Remote Procedure Calls

Benefits & Requirements of RPC

RPC is intended to simplify the development of cross address space and cross machine interactions.

• It provides higher level interface for data movement and communication.

• It provides error handling.

• It hides the complexity of cross-machine interactions.

In order to implement RPC, one should satisfy the following requirements.

• Logic for client and server interactions

• Procedure call interface should be synchronous

• Type checking for error handling and packet bytes interpretation

• Cross-machine conversion like big to little endian for integers

• Higher-level protocol for access control, fault tolerance, and different transport protocols

Interaction

The following is a generic RPC structure for machine to machine interaction using RPC.

Structure of RPC

1. Binding - Client finds and binds to the desired server, e.g. establishing TCP connection.

2. Call - Client makes RPC call, control passed to stub, client code blocks.

3. Marshal - Client stub marshals arguments, i.e. serialize arguments into buffer.

4. Send - Client sends messages to server.

5. Receive - Server receives message, passes message to server stub and access control.

6. Unmarshal - Server stub unmarshals arguments, i.e. deserialize arguments into data structures.

7. Actual Call - Server stub calls local procedure implementation.

8. Result - Server performs operation and computes result of RPC operation.

The remaining steps are to return data back to client following the similiar steps above.

Interface Definition Language

In the context of gRPC, protobuf is the IDL. An IDL used to describe the interface server exports should provide the following features.

• Procedure name and arguments

• Version number

• Language agnostic

Protobuf provides serialization/deserialization that satisfies the features above. Notice that sending pointers to another machine with a different address space is meaningless. Typically, IDL would either

• Disallow all pointers

• Serialize pointers and copy referenced data structure

Error Handling

RPC cannot provide a specific answer to why something fails. Usually RPC would provide a special RPC error notification that would capture any of the following failures.

• client packet lost

• server packet lost

• network link down

• server machine down

• server process failed

• server process overloaded

Not going to dive too much into the details of SunRPC and XDR because they are irrelevant technologies now.