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CS 501: Software Engineering

Lecture 16

System Architecture and Design II

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Administration

• Schedule your second presentations with Anat Nidar-Levi

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Distributed ComputingExample 4: UseNet

This is an example of an epidemic protocol. Such protocols are especially useful in networks with intermittent connectivity, e.g., mobile computing.

The biggest problem is ensuring that the data is distributed effectively.

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Distributed ComputingExample 5: The Domain Name System

.edu server

cornell.edu server

cs.cornell.edu server

First attempt to resolve www.cs.cornell.edu

1

2

3

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Distributed ComputingThe Domain Name System

.edu server

cornell.edu server

cs.cornell.edu server

Better method

3

1

almaden.ibm.comcornell.eduece.cmu.eduibm.comacm.org.edu

2

Localcache

local DNS server

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Distributed ComputingDomain Name System

For details of the actual protocol read:

Paul Mockapetris, "Domain Names - Implementation and Specification". IETF Network Working Group, Request for Comments: 1035, November 1987.

http://www.ietf.org/rfc/rfc1035.txt?number=1035

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Distributed ComputingExample 5: Web Server

http messagedaemon

spawned processesTCP port 80

The daemon listens at port 80

When a message arrives it:spawns a processes to handle the messagereturns to listening at port 80

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Time-Critical Systems

A real time (time-critical) 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

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Time-Critical System Example 1: Autonomous Land Vehicle

Sensors

GPS

Sonar

Laser

Signal processing

Model Control signals

Steer

Throttle

Controls

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Time-Critical System Example 2: Routers and Other Network Computing

• Interoperation with third party devices

• Support for several versions of protocols

• Restart after total failure

• Defensive programming -- must survive

=> erroneous or malicious messages

=> extreme loads

• Time outs, dropped packets, etc.

• Evolution of network systems

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Techniques: Software Development

Developers of advanced time-critical software spend almost all their effort developing the software environment:

• Monitoring and testing -- debuggers

• Crash restart -- component and system-wide

• Downloading and updating

• Hardware troubleshooting and reconfiguration

etc., etc., etc.

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Time-Critical System Example 3:CD Controller

Input block Output

block

12

345

67

Circular buffer

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Time-Critical System Example 4:Embedded Real-time Systems

Software and hardware are combined to provide an integrated unit, usually dedicated to a specific task:

• Digital telephone

• Automobile engine control

• GPS

• Scientific instruments

• Seat bag controller

The software may be embedded in the device in a manner that cannot be altered after manufacture.

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Time-Critical System Example 4:Embedded Real-time Systems

Design of embedded systems requires close understanding of hardware characteristics

• Special purpose hardware requires special tools and expertise.

• Some functions may be implemented in either hardware of software (e.g., floating point unit)

• Design requires separation of functions

Distinction between hardware and software may be blurred.

Hardware v. Software

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Time-Critical System Example 5:Shared Systems

Many users are using the same equipment at the same time

• Multi-user data processing (common task)

• Time sharing (independent tasks)

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Techniques

• Special purpose hardware

• Multi-threading and multi-tasking

• Parallel processing

=> digital signal processing

• Interrupts

=> levels and priorities

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Techniques: Multi-Threading

Several similar threads operating concurrently:

• Re-entrant code -- separation of pure code from data for each thread

• Testing -- single thread and multi thread

May be real-time (e.g., telephone switch) or non-time critical

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Techniques: Real Time Executive

Schedules and dispatches tasks in a real time system

• Real time clock

• Interrupt handler

• Scheduler

• Resource manager

• Dispatcher

Must be extremely reliable

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Techniques: Timing

Timing mechanisms

• Synchronous (clocked) -- periodic stimuli

• Asynchronous -- wait for next signal

Example: Communications protocols may be synchronous or asynchronous

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Techniques: Software Considerations

Resource considerations may dictate software design and implementation:

• Low level language (e.g., C) where programmer has close link to machine

• Inter-process communication may be too slow (e.g., C fork).

• May implement special buffering, etc., to control timings

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Continuous Operation

Many systems must operate continuously

• Software update while operating

• Hardware monitoring and repair

• Alternative power supplies, networks, etc.

• Remote operation

These functions must be designed into the fundamental architecture.