Distributed Systems Lecture #3: Remote Communication

Communication Communication across address spaces –Local –Remote Same Rules apply –Transparency –Fault-isolation –Consistency

Overview Mechanism –Message Passing Abstractions –Message Passing –Synchronous: Procedure Calls –Asynchronous: Events –Coupled: Shared Memory –De-coupled: Tuple-spaces

Local Communication How would 2 processes communicate? Message Passing: Sockets Shared Memory: Files Procedural call (across address space)? Ever used local RPC? Heard of OLE? COM?

RPC Enables Modularization Clients and Servers –Better software engineering Well-defined interfaces between Client Service provider –Fault-isolation Client failure does not affect the server Centralized consistency management –Security –Scalable Performance: Server not constrained by client

Local RPC Transparency! –Makes sense? Issues: –What transpires in a subroutine call?

(Local) RPC Issues Transfer of control and data across address spaces –Binding –Data Marshalling –Thread Migration –Synchronization

Remote RPC Same Issues –BUT transparency makes sense? To what degree? –Modularization –Fault-isolation But masking? Failure may be partial –Time? Consistency –Scalable Performance?

Case Study: Sun RPC Used in NFS What does an RPC system consist of? 1. Standards for wire format of RPC msgs and data types. XDR, XML 2. Library of routines to marshal / unmarshal data. 3. Stub generator, Client: marshal, invoke, wait, unmarshal reply. For server: unmarshal arguments, invoke server function, marshal reply. 4. Server framework: Dispatch each call message to correct server stub. 5. Client framework: Give each reply to correct waiting thread / callback. 6. Binding: how does client find the right server?

Sun RPC What does a Sun RPC request contain? Wire format –Xid –Call orreply –RPC version –program # –program version –procedure # –auth bits –Marshalled arguments

Marshalling: Sun RPC Marshaling arguments "Linearize" data "Externalize" data: Formats defined by XDR standard Easy for e.g. int -- same representation, though portable byte order... Collections? include a length. Pointers? Pass by value of reference? Return Values –How to return? IDL: In, Out, INOut RMI: Objects –Recursive packing

NFS Transparency Tension between performance and consistency –Syntax preserved –Not Semantics –File read/write semantics Only check on open() Consistency? –Events! Expose Distribution: Java RMI