Distributed Systems

Lecture # 3

Administrivia

• Projects– Design and Implement a distributed file

system

• Paper Discussions– Discuss papers as case studies

Paper Discussions

• Attended by ~40 students• 15% grade (10 + 5)• Paper discussion modeled after program committee

meetings • Each Paper presented by 4 students

– Motivator (context of work)– Presenter– Advocate– Devil’s advocate– Class discussion and vote

Assignments

• Assignments:– One big project

• Teams of 5

– Network File System• Both paper design and implementation• Communication (2 weeks: Design and implement)• Naming (2 weeks: design and implement)• Storage: Multiple users (2 weeks: design and implement)• Reliability (make it work in 3 weeks)

Design Documents

• 400 words: One pager

• Two main sections– What is your design?– What were the alternatives

• Only bullet points– A picture is worth a thousand words

Implementation

• .. then you implement your design

• No copying of code– This is a serious offense!

• Use Java: Lots of existing code and libraries– Code submission at the end of the week– TAs will need to see a demo

Layered Communication

Application

Distributed System (RPC)

Socket

OSTCP/IPUDP/IP

Sun RPC

• What does a Sun RPC request contain?• Wire format

– Xid

– Call or reply

– RPC version

– program #

– program version

– procedure #

– auth bits

– Marshaled 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

RPC Components

Client Server

Stub Skeleton

Client Framework Server Framework

Network

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

Your distributed file system

• What would you use?

Your file system

Java

Operating System

-Client-servercommunication- Naming of files-Storage of files- Consistency- Availability

Your file-system

File Server

Remote Client• Communication?• Name Server?

• Directories?• Storage

• Local file-system?• Consistency of data/names• Fault-tolerance