Software Engineering Lecture 15: System Architecture II - Distributed & Real Time (CS 501), Slides of Software Engineering

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CS 501: Software Engineering

Fall 2000

Lecture 15

System Architecture II Distributed and Real Time Systems

Administration

Assignment 2: Requirements

• Grades -- presentation, report, individual
• Comments at presentation
• Comments from teaching assistant

Assignment 3: Design

Sequence Diagram: Notation

BookBorrower

libMem: LibraryMember theCopy:Copy

theBook:Book

borrow(theCopy) okToBorrow

borrow borrow

dotted line shows object lifetime

rectangle shows focus of control

Sequence Diagram: Branching

BookBorrower

libMem: LibraryMember theCopy:Copy

theBook:Book

1:borrow(theCopy) 2:okToBorrow

[ok]3:borrow 4:borrow

branch

[not ok]3:noborrow

Distributed Data and Replication

Distributed Data

Data is held on several computer systems. A transaction may need to assemble data from several sources.

Replication

Several copies of the data are held in different locations.

Mirror: Complete data set is replicated

Cache: Dynamic set of data is replicated (e.g., most recently used)

With replicated data, the biggest problem is consistency.

Example: Broadcast Search

User interface

User^ server

Databases

Stateless Protocol v. Stateful

Stateless protocol Example: http Open connection Send message Return reply Close connection State in http must be sent with every message (e.g., as parameter string or in a cookie)

Stateless Protocol v. Stateful

Stateful (session) protocol

Example: Z39.

Open connection Begin session Interactive session End session Close connection

Server remembers the results of previous transactions (e.g., authentication, partial results) until session is closed.

The Domain Name System

.edu server

cornell.edu server

cs.cornell.edu server

First attempt to resolve www.cs.cornell.edu

1

Discussion of the First Attempt

Problems?

Real Time System

A real time system is a software system whose correct functioning depends upon the results produced and the time at which they are produced.

• A soft real time system is degraded if the results are not produced within required time constraints
• A hard real time system fails if the results are not produced within required time constraints

Example: Web Server

http message daemon

spawned processes

TCP port 80

The daemon listens at port 80.

When a message arrives it:

spawns a processes to handle the message returns to listening at port 80

Example: Autonomous Land Vehicle

Sensors

GPS

Sonar

Laser

Signal processing

Model Control signals

Steer

Throttle

Controls

Other Applications

Response critical

• Network router
• Telephone switch
• Seat bag controller

Shared systems

• Multi-user data processing
• Time sharing