work, returns result to the stub 7. Server stub packs it in message, calls local OS 8. Server's OS sends message to client's OS 9. Client's OS gives message to client stub 10. Stub unpacks result, returns to client

 Client: ◦ Initiates RPC ◦ Makes a local call that invokes a corresponding procedure in the client stub  Client Stub: ◦ Receiving a call request from client:  Packs the specification of the target procedure & the arguments into a message  Asks local RPCRuntime to send it to server stub ◦ Receiving result of procedure execution  Unpacks the result & passes it to the client

 RPCRuntime: ◦ Client Machine  Receive call request message from Client Stub & sends to server machine  Receives the message containing the result of procedure execution from the server machine & sends to client stub ◦ Server Machine  Receives the message containing the result of procedure execution from the server stub & sends it to client machine  Receive call request message from the client machine & sends it to server stub

 Server Stub: ◦ Receiving a call request from local RPCRuntime:  Unpacks & makes a normal call to invoke the appropriate procedure in Server ◦ Receiving result of procedure execution  Packs the result into a message  Asks local RPCRuntime to send it to client stub  Server: ◦ Receiving a call request from the server stub  Executes the appropriate procedure  Returns the result of execution to the server stub

 Manually ◦ The RPC implementor provides a set of translation functions from which a user can construct his/her own stubs ◦ It is simple to implement and can handle very complex parameter types  Automatically ◦ Uses Interface Definition Language(IDL)

 The transfer of message data between two computers requires encoding & decoding of the message data  This operation in RPCs is known as Marshaling ◦ Actions involved in Marshaling  Taking the arguments  Encoding the message data on sender’s computer  Decoding the message data on receiver’s computer

 Two issues in server management ◦ Server Implementation  Stateful Server  Stateless Server ◦ Server Creation  Instance-per-call server  Instance-per-session server  Persistent servers

 Based on the style of implementation, it is of two types  Stateful Servers ◦ Maintains clients’ state information ◦ For eg: Consider a server for byte stream files that allows the following operations on file:  Open(filename, mode)  Read(fid, n, buffer)  Write(fid, n, buffer)  Seek(fid, position)  Close(fid)

Open(filename,mode) Return(fid) Return(bytes 0 to 99) Return(bytes100 to199) Read (fid, 100, buf) fid Mode R/W ptr Client Process Server Process Stateful File Server

 Stateless Servers ◦ Does not maintain any client state information ◦ For eg: Consider a server for byte stream files that allows the following operations on files is stateless:  Read(filename, position, n,buffer)  write(filename, position, n,buffer)

Return(bytes 0 to 99) Return(bytes100 to199) Read (fid, 100, buf) Read (filename, 100, buf) Client Process Server Process Stateless File Server fid Mode R/W ptr

 Based on time duration for which RPC servers survive, it is classified into three  Instance-per-Call Servers ◦ Exist only for the duration of a single call ◦ Created by RPCRuntime -> when a call message arrives  Instance-per-Session Servers ◦ Exist for the entire session for which a client & a server intract  Persistent Servers ◦ Remains in existence indefinetely