distributed systems

Distributed Systems

Lecture #1: 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

Layered Communication

Application

Distributed System (RPC)

Socket

OSTCP/IPUDP/IP

Local CommunicationHow 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?

RPC Components

Client Server

Stub Skeleton

Client Framework Server Framework

Network

(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 or reference?

• Return Values– How to return?

• IDL: In, Out, INOut

• RMI: Objects– Recursive packing

RPC: Summary• Synchronous, Blocking• Mimics a procedural call• Involves:

– Data transfer: Marshaling– Thread Migration

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

Events• Subscribe/notify

• Asynchronous

• Non-blocking

Subscribe

Notify

Events• Better performance than polling for larger tasks

– Do it, let me know when you are done

• Preferred mode of communication for interactive applications: none-blocking

• But Harder to program– Synchronization– Spaghetti code

Programming Events

Main thread

Events Handler

Subscribe

Notify

Distributed Shared Memory

Int x;x = 10;

Int x;..print x;

X = 10• Consistency?• Deadlock?• DSM abstraction?

• Make it explicit

Tuple-spaces

Tuple-space

• Explicit DSM• Tuple as the atomic data object • Clearly defined semantics for insert/read/peak