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CS 501: Software Engineering

Lecture 14

System Architecture and Design 2

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Administration

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Data Intensive SystemsExample: A Small-town Stockbroker

• Transactions

Received by mail or over telephone

For immediate or later action

• Complex customer inquiries

• Highly competitive market

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A Database Architecture

Databases:

• Customer and account database

• Financial products (e.g., account types, pension plans, savings schemes)

• Links to external databases (e.g., stock markets, mutual funds, insurance companies)

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Real-time Transactions

Customer & account database

Products & services database

External services

Real-time transactions

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Real-time Transactions & Batch Processing

Customer & account database

Products & services database

External services

Real-time transactions

Batch processing

Data input

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Stock Broker: Interface Diagram

CustomerDBProductDB

OnLineTR BatchTR

External

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Practical considerations to include in Architecture and Specification

• Real-time service during scheduled hours with batch processing overnight

• Database consistency after any type of failure

two-phase commitreload from checkpoint + logdetailed audit trail

• How will transaction errors be avoided and identified?

• How will transaction errors be corrected?

• How will staff dishonesty be controlled?

These practical considerations may be major factors in the choice of architecture.

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Architectural Styles

An architectural style is system architecture that recurs in many different applications.

See: Mary Shaw and David Garlan, Software architecture: perspectives on an emerging discipline. Prentice Hall, 1996

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Architectural Style: Pipe

Example: A three-pass compiler

ParserLexical analysis

Code generation

Output from one subsystem is the input to the next.

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Architectural Style: Master File Update

Master file update

Data input and validation

Mailing and reports

Example: billing system for electric utility

Advantages: Efficient way to process batches of transactions.

Disadvantages: Information in master file is not updated immediately.

Customer services

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Architectural Style: Repository

Repository

Input components

Transactions

Example: A digital library

Advantages: Flexible architecture for data-intensive systems.

Disadvantages: Difficult to modify repository since all other components are coupled to it.

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Architectural Style: Repository with Storage Access Layer

Data Store

Input components

Transactions

Advantages: Data Store subsystem can be changed without modifying any component except the Storage Access.

Storage Access

This is sometimes called a "glue" layer

Repository

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Data Intensive Systems: Merger of Two Banks

Each bank has a database with its customer accounts. The databases are used by staff at many branches and for back-office processing. These systems are examples of Repository Architectural Style.

The requirement is to integrate the two banks so that they appear to the customers to be a single organization and to provide integrated service from all branches.

This is an example of working with legacy systems.

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Merger of Two Banks: Options

???

???

AA BB

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Merger of Two Banks: Interface between the Databases

Accounts database

Batch input

Teller transactions

Accounts database

Batch input

Teller transactions

Bank A Bank B

Both systems follow the repository style

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Merger of Two Banks: Architectural Options

I. Convert everything to System A

convert databasesretrain staffenhance System A (software and

hardware)discard System B

II. Build an interface between the databases in System A and System B

III. Extend client software so that it can interact with either System A or System B database

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Merger of Two Banks: Interface between the Databases

Accounts database

Batch input

Teller transactions

Accounts database

Batch input

Teller transactions

Bank A Bank B

Data exchange

API

Problem Accounts databases are rarely exactly equivalent.

Data trans-form

Data trans-form

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Data Intensive Systems: Distributed Data

Distributed Data

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

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Architectures for Distributed Computing

An application that is running on one computer wishes to use data or services provided by another:

• Network connectionprivate, public, or virtual private networklocation of firewalls

• Protocolspoint-to-point, multicast, broadcastmessage passing, RPC, distributed objectsstateful or stateless

• Performance

quality of service

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Architectural Style: Client/Server

Web example: Serving static pages

Firefox client

Apache server

The control flows in the client and the server are independent. communication between client and server follows a protocol.

In a peer-to-peer architecture, the same component acts as both a client and a server.

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Architectural Style: Three Tier Architecture

Web example: Serving dynamic pages

Each of the tiers can be replaced by other components that implement the same interfaces

Presentation tier

Application tier

Database tier

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Distributed Computing: Multicast

User interfaceservice

User

Databases

This is an example of a multicast protocol.

The primary difficulty is to avoid troubles at one site degrading the entire system (e.g., every transaction cannot wait for a system to time out).

Three tier architecture: Broadcast searching

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Networks:Quality of Network Services

Criteria in choosing a network architecture

Performance

Maximum throughputLatencyVariations in throughputReal-time media (e.g., audio)

Business

Suppliers, costTrouble shooting and maintenance

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Networks: Choices

Public Internet:

Ubiquitous -- worldwideLow cost

Private network:

Security / reliabilityPredictable performanceChoice of protocols (not constrained to TCP/IP)

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Networks:Routers and Other Network Computing

• Interoperation with third party devices

=> remote devices may have faulty software

• Restart after total failure

• Defensive programming -- must survive

=> erroneous or malicious messages

=> extreme loads

=> time outs, dropped packets, etc.

• Evolution of network systems

=> Support for several versions of protocols

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Networks: Firewall

Public network

Private network

Firewall

A firewall is a computer at the junction of two network segments that:

• Inspects every packet that attempts to cross the boundary

• Rejects any packet that does not satisfy certain criteria, e.g.,

an incoming request to open a TCP connectionan unknown packet type

Firewalls provide security at a loss of flexibility and a cost of system administration.

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Distributed Data: Replication

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 problems are concurrency and consistency.

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Distributed Computing: Distributed Caches

.edu server

cornell.edu server

cs.cornell.edu server

First attempt to resolve www.cs.cornell.edu

1

2

3

The Domain Name System

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Distributed Computing:Distributed Caches

.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

The Domain Name System

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Distributed Computing:Distributed Caches

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

The Domain Name System

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Distributed Computing: Intermittent Connectivity

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.

Example: Usenet

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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: Buffering a CD Controller for Automobile

Input block Output

block

12

345

67

Circular buffer

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Time Critical System: Architectural Style - Daemon

Daemon

Example: Web server

The daemon listens at port 80

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

Spawned process

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Time Critical System: Architectural Style - Model/Controller/View

ModelController View

Example: An unmanned aircraft

Controller: Sends control signals to the aircraft and receives instrument readings.

Model: Translates data received from and sent to the aircraft into a model of flight performance. It uses domain knowledge about the aircraft and flight.

View: Displays information about the aircraft to the user.

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Time-Critical System: Autonomous Land Vehicle

Sensors

GPS

Sonar

Laser

Signal processing

Model Control signals

Steer

Throttle

Controls

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Time Critical System:Autonomous Land Vehicle

Model

Sensors

View

Extend model/controller/view

Controller

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Software Considerations

In some types of system architecture, non-functional requirements of the system may dictate the software design and development process.

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Software Considerations:Time-Critical System

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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Software Considerations: Performance

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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Software Considerations: Multi-Threading

Several similar threads operating concurrently:

• Re-entrant code -- separation of pure code from data for each thread• May be real-time (e.g., telephone switch) or non-time critical

The difficult of testing real-time, multi-threaded systems may determine the entire software architecture.

• Division into components, each with its own acceptance test.

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Software Considerations: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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Software Considerations: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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Software Considerations: 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.

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Coupling and Cohesion

Coupling is a measure of the dependencies between two subsystems. If two systems are strongly coupled, it is hard to modify one without modifying the other.

Cohesion is a measure of dependencies within a subsystem. If a subsystem contains many closely related functions its cohesion is high.

An ideal breakdown of a complex system into subsystems has low coupling between subsystems with high cohesion within subsystems.

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