Chapter 7 Addressing Design Goals

Transcription

1 Object-Oriented Software Engineering Using UML, Patterns, and Java Chapter 7 Addressing Design Goals Overview System Design I 0. Overview of System Design 1. Design Goals 2. Subsystem Decomposition 3. Refine the subsystem decomposition until all design goals are addressed. System Design II 3. Concurrency 4. Hardware/Software Mapping 5. Persistent Data Management 6. Global Resource Handling and Access Control 7. Software Control 8. Boundary Conditions Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 2

2 Redundancy in the Space Shuttle computer system Unlike previous spacecraft, the space shuttle was designed to be autonomous. the multiple missions be longer and crews larger than on previous Apollo missions. the mission of this shuttle needs to tolerate before abort. many redundant features including a fault-tolerant computer system responsible for guidance, navigation, and altitude control 1) The Saturn rocket (for launching the Apollo spacecraft) used triple modular redundancy for guidance system - three components - the failure of a single component was detected when it produced a different output than the other two. for example, it would not have survived a massive failure, such as, the exposition on Apollo 13. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 3 The Skylab Space station took a different approach: - the computer systems are duplicated and located at different ends of the station. - when one computer failed, the other will be switched on take over. - whereas a slow switch-over for a space station, (i.e., the space station could loose some altitude before safety), it would not acceptable for the space shuttle, whose computer system was responsible for high-frequency tasks such as guidance during take-off and landing. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 4

3 The initial requirements By NASA, the Shuttle should be able to expensive two consecutive failures before the mission was aborted. - Five identical computers running the same software, if two individual computers failed, the last three would constitute a triple redundancy system for landing. if the third one failed, the last two would be enough to ensure a safe landing. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 5 ** Due to cost consideration, NASE later decided to lower its requirement to one failure before mission abort. - Five computers, But fifth computer for a back-up system. - While the quadruple redundancy against H/W failure, it does not increase reliability against software faults, as all four computers run the same software. However, the back-up system runs a simpler version of the software that is only able to guide the shuttle during take-off and landing. How architectural decisions were made during the design of a complex computer system. Driven by design goals and nonfunctional requirements. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 6

4 The activities of system design that address the design goals. Define design goals Define subsystems Implement subsystems Map subsystems to hardware/ software platform Manage persistent data Define access control policies Select a global control flow Describe boundary conditions Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 7 UML Deployment Diagram Used to depict the relationship among run-time components and hardware nodes. Components are self-contained entities that provide services to other components or actors. Deployment Diagram focuses on the allocation of components to different hardware nodes and provides a high-level view of each component. Components includes information about interfaces they provide and the classes they contain. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 8

5 A UML deployment diagram representing the allocation of components to different nodes and the dependencies among components. mymac:mac :UnixHost :Safari dependency :WebServer Component apc:pc :UnixHost :IExplorer :Database Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 9 Refined view of the WebServer component. WebServer HttpRequest File Classes URL DBQuery DBResult GET POST Interfaces Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 10

6 7.4 System Design Activities: Addressing Design Goals These activities needed to ensure that subsystem decomposition addresses all the nonfunctional requirements and any constraints during implementation phase. [Section 6.4] for MyTrip -Already identify Design Goals - designed an initial subsystem decomposition. Refine The subsystem decomposition by 1) Mapping Subsystem to Processors and Components (7.4.1) 2) Identifying and Storing Persistent Data (7.4.2) 3) Providing Access Control (7.4.3) 4) Designing the Global Control Flow (7.4.4) 5) Reviewing the System Design Model (7.4.6) Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 11 Analysis Model for the Mytrip route planning and execution RouteAssistant Location Crossing Trip Direction Segment PlanningService Destination Crossing: A Crossing is a geographical point where several Segments meet. Destination: A Destination represents a location where the driver wishes to go. Direction: Given a Crossing and an adjacent Segment, a Direction describes in natural language how to steer the car onto the given Segment. Location: A Location is the position of the car as known by the onboard GPS system the number of turns of the wheels. PlanningService: A PlanningService is a Web server that can supply a trip, linking a number of destinations in the form of a sequence of Crossings and Segments. RouteAssistant: A RouteAssistant givens Directions to the driver, given the current Location and upcoming Crossing. Segment: A Segment represents the road between two Crossings. Trip: A Trip is a sequence of Directions between two Destinations. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 12

7 Nonfunctional requirements for Myrip 1) MyTrip is in contact with the PlanningService via a wireless modem. Assume that the wireless modem functions properly at the initial destination. 2) Once the trip has been started. Mytrip should give a correct directions even if modem fails to maintain a connection with the PlanningService. 3) MyTrip should minimize connection time to reduce operation costs. 4) Replanning is possible only if the connection to the PlanningService is possible. 5) The PlanningService can support at least 50 different drivers and 1,000trips. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 13 Design goals for Myrip Reliability: MyTrip should be reliable [generalization of NFR 2] Fault Tolerance: Mytrip should give fault tolerant to loss of connectivity with routing service [rephrased NFR 2] Security: MyTrip should be se cure,i.e., not allow other drivers or nonauthorized uses to access a driver s trips [deduced from application domain] Modifiability: MyTrip should be modifiable to use different routing services [anticipation of change by developers] Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 14

8 Mapping Subsystem to Hardware and Components Selecting a hardware configuration and a platform Allocation objects and subsystem to Hardware Nodes Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 15 Allocation of MyTrip subsystems to hardware. The Web browsers, safari, and Internet explorers as a virtual machine a Unix system as a virtual machine :OnBoardComputer RoutingSubsystem :WebServer PlanningSubsystem (RouingSbusystem runs on the OnBoardComputer; PlanningSubsystem runs on a WebServer.) Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 16

9 Revised design model for MyTrip. RoutingSubsystem PlanningSubsystem RouteAssistant PlanningService Location Trip Destination TripProxy SegmentProxy Add new classes CommunicationSubsystem Crossing Direction Segment Message Connection Add New subsystem for managing the communication between them Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java Hardware Software Mapping This activity addresses two questions: How shall we realize the subsystems: Hardware or Software? How is the object model mapped on the chosen hardware & software? Mapping Objects onto Reality: Processor, Memory, Input/Output Mapping Associations onto Reality: Connectivity Much of the difficulty of designing a system comes from meeting externally-imposed hardware and software constraints. Certain tasks have to be at specific locations Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 18

10 Mapping the Objects Processor issues: Is the computation rate too demanding for a single processor? Can we get a speedup by distributing tasks across several processors? How many processors are required to maintain steady state load? Memory issues: Is there enough memory to buffer bursts of requests? I/O issues: Do you need an extra piece of hardware to handle the data generation rate? Does the response time exceed the available communication bandwidth between subsystems or a task and a piece of hardware? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 19 Mapping the Subsystems Associations: Connectivity Describe the physical connectivity of the hardware Often the physical layer in ISO s OSI Reference Model Which associations in the object model are mapped to physical connections? Which of the client-supplier relationships in the analysis/design model correspond to physical connections? Describe the logical connectivity (subsystem associations) Identify associations that do not directly map into physical connections: How should these associations be implemented? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 20

11 Typical Informal Example of a Connectivity Drawing Application Client Application Client Application Client Physical Connectivity Logical Connectivity Communication Agent for Application Clients TCP/IP Communication Agent for Application Clients Backbone Network LAN Communication Agent for Data Server Ethernet LAN Global Data Server OODBMS Communication Agent for Data Server Global Data Server LAN RDBMS Local Data Server Global Data Server Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 21 Logical vs Physical Connectivity and the relationship to Subsystem Layering Application Layer Application Layer Presentation Layer Session Layer Transport Layer Bidirectional associations for each layer Presentation Layer Session Layer Transport Layer Logical Connectivity Layers Network Layer Network Layer Data Link Layer Physical Layer Data Link Layer Physical Layer Physical Connectivity Processor 1 Processor 2 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 22

12 Layer 1 Subsystem 1 Subsystem 2 Layer 2 Layer 3 Layer 4 Layer 1 Layer 2 Layer 3 Application Layer Application Layer Presentation Layer Presentation Layer Session Layer Transport Layer Network Layer Bidirectional associations for each layer Session Layer Transport Layer Network Layer Data Link Layer Data Link Layer Hardware Hardware Processor 1 Processor 2 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 23 Hardware/Software Mapping Questions What is the connectivity among physical units? Tree, star, matrix, ring What is the appropriate communication protocol between the subsystems? Function of required bandwidth, latency and desired reliability, desired quality of service (QOS) Is certain functionality already available in hardware? Do certain tasks require specific locations to control the hardware or to permit concurrent operation? Often true for embedded systems General system performance question: What is the desired response time? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 24

13 Connectivity in Distributed Systems If the architecture is distributed, we need to describe the network architecture (communication subsystem) as well. Questions to ask What are the transmission media? (Ethernet, Wireless) What is the Quality of Service (QOS)? What kind of communication protocols can be used? Should the interaction asynchronous, synchronous or blocking? What are the available bandwidth requirements between the subsystems? Stock Price Change -> Broker Icy Road Detector -> ABS System Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 25 Drawing Hardware/Software Mappings in UML System design must model static and dynamic structures: Component Diagrams for static structures show the structure at design time or compilation time Deployment Diagram for dynamic structures show the structure of the run-time system Note the lifetime of components Some exist only at design time Others exist only until compile time Some exist at link or runtime Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 26

14 Identifying and Storing Persistent Objects Identifying persistent objects Selecting a Storage a management strategy Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 27 Subsystem decomposition of MyTrip after deciding on the issue of data stores. RoutingSubsystem PlanningSubsystem CommunicationSubsystem TripFileStoreSubsystem MapDBStoreSubsystem TripFileStoreSubsystem: responsible for storing trips in files on the onboard computer. because this functionality is only used for storing trips when the car shuts down, this subsystem only supports the fast storage and loading of the whole trips. MapDBStoreSubsystem: responsible for storing maps and trips in database for the PlanningSubsystem. This subsystem supports multiple concurrent Drivers and Planning agents Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 28

15 5. Data Management Some objects in the models need to be persistent Provide clean separation points between subsystems with welldefined interfaces. A persistent object can be realized with one of the following Data structure Files Database If the data can be volatile Cheap, simple, permanent storage Low level (Read, Write) Applications must add code to provide suitable level of abstraction Powerful, easy to port Supports multiple writers and readers Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 29 File or Database? When should you choose a file? Are the data voluminous (bit maps)? Do you have lots of raw data (core dump, event trace)? Do you need to keep the data only for a short time? Is the information density low (archival files,history logs)? When should you choose a database? Do the data require access at fine levels of details by multiple users? Must the data be ported across multiple platforms (heterogeneous systems)? Do multiple application programs access the data? Does the data management require a lot of infrastructure? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 30

16 Database Management System Contains mechanisms for describing data, managing persistent storage and for providing a backup mechanism Provides concurrent access to the stored data Contains information about the data ( meta-data ), also called data schema. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 31 Issues To Consider When Selecting a Database Storage space Database require about triple the storage space of actual data Response time Mode databases are I/O or communication bound (distributed databases). Response time is also affected by CPU time, locking contention and delays from frequent screen displays Locking modes Pessimistic locking: Lock before accessing object and release when object access is complete Optimistic locking: Reads and writes may freely occur (high concurrency!) When activity has been completed, database checks if contention has occurred. If yes, all work has been lost. Administration Large databases require specially trained support staff to set up security policies, manage the disk space, prepare backups, monitor performance, adjust tuning. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 32

17 Object-Oriented Databases Support all fundamental object modeling concepts Classes, Attributes, Methods, Associations, Inheritance Mapping an object model to an OO-database Determine which objects are persistent. Perform normal requirement analysis and object design Create single attribute indices to reduce performance bottlenecks Do the mapping (specific to commercially available product). Example: In ObjectStore, implement classes and associations by preparing C++ declarations for each class and each association in the object model Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 33 Relational Databases Based on relational algebra Data is presented as 2-dimensional tables. Tables have a specific number of columns and and arbitrary numbers of rows Primary key: Combination of attributes that uniquely identify a row in a table. Each table should have only one primary key Foreign key: Reference to a primary key in another table SQL is the standard language defining and manipulating tables. Leading commercial databases support constraints. Referential integrity, for example, means that references to entries in other tables actually exist. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 34

18 Data Management Questions Should the data be distributed? Should the database be extensible? How often is the database accessed? What is the expected request (query) rate? In the worst case? What is the size of typical and worst case requests? Do the data need to be archived? Does the system design try to hide the location of the databases (location transparency)? Is there a need for a single interface to access the data? What is the query format? Should the database be relational or object-oriented? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java Concurrency Identify concurrent threads and address concurrency issues. Design goal: response time, performance. Threads A thread of control is a path through a set of state diagrams on which a single object is active at a time. A thread remains within a state diagram until an object sends an event to another object and waits for another event Thread splitting: Object does a nonblocking send of an event. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 36

19 Providing Access Control Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 37 Defining Access Control In multi-user systems different actors have access to different functionality and data. During analysis we model these different accesses by associating different use cases with different actors. During system design we model these different accesses by examing the object model by determining which objects are shared among actors. Depending on the security requirements of the system, we also define how actors are authenticated to the system and how selected data in the system should be encrypted. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 38

20 Access Matrix We model access on classes with an access matrix. The rows of the matrix represents the actors of the system The column represent classes whose access we want to control. Access Right: An entry in the access matrix. It lists the operations that can be executed on instances of the class by the actor. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 39 Access Matrix Implementations Global access table: Represents explicitly every cell in the matrix as a (actor,class, operation) tuple. Determining if an actor has access to a specific object requires looking up the corresponding tuple. If no such tuple is found, access is denied. Access control list associates a list of (actor,operation) pairs with each class to be accessed. Every time an object is accessed, its access list is checked for the corresponding actor and operation. Example: guest list for a party. A capability associates a (class,operation) pair with an actor. A capability provides an actor to gain control access to an object of the class described in the capability. Example: An invitation card for a party. Which is the right implementation? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 40

21 Global Resource Questions Does the system need authentication? If yes, what is the authentication scheme? User name and password? Access control list Tickets? Capability-based What is the user interface for authentication? Does the system need a network-wide name server? How is a service known to the rest of the system? At runtime? At compile time? By port? By name? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java Decide on Software Control Choose implicit control (non-procedural, declarative languages) Rule-based systems Logic programming Choose explicit control (procedural languages): Centralized or decentralized Centralized control: Procedure-driven or event-driven Procedure-driven control Control resides within program code. Example: Main program calling procedures of subsystems. Simple, easy to build, hard to maintain (high recompilation costs) Event-driven control Control resides within a dispatcher calling functions via callbacks. Very flexible, good for the design of graphical user interfaces, easy to extend Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 42

22 Concurrency (continued) Two objects are inherently concurrent if they can receive events at the same time without interacting Inherently concurrent objects should be assigned to different threads of control Objects with mutual exclusive activity should be folded into a single thread of control (Why?) Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 43 Concurrency Questions Which objects of the object model are independent? What kinds of threads of control are identifiable? Does the system provide access to multiple users? Can a single request to the system be decomposed into multiple requests? Can these requests be handled in parallel? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 44

23 Implementing Concurrency Concurrent systems can be implemented on any system that provides physical concurrency (hardware) or logical concurrency (software): Scheduling problem (Operating systems) Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 45 Designing Global Control Flow Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 46

24 Global Resource Questions Does the system need authentication? If yes, what is the authentication scheme? User name and password? Access control list Tickets? Capability-based What is the user interface for authentication? Does the system need a network-wide name server? How is a service known to the rest of the system? At runtime? At compile time? By port? By name? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java Decide on Software Control Choose implicit control (non-procedural, declarative languages) Rule-based systems Logic programming Choose explicit control (procedural languages): Centralized or decentralized Centralized control: Procedure-driven or event-driven Procedure-driven control Control resides within program code. Example: Main program calling procedures of subsystems. Simple, easy to build, hard to maintain (high recompilation costs) Event-driven control Control resides within a dispatcher calling functions via callbacks. Very flexible, good for the design of graphical user interfaces, easy to extend Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 48

25 Event-Driven Control Example: MVC Model-View-Controller Paradigm (Adele Goldberg, Smalltalk 80) :Control Model has changed Update Update Update :View :Model :View :View Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 49 Software Control (continued) Decentralized control Control resides in several independent objects. Possible speedup by mapping the objects on different processors, increased communication overhead. Example: Message based system. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 50

26 Centralized vs. Decentralized Designs Should you use a centralized or decentralized design? Take the sequence diagrams and control objects from the analysis model Check the participation of the control objects in the sequence diagrams If sequence diagram looks more like a fork: Centralized design The sequence diagram looks more like a stair: Decentralized design Centralized Design One control object or subsystem ("spider") controls everything Pro: Change in the control structure is very easy Con: The single conctrol ojbect is a possible performance bottleneck Decentralized Design Not a single object is in control, control is distributed, That means, there is more than one control object Con: The responsibility is spread out Pro: Fits nicely into object-oriented development Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 51 Identifying Boundary Conditions Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 52

27 8. Boundary Conditions Most of the system design effort is concerned with steady-state behavior. However, the system design phase must also address the initiation and finalization of the system. This is addressed by a set of new uses cases called administration use cases Initialization Describes how the system is brought from an non initialized state to steady-state ("startup use cases ). Termination Describes what resources are cleaned up and which systems are notified upon termination ("termination use cases"). Failure Many possible causes: Bugs, errors, external problems (power supply). Good system design foresees fatal failures ( failure use cases ). Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 53 Example: Administrative Use cases for MyTrip Administration use cases for MyTrip (UML use case diagram). An additional subsystems that was found during system design is the server. For this new subsystem we need to define use cases. ManageServer includes all the functions necessary to start up and shutdown the server. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 54

28 ManageServer Use Case <<include>> PlanningService Administrator <<include>> StartServer ManageServer ShutdownServer <<include>> ConfigureServer Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 55 Boundary Condition Questions 8.1 Initialization How does the system start up? What data need to be accessed at startup time? What services have to registered? What does the user interface do at start up time? 8.2 Termination How does it present itself to the user? Are single subsystems allowed to terminate? Are other subsystems notified if a single subsystem terminates? How are local updates communicated to the database? 8.3 Failure How does the system behave when a node or communication link fails? Are there backup communication links? How does the system recover from failure? Is this different from initialization? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 56

29 Modeling Boundary Conditions Boundary conditions are best modeled as use cases with actors and objects. Actor: often the system administrator Interesting use cases: Start up of a subsystem Start up of the full system Termination of a subsystem Error in a subystem or component, failure of a subsystem or component Task: Model the startup of the ARENA system as a set of use cases. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java Hardware Software Mapping This activity addresses two questions: How shall we realize the subsystems: Hardware or Software? How is the object model mapped on the chosen hardware & software? Mapping Objects onto Reality: Processor, Memory, Input/Output Mapping Associations onto Reality: Connectivity Much of the difficulty of designing a system comes from meeting externally-imposed hardware and software constraints. Certain tasks have to be at specific locations Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 58

30 Mapping the Objects Processor issues: Is the computation rate too demanding for a single processor? Can we get a speedup by distributing tasks across several processors? How many processors are required to maintain steady state load? Memory issues: Is there enough memory to buffer bursts of requests? I/O issues: Do you need an extra piece of hardware to handle the data generation rate? Does the response time exceed the available communication bandwidth between subsystems or a task and a piece of hardware? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 59 Mapping the Subsystems Associations: Connectivity Describe the physical connectivity of the hardware Often the physical layer in ISO s OSI Reference Model Which associations in the object model are mapped to physical connections? Which of the client-supplier relationships in the analysis/design model correspond to physical connections? Describe the logical connectivity (subsystem associations) Identify associations that do not directly map into physical connections: How should these associations be implemented? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 60

31 Typical Informal Example of a Connectivity Drawing Application Client Application Client Application Client Physical Connectivity Logical Connectivity Communication Agent for Application Clients TCP/IP Communication Agent for Application Clients Backbone Network LAN Communication Agent for Data Server Ethernet LAN Global Data Server OODBMS Communication Agent for Data Server Global Data Server LAN RDBMS Local Data Server Global Data Server Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 61 Logical vs Physical Connectivity and the relationship to Subsystem Layering Application Layer Application Layer Presentation Layer Session Layer Transport Layer Bidirectional associations for each layer Presentation Layer Session Layer Transport Layer Logical Connectivity Layers Network Layer Network Layer Data Link Layer Physical Layer Data Link Layer Physical Layer Physical Connectivity Processor 1 Processor 2 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 62

32 Layer 1 Subsystem 1 Subsystem 2 Layer 2 Layer 3 Layer 4 Layer 1 Layer 2 Layer 3 Application Layer Application Layer Presentation Layer Presentation Layer Session Layer Transport Layer Network Layer Bidirectional associations for each layer Session Layer Transport Layer Network Layer Data Link Layer Data Link Layer Hardware Hardware Processor 1 Processor 2 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 63 Hardware/Software Mapping Questions What is the connectivity among physical units? Tree, star, matrix, ring What is the appropriate communication protocol between the subsystems? Function of required bandwidth, latency and desired reliability, desired quality of service (QOS) Is certain functionality already available in hardware? Do certain tasks require specific locations to control the hardware or to permit concurrent operation? Often true for embedded systems General system performance question: What is the desired response time? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 64

33 Connectivity in Distributed Systems If the architecture is distributed, we need to describe the network architecture (communication subsystem) as well. Questions to ask What are the transmission media? (Ethernet, Wireless) What is the Quality of Service (QOS)? What kind of communication protocols can be used? Should the interaction asynchronous, synchronous or blocking? What are the available bandwidth requirements between the subsystems? Stock Price Change -> Broker Icy Road Detector -> ABS System Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 65 Drawing Hardware/Software Mappings in UML System design must model static and dynamic structures: Component Diagrams for static structures show the structure at design time or compilation time Deployment Diagram for dynamic structures show the structure of the run-time system Note the lifetime of components Some exist only at design time Others exist only until compile time Some exist at link or runtime Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 66

34 Component Diagram Component Diagram A graph of components connected by dependency relationships. Shows the dependencies among software components source code, linkable libraries, executables Dependencies are shown as dashed arrows from the client component to the supplier component. The kinds of dependencies are implementation language specific. A component diagram may also be used to show dependencies on a façade: Use dashed arrow the corresponding UML interface. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 67 Component Diagram Example Scheduler reservations UML Component UML Interface Planner update GUI Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 68

35 Deployment Diagram Deployment diagrams are useful for showing a system design after the following decisions are made Subsystem decomposition Concurrency Hardware/Software Mapping A deployment diagram is a graph of nodes connected by communication associations. Nodes are shown as 3-D boxes. Nodes may contain component instances. Components may contain objects (indicating that the object is part of the component) Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 69 Deployment Diagram Example Compile Time Dependency :HostMachine <<database>> meetingsdb :Scheduler Runtime Dependency :PC :Planner Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 70

36 5. Data Management Some objects in the models need to be persistent Provide clean separation points between subsystems with welldefined interfaces. A persistent object can be realized with one of the following Data structure Files Database If the data can be volatile Cheap, simple, permanent storage Low level (Read, Write) Applications must add code to provide suitable level of abstraction Powerful, easy to port Supports multiple writers and readers Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 71 File or Database? When should you choose a file? Are the data voluminous (bit maps)? Do you have lots of raw data (core dump, event trace)? Do you need to keep the data only for a short time? Is the information density low (archival files,history logs)? When should you choose a database? Do the data require access at fine levels of details by multiple users? Must the data be ported across multiple platforms (heterogeneous systems)? Do multiple application programs access the data? Does the data management require a lot of infrastructure? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 72

37 Database Management System Contains mechanisms for describing data, managing persistent storage and for providing a backup mechanism Provides concurrent access to the stored data Contains information about the data ( meta-data ), also called data schema. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 73 Issues To Consider When Selecting a Database Storage space Database require about triple the storage space of actual data Response time Mode databases are I/O or communication bound (distributed databases). Response time is also affected by CPU time, locking contention and delays from frequent screen displays Locking modes Pessimistic locking: Lock before accessing object and release when object access is complete Optimistic locking: Reads and writes may freely occur (high concurrency!) When activity has been completed, database checks if contention has occurred. If yes, all work has been lost. Administration Large databases require specially trained support staff to set up security policies, manage the disk space, prepare backups, monitor performance, adjust tuning. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 74

38 Object-Oriented Databases Support all fundamental object modeling concepts Classes, Attributes, Methods, Associations, Inheritance Mapping an object model to an OO-database Determine which objects are persistent. Perform normal requirement analysis and object design Create single attribute indices to reduce performance bottlenecks Do the mapping (specific to commercially available product). Example: In ObjectStore, implement classes and associations by preparing C++ declarations for each class and each association in the object model Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 75 Relational Databases Based on relational algebra Data is presented as 2-dimensional tables. Tables have a specific number of columns and and arbitrary numbers of rows Primary key: Combination of attributes that uniquely identify a row in a table. Each table should have only one primary key Foreign key: Reference to a primary key in another table SQL is the standard language defining and manipulating tables. Leading commercial databases support constraints. Referential integrity, for example, means that references to entries in other tables actually exist. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 76

39 Data Management Questions Should the data be distributed? Should the database be extensible? How often is the database accessed? What is the expected request (query) rate? In the worst case? What is the size of typical and worst case requests? Do the data need to be archived? Does the system design try to hide the location of the databases (location transparency)? Is there a need for a single interface to access the data? What is the query format? Should the database be relational or object-oriented? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java Global Resource Handling Discusses access control Describes access rights for different classes of actors Describes how object guard against unauthorized access Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 78

40 Defining Access Control In multi-user systems different actors have access to different functionality and data. During analysis we model these different accesses by associating different use cases with different actors. During system design we model these different accesses by examing the object model by determining which objects are shared among actors. Depending on the security requirements of the system, we also define how actors are authenticated to the system and how selected data in the system should be encrypted. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 79 Access Matrix We model access on classes with an access matrix. The rows of the matrix represents the actors of the system The column represent classes whose access we want to control. Access Right: An entry in the access matrix. It lists the operations that can be executed on instances of the class by the actor. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 80

41 Access Matrix Implementations Global access table: Represents explicitly every cell in the matrix as a (actor,class, operation) tuple. Determining if an actor has access to a specific object requires looking up the corresponding tuple. If no such tuple is found, access is denied. Access control list associates a list of (actor,operation) pairs with each class to be accessed. Every time an object is accessed, its access list is checked for the corresponding actor and operation. Example: guest list for a party. A capability associates a (class,operation) pair with an actor. A capability provides an actor to gain control access to an object of the class described in the capability. Example: An invitation card for a party. Which is the right implementation? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 81 Global Resource Questions Does the system need authentication? If yes, what is the authentication scheme? User name and password? Access control list Tickets? Capability-based What is the user interface for authentication? Does the system need a network-wide name server? How is a service known to the rest of the system? At runtime? At compile time? By port? By name? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 82

42 7. Decide on Software Control Choose implicit control (non-procedural, declarative languages) Rule-based systems Logic programming Choose explicit control (procedural languages): Centralized or decentralized Centralized control: Procedure-driven or event-driven Procedure-driven control Control resides within program code. Example: Main program calling procedures of subsystems. Simple, easy to build, hard to maintain (high recompilation costs) Event-driven control Control resides within a dispatcher calling functions via callbacks. Very flexible, good for the design of graphical user interfaces, easy to extend Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 83 Event-Driven Control Example: MVC Model-View-Controller Paradigm (Adele Goldberg, Smalltalk 80) :Control Model has changed Update Update Update :View :Model :View :View Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 84

43 Software Control (continued) Decentralized control Control resides in several independent objects. Possible speedup by mapping the objects on different processors, increased communication overhead. Example: Message based system. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 85 Centralized vs. Decentralized Designs Should you use a centralized or decentralized design? Take the sequence diagrams and control objects from the analysis model Check the participation of the control objects in the sequence diagrams If sequence diagram looks more like a fork: Centralized design The sequence diagram looks more like a stair: Decentralized design Centralized Design One control object or subsystem ("spider") controls everything Pro: Change in the control structure is very easy Con: The single conctrol ojbect is a possible performance bottleneck Decentralized Design Not a single object is in control, control is distributed, That means, there is more than one control object Con: The responsibility is spread out Pro: Fits nicely into object-oriented development Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 86

44 8. Boundary Conditions Most of the system design effort is concerned with steady-state behavior. However, the system design phase must also address the initiation and finalization of the system. This is addressed by a set of new uses cases called administration use cases Initialization Describes how the system is brought from an non initialized state to steady-state ("startup use cases ). Termination Describes what resources are cleaned up and which systems are notified upon termination ("termination use cases"). Failure Many possible causes: Bugs, errors, external problems (power supply). Good system design foresees fatal failures ( failure use cases ). Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 87 Example: Administrative Use cases for MyTrip Administration use cases for MyTrip (UML use case diagram). An additional subsystems that was found during system design is the server. For this new subsystem we need to define use cases. ManageServer includes all the functions necessary to start up and shutdown the server. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 88

45 ManageServer Use Case <<include>> PlanningService Administrator <<include>> StartServer ManageServer ShutdownServer <<include>> ConfigureServer Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 89 Boundary Condition Questions 8.1 Initialization How does the system start up? What data need to be accessed at startup time? What services have to registered? What does the user interface do at start up time? 8.2 Termination How does it present itself to the user? Are single subsystems allowed to terminate? Are other subsystems notified if a single subsystem terminates? How are local updates communicated to the database? 8.3 Failure How does the system behave when a node or communication link fails? Are there backup communication links? How does the system recover from failure? Is this different from initialization? Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 90

46 Modeling Boundary Conditions Boundary conditions are best modeled as use cases with actors and objects. Actor: often the system administrator Interesting use cases: Start up of a subsystem Start up of the full system Termination of a subsystem Error in a subystem or component, failure of a subsystem or component Task: Model the startup of the ARENA system as a set of use cases. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 91 Summary In this lecture, we reviewed the activities of system design : Concurrency identification Hardware/Software mapping Persistent data management Global resource handling Software control selection Boundary conditions Each of these activities revises the subsystem decomposition to address a specific issue. Once these activities are completed, the interface of the subsystems can be defined. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 92