Transcript Architectural Design - National Chung Cheng University

Chapter 5
Software Design
This will cover architectural design and
application architectures, which is
mainly Chapters 11 and 13 of Ian
Sommerville’s Software Engineering.
Architectural Design
Ian Sommerville, Software Engineering,
Chapter 11
Objectives
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To introduce architectural design and to
discuss its importance
To explain the architectural design decisions
that have to be made
To introduce three complementary
architectural styles covering organisation,
decomposition and control
To discuss reference architectures used to
communicate and compare architectures
Topics covered
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Architectural design decisions
System organisation
Decomposition styles
Control styles
Reference architectures
Software architecture
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The design process for identifying the subsystems making up a system and the
framework for sub-system control and
communication is architectural design.
The output of this design process is a
description of the software architecture.
Architectural design
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An early stage of the system design process.
Represents the link between specification
and design processes.
Often carried out in parallel with some
specification activities.
It involves identifying major system
components and their communications.
Advantages of explicit architecture
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Stakeholder communication
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System analysis
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Architecture may be used as a focus of discussion
by system stakeholders.
Analysis of whether a system can meet its nonfunctional requirements.
Large-scale reuse
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The architecture may be reusable across a range
of systems.
Architecture and system characteristics
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Performance
 Localise critical operations and minimise communications. Use
large rather than fine-grain components.
Security
 Use a layered architecture with critical assets in the inner layers.
Safety
 Localise safety-critical features in a small number of subsystems.
Availability
 Include redundant components and mechanisms for fault
tolerance.
Maintainability
 Use fine-grain, replaceable components.
Architectural conflicts
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Using large-grain components improves
performance but reduces maintainability.
Introducing redundant data improves
availability but makes security more difficult.
Localising safety-related features usually
means more communication so degraded
performance.
System structuring
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Concerned with decomposing the system into
interacting sub-systems.
The architectural design is normally
expressed as a block diagram presenting an
overview of the system structure.
More specific models showing how subsystems share data, are distributed and
interface with each other may also be
developed.
Packing robot control system
Box and line diagrams
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Very abstract - they do not show the nature of
component relationships nor the externally
visible properties of the sub-systems.
However, useful for communication with
stakeholders and for project planning.
Architectural design decisions
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Architectural design is a creative process so
the process differs depending on the type of
system being developed.
However, a number of common decisions
span all design processes.
Architectural design decisions
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Is there a generic application architecture that can
be used?
How will the system be distributed?
What architectural styles are appropriate?
What approach will be used to structure the system?
How will the system be decomposed into modules?
What control strategy should be used?
How will the architectural design be evaluated?
How should the architecture be documented?
Architecture reuse
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Systems in the same domain often have
similar architectures that reflect domain
concepts.
Application product lines are built around a
core architecture with variants that satisfy
particular customer requirements.
Application architectures are covered in
Chapter 13 and product lines in Chapter 18.
Architectural styles
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The architectural model of a system may
conform to a generic architectural model or
style.
An awareness of these styles can simplify the
problem of defining system architectures.
However, most large systems are
heterogeneous and do not follow a single
architectural style.
Architectural models
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Used to document an architectural design.
Static structural model that shows the major system
components.
Dynamic process model that shows the process
structure of the system.
Interface model that defines sub-system interfaces.
Relationships model such as a data-flow model that
shows sub-system relationships.
Distribution model that shows how sub-systems are
distributed across computers.
System organisation
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Reflects the basic strategy that is used to
structure a system.
Three organisational styles are widely used:
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A shared data repository style;
A shared services and servers style;
An abstract machine or layered style.
The repository model
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Sub-systems must exchange data. This may
be done in two ways:
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Shared data is held in a central database or
repository and may be accessed by all subsystems;
Each sub-system maintains its own database and
passes data explicitly to other sub-systems.
When large amounts of data are to be shared,
the repository model of sharing is most
commonly used.
CASE toolset architecture
Repository model characteristics
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Advantages
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Efficient way to share large amounts of data;
Sub-systems need not be concerned with how data is
produced
Centralised management e.g. backup, security, etc.
Sharing model is published as the repository schema.
Disadvantages
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Sub-systems must agree on a repository data model.
Inevitably a compromise;
Data evolution is difficult and expensive;
No scope for specific management policies;
Difficult to distribute efficiently.
Client-server model
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Distributed system model which shows how
data and processing is distributed across a
range of components.
Set of stand-alone servers which provide
specific services such as printing, data
management, etc.
Set of clients which call on these services.
Network which allows clients to access
servers.
Film and picture library
Client-server characteristics
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Advantages
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Distribution of data is straightforward;
Makes effective use of networked systems. May require
cheaper hardware;
Easy to add new servers or upgrade existing servers.
Disadvantages
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No shared data model so sub-systems use different data
organisation. Data interchange may be inefficient;
Redundant management in each server;
No central register of names and services - it may be hard
to find out what servers and services are available.
Abstract machine (layered) model
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Used to model the interfacing of sub-systems.
Organises the system into a set of layers (or
abstract machines) each of which provide a
set of services.
Supports the incremental development of
sub-systems in different layers. When a layer
interface changes, only the adjacent layer is
affected.
However, often artificial to structure systems
in this way.
Version management system
Configur ation m a na ge m ent system lay e r
Obj ec t m a na ge m ent syste m lay e r
Database syste m la ye r
Ope ra ting sy ste m la ye r
Sub-systems and modules
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A sub-system is a system in its own right
whose operation is independent of the
services provided by other sub-systems.
A module is a system component that
provides services to other components but
would not normally be considered as a
separate system.
Modular decomposition
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Another structural level where sub-systems are
decomposed into modules.
Two modular decomposition models covered
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An object model where the system is decomposed into
interacting object;
A pipeline or data-flow model where the system is
decomposed into functional modules which transform
inputs to outputs.
If possible, decisions about concurrency should be
delayed until modules are implemented.
Object models
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Structure the system into a set of loosely
coupled objects with well-defined interfaces.
Object-oriented decomposition is concerned
with identifying object classes, their attributes
and operations.
When implemented, objects are created from
these classes and some control model used
to coordinate object operations.
Invoice processing system
Object model advantages
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Objects are loosely coupled so their
implementation can be modified without
affecting other objects.
The objects may reflect real-world entities.
OO implementation languages are widely
used.
However, object interface changes may
cause problems and complex entities may be
hard to represent as objects.
Function-oriented pipelining
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Functional transformations process their
inputs to produce outputs.
May be referred to as a pipe and filter model
(as in UNIX shell).
Variants of this approach are very common.
When transformations are sequential, this is
a batch sequential model which is extensively
used in data processing systems.
Not really suitable for interactive systems.
Invoice processing system
Pipeline model advantages
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Supports transformation reuse.
Intuitive organisation for stakeholder
communication.
Easy to add new transformations.
Relatively simple to implement as either a
concurrent or sequential system.
However, requires a common format for data
transfer along the pipeline and difficult to
support event-based interaction.
Control styles
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Are concerned with the control flow between
sub-systems. Distinct from the system
decomposition model.
Centralised control
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One sub-system has overall responsibility for
control and starts and stops other sub-systems.
Event-based control
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Each sub-system can respond to externally
generated events from other sub-systems or the
system’s environment.
Centralised control
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A control sub-system takes responsibility for
managing the execution of other sub-systems.
Call-return model
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Top-down subroutine model where control starts at the top
of a subroutine hierarchy and moves downwards.
Applicable to sequential systems.
Manager model
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Applicable to concurrent systems. One system component
controls the stopping, starting and coordination of other
system processes. Can be implemented in sequential
systems as a case statement.
Call-return model
Real-time system control
Event-driven systems
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Driven by externally generated events where the
timing of the event is outwith the control of the subsystems which process the event.
Two principal event-driven models
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Broadcast models. An event is broadcast to all subsystems. Any sub-system which can handle the event may
do so;
Interrupt-driven models. Used in real-time systems where
interrupts are detected by an interrupt handler and passed
to some other component for processing.
Other event driven models include spreadsheets
and production systems.
Broadcast model
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Effective in integrating sub-systems on different
computers in a network.
Sub-systems register an interest in specific events.
When these occur, control is transferred to the subsystem which can handle the event.
Control policy is not embedded in the event and
message handler. Sub-systems decide on events of
interest to them.
However, sub-systems don’t know if or when an
event will be handled.
Selective broadcasting
Interrupt-driven systems
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Used in real-time systems where fast
response to an event is essential.
There are known interrupt types with a
handler defined for each type.
Each type is associated with a memory
location and a hardware switch causes
transfer to its handler.
Allows fast response but complex to program
and difficult to validate.
Interrupt-driven control
Reference architectures
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Architectural models may be specific to some
application domain.
Two types of domain-specific model
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Generic models which are abstractions from a number of
real systems and which encapsulate the principal
characteristics of these systems. Covered in Chapter 13.
Reference models which are more abstract, idealised
model. Provide a means of information about that class of
system and of comparing different architectures.
Generic models are usually bottom-up models;
Reference models are top-down models.
Reference architectures
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Reference models are derived from a study
of the application domain rather than from
existing systems.
May be used as a basis for system
implementation or to compare different
systems. It acts as a standard against which
systems can be evaluated.
OSI model is a layered model for
communication systems.
OSI reference model
7
Ap plication
Ap plication
6
Pres en ta tion
Pres en ta tion
5
Sess io n
Sess io n
4
Tran sp or t
Tran sp or t
3
Netw o rk
Netw o rk
Netw o rk
2
Data lin k
Data lin k
Data lin k
1
Phy sical
Phy sical
Phy sical
Comm u nication s med iu m
Case reference model
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Data repository services
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Data integration services
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Definition and enaction of process models.
Messaging services
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Managing groups of entities.
Task management services
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Storage and management of data items.
Tool-tool and tool-environment communication.
User interface services
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User interface development.
The ECMA reference model
Application Architectures
Ian Sommerville, Software Engineering,
Chapter 13
Objectives
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To explain the organisation of two
fundamental models of business systems batch processing and transaction processing
systems
To describe the abstract architecture of
resource management systems
To explain how generic editors are event
processing systems
To describe the structure of language
processing systems
Topics covered
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Data processing systems
Transaction processing systems
Event processing systems
Language processing systems
Generic application architectures
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Application systems are designed to meet an
organisational need.
As businesses have much in common, their
application systems also tend to have a
common architecture that reflects the
application requirements.
A generic architecture is configured and
adapted to create a system that meets
specific requirements.
Use of application architectures
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As a starting point for architectural design.
As a design checklist.
As a way of organising the work of the
development team.
As a means of assessing components for
reuse.
As a vocabulary for talking about application
types.
Application types
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Data processing applications
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Transaction processing applications
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Data-centred applications that process user requests and update
information in a system database.
Event processing systems
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Data driven applications that process data in batches without
explicit user intervention during the processing.
Applications where system actions depend on interpreting events
from the system’s environment.
Language processing systems
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Applications where the users’ intentions are specified in a formal
language that is processed and interpreted by the system.
Application type examples
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Data processing systems
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Transaction processing systems
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E-commerce systems;
Reservation systems.
Event processing systems
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Billing systems;
Payroll systems.
Word processors;
Real-time systems.
Language processing systems
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Compilers;
Command interpreters.
Data processing systems
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Systems that are data-centred where the
databases used are usually orders of
magnitude larger than the software itself.
Data is input and output in batches
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Input: A set of customer numbers and associated
readings of an electricity meter;
Output: A corresponding set of bills, one for each
customer number.
Data processing systems usually have an
input-process-output structure.
Input-process-output model
Sy stem
In pu t
Proces s
Outp u t
Prin ter
Database
Input-process-output
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The input component reads data from a file or
database, checks its validity and queues the
valid data for processing.
The process component takes a transaction
from the queue (input), performs
computations and creates a new record with
the results of the computation.
The output component reads these records,
formats them accordingly and writes them to
the database or sends them to a printer.
Data-flow diagrams
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Show how data is processed as it moves
through a system.
Transformations are represented as roundedged rectangles, data-flows as arrows
between them and files/data stores as
rectangles.
Salary payment DFD
Transaction processing systems
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Process user requests for information from a
database or requests to update the database.
From a user perspective a transaction is:
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Any coherent sequence of operations that
satisfies a goal;
For example - find the times of flights from London
to Paris.
Users make asynchronous requests for
service which are then processed by a
transaction manager.
Transaction processing
I/O
p roces sin g
Application
lo gic
Trans action
man ag er
Database
ATM system organisation
Transaction processing middleware
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Transaction management middleware or
teleprocessing monitors handle
communications with different terminal types
(e.g. ATMs and counter terminals), serialises
data and sends it for processing.
Query processing takes place in the system
database and results are sent back through
the transaction manager to the user’s
terminal.
Transaction management
Information systems architecture
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Information systems have a generic
architecture that can be organised as a
layered architecture.
Layers include:
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The user interface
User communications
Information retrieval
System database
Information system structure
Use r inte r fa ce
Use r c om m unica tions
Inform a tion r etrie va l a nd m odif ic ation
Tra nsac tion m anagem e nt
Database
LIBSYS architecture
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The library system LIBSYS is an example of an
information system.
User communications layer:
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LIBSYS login component;
Form and query manager;
Print manager;
Information retrieval layer
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Distributed search;
Document retrieval;
Rights manager;
Accounting.
LIBSYS organisation
We b browse r inte r fa ce
LIBSYS
login
Distributed
se ar ch
Form s and
quer y manager
Doc um e nt
re tr ieval
P rint
m ana ge r
Rights
m ana ge r
Acc ounting
Libr ar y inde x
DB1
DB2
DB3
DB4
DBn
Resource allocation systems
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Systems that manage a fixed amount of some
resource (football game tickets, books in a
bookshop, etc.) and allocate this to users.
Examples of resource allocation systems:
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Timetabling systems where the resource being allocated is
a time period;
Library systems where the resource being managed is
books and other items for loan;
Air traffic control systems where the resource being
managed is the airspace.
Resource allocation architecture
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Resource allocation systems are also layered
systems that include:
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A resource database;
A rule set describing how resources are allocated;
A resource manager;
A resource allocator;
User authentication;
Query management;
Resource delivery component;
User interface.
Layered resource allocation
Use r inte r fa ce
Use r
a uthentic ation
Resourc e
m anage me nt
Resourc e
delive ry
Que ry
management
Resourc e policy
c ontrol
Tra nsac tion m anagem e nt
Resourc e database
Resourc e
a lloca tion
Layered system implementation
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Each layer can be implemented as a large
scale component running on a separate
server. This is the most commonly used
architectural model for web-based systems.
On a single machine, the middle layers are
implemented as a separate program that
communicates with the database through its
API.
Fine-grain components within layers can be
implemented as web services.
E-commerce system architecture
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E-commerce systems are Internet-based
resource management systems that accept
electronic orders for goods or services.
They are usually organised using a multitier architecture with application layers
associated with each tier.
Web
b rowser
Web server
App lication
serv er
Database
serv er
Event processing systems
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These systems respond to events in the
system’s environment.
Their key characteristic is that event timing is
unpredictable so the architecture has to be
organised to handle this.
Many common systems such as word
processors, games, etc. are event processing
systems.
Editing systems
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Real-time systems (Chapter 15) and editing
systems are the most common types of event
processing system.
Editing system characteristics:
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Single user systems;
Must provide rapid feedback to user actions;
Organised around long transactions so may
include recovery facilities.
Editing system components
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Editing systems are naturally object-oriented:
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Screen - monitors screen memory and detects events;
Event - recognises events and passes them for processing;
Command - executes a user command;
Editor data - manages the editor data structure;
Ancillary data - manages other data such as styles and
preferences;
File system - manages file I/O;
Display - updates the screen display.
Editing system architecture
File Sy ste m
Save
Ope n
Anc illar y da ta
Editor da ta
Anc illar y
c om m ands
Editing
c om m ands
Com m a nd
Displa y
Inte rpre t
Update
Eve nt
P roce ss
Scre e n
Ref re sh
Language processing systems
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Accept a natural or artificial language as input and
generate some other representation of that
language.
May include an interpreter to act on the instructions
in the language that is being processed.
Used in situations where the easiest way to solve a
problem is to describe an algorithm or describe the
system data
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Meta-case tools process tool descriptions, method
rules, etc and generate tools.
A language processing system
Tra nsla tor
Instructions
Che ck sy ntax
Chec ksemantics
Gener ate
Abstra ct m /c
instr uc tions
Inte rpre ter
Data
Fe tch
Exe cute
Results
Language processing components
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Lexical analyser
Symbol table
Syntax analyser
Syntax tree
Semantic analyser
Code generator
Data-flow model of a compiler
Repository model of a compiler