A short summary of Comm. in Dist. Systems Part C

When everything works correctly, Remote Method Invocation (RMI) behaves exactly like a local method call. The client invokes a remote method transparently and receives the result as if the object were local.

The RMI omission failure model considers:

• Lost request messages
• Lost reply messages
• Server crashes
• Client crashes

Messages are either delivered correctly or lost, and processes either execute correctly or crash.

The client starts a timer when sending a request. If the timer expires before a reply or acknowledgment is received, the client resends the request.

If a request is not actually lost but delayed, the server may receive it more than once. This can cause the server to execute the same operation multiple times, leading to incorrect behavior.

Each request is assigned a unique identifier.

• If a duplicate arrives before the reply is sent, the server resends the reply.
• If it arrives after the reply was sent, the server uses history to resend the stored reply.

An idempotent operation can be executed multiple times without changing the final system state.

Examples:

• setName("Alice")
• deleteFile("A.txt")
• HTTP PUT

Non-idempotent operations change system state every time they are executed.

Examples:

• withdrawMoney(100)
• createUser("Ali")
• HTTP POST

Executing them multiple times may lead to incorrect results.

Exactly-once semantics can be achieved by:

• Resending requests until a reply is received
• Filtering duplicate requests
• Maintaining a history (log) of executed requests and replies

If a server crashes after executing an operation but before sending the reply, and loses memory on restart, it cannot know whether the operation was already executed.

In direct communication, the sender knows the identity and address of the receiver and sends messages directly.

Examples include HTTP client-server communication and TCP sockets.

Indirect communication uses an intermediary. The sender does not need to know the identity of the receiver.

It provides space and time decoupling.

Group communication allows a sender to send a message to a group of receivers in a single operation, rather than sending separate messages.

It is useful for:

• Fault tolerance through replication
• Service discovery
• Managing replicated data
• Interest groups and mailing lists

Publishers generate events, and subscribers receive only the events they are interested in through a notification service. Publishers and subscribers are decoupled.

The notification service stores subscriptions, receives events from publishers, and dispatches notifications to interested subscribers.

RPCs use arbitrary ports, are often platform-dependent, were designed for LANs, and do not handle scalability, latency, or statelessness well.

Web services are platform-independent, use standard protocols (HTTP), text-based data formats (XML/JSON), tolerate high latency, and support large numbers of clients.

• SOAP: XML-based, uses WSDL, strict standards, heavier messages
• REST: Uses HTTP verbs, JSON, lightweight, stateless, easier to scale

• SOA: Coarse-grained services, shared database, SOAP-based communication
  
• Microservices: Fine-grained services, independent databases, REST-based APIs, independent deployment