1 cs 501 spring 2004 cs 501: software engineering lecture 15 system architecture and design 2
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1 CS 501 Spring 2004
CS 501: Software Engineering
Lecture 15
System Architecture and Design 2
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Administration
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Quiz 2, Question 1
You are developing the requirements for an online shopping system. To place an order, a user connects to the system, searches to find items to purchase, selects one or more items, and supplies credit card information to pay for them.
(i) Create a scenario for a user making a purchase.
A scenario should follow an individual user through a single interaction with the system.
See Lecture 8, slides 11 to 14 for a typical example.
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Quiz 2, Question 1
(ii) Develop a use case diagram and brief specification for a use case, PlaceOrder, which is modeled on this scenario. The use case should show a relationship to a previously specified use case, Pay, which models credit card payments. (You do not need to specify the Pay use case.)
PayPlaceOrder<<includes>>
Shopper
The subject of a use case is an actor, with a generic role.
For an example of a specification, see Lecture 8 slides 26-27
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Quiz 2, Question 1
CheckOrder
(iii) A user might interact with the online shopping system in other ways. Draw a diagram for a different use case in which the same actor interacts with the online shopping system.
Shopper
Note same actor for this different use case.
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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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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: 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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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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Architectural Style: Client/Server
Example: the Web
Mozilla 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: Pipe
Example: A three-pass compiler
ParserLexical analysis
Code generation
Output from one subsystem is the input to the next.
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Distributed Computing: General Problem
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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Network Choices
Public Internet:
Ubiquitous -- worldwideLow cost
Private network:
Security / reliabilityPredictable performanceChoice of protocols (not constrained to TCP/IP)
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Quality of Network Services
Criteria in choosing a system architecture
Performance
Maximum throughputLatencyVariations in throughputReal-time media (e.g., audio)
Business
Suppliers, costTrouble shooting and maintenance
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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
two copies of the same data
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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 problems are concurrency and consistency.
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Distributed Computing: Searching
User interfaceserverUser
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).
Broadcast Searching
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Distributed Computing:Stateless Protocol v. Stateful
Stateless protocol
Example: http
Open connectionSend message Return replyClose connection
State in http must be sent with every message (e.g., as parameter string)
Cookies are a primitive way of retaining some state
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Stateless Protocol v. Stateful
Stateful (session) protocol
Example: Z39.50
Open connectionBegin sessionInteractive sessionEnd sessionClose connection
Client and server remember the results of previous transactions (e.g., authentication, partial results) until session is closed.
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Distributed Computing: 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 Computing: 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 Computing:The 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
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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 Computing: 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: Autonomous Land Vehicle
Sensors
GPS
Sonar
Laser
Signal processing
Model Control signals
Steer
Throttle
Controls
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Time-Critical System:CD Controller
Input block Output
block
12
345
67
Circular buffer
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Software Considerations of System Architectures
In some types of system architecture, non-functional requirements of the system may dictate the software design and development process.
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Time-Critical System: 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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Time-Critical System: 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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Software Considerations of System Architectures: 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 of System Architectures: 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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Time-Critical System: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 of System Architectures: 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.