Idempotent operations

An idempotent operation is an operation that can be performed repeatedly with the same effect as if it had been performed exactly once.

History

If operations are not idempotent, the server must ensure that the same request is not executed twice. Keep a history of all requests and replies. The same reply can be sent without re-executing if a request is resent.

• At-most-once: the request has been executed once. Implemented using a history or simply not resending requests.
• At-least-once: the request has been executed at least once. No need for a history; simply resend requests until a reply is received.

HTTP

Request = Request-Line *(header CRLF) CRLF [ message-body ]

Request-Line = Method space Request-URI space HTTP-Version CRLF

GET /index.html HTTP/1.1\r\n foo 42 \r\n\r\nHello

CRLF: carriage return; line feed.

RPC: Remote Procedure Call

RPC is a mechanism that allows a program running on one computer (VM) to cause a procedure to be executed on another computer (VM) without the programmer needing to code for this explicitly.

Two processes are involved:

• Caller (RPC client) is a calling process that initiates an RPC to a server.
• Callee (RPC server) is a called process that accepts the call.

Executing RPC

On each RPC, the server starts a new process to execute the call.

• The new process terminates when the procedure returns and results are sent to the caller.
• Calls from the same caller and calls from different callers are serviced by different concurrent processes on the server.

Concurrent invocations might interfere when accessing shared objects – might need synchronization.

Java RMI

A remote object is passed as a reference (by reference), i.e., it remains at the original place it was created.

A serializable object is passed as a copy (by value), i.e., the object is duplicated.