© Andrew IrelandSoftware Design F28SD2 Architectural Design Andrew Ireland School of Mathematical &...

© Andrew IrelandSoftware Design F28SD2

Architectural Design

Andrew IrelandSchool of Mathematical & Computer Sciences

Heriot-Watt UniversityEdinburgh


Outline

• Motivations • Structure• Control• Decomposition


Motivations • Architectural design represents the most abstract

level of design• Involves the identification of subsystems and their

interconnections• Just like in designing a building or a bridge, sound

architectural decisions are crucial to the success of a project

– a bad architecture can’t be saved by good construction methods!


Architectural Activities• Systems structuring:

– Subsystems– Communication between subsystems

• Control modelling:– Subsystems and control

• Subsystem decomposition:– Subsystems and modules– Module interconnections


Repository Model

Automotive Parts: repository

Client Interface

Invoice Subsystem

Supplier Interface

Stock Subsystem

Note: often referred to as data-centred architecture


Repository Model • The repository model is appropriate when there exists a

clear producer/consumer relationship between subsystems and data, e.g. command and control, inventory management, information management

• The repository imposes a data model that all subsystems will have to operate with

• Advantages: – Efficient mechanism for sharing large amounts of data between

subsystems – no need for data to be communicated between subsystems directly

– Data producers (subsystems) are decoupled from the needs of data consumers (subsystems)

– Security and archive issues are centralized


Repository Model • Advantages (cont’d):

– Having a clearly defined data model makes integration of new subsystems a well defined task

• Disadvantages: – Imposing a single data model on all systems may impact on

performance, as well as adaptability, i.e. integration of new subsystems

– Evolution may be impeded by the use of a standard data model, i.e. converting to a new standard, may be expensive and may lead to the lose of data (data warehousing)

– Distributing a repository model over multiple machines gives rise to additional challenges, i.e. dealing with redundancy and inconsistencies


Data-flow Model • A data-flow architecture is appropriate when input data is

transformed through a series of computational components, i.e. filters connected by pipes

• Filters operate independently of each other

• In its simplest form, a data-flow architecture takes the form of a single line of transformations

filter

filter

filter filter

filter filter

filterpipes


Data-flow Model • Advantages:

– Promotes parallelism– Promotes component style development

• Disadvantages: – Where parallelism is exploited, load balancing and

synchronization issues may impact negatively on system performance

– Deadlock is also a potential problem via parallelization– Not well suited to error handling, e.g. ignore erroneous data or

restart the processing with the erroneous data eliminated – neither is very satisfactory

• Applications: image processing, parallel search, …


Client-server Model

Network

Client 1 Client M…

Server 1 Server N…


Client-server Model • A distributed systems model contains a:

– set of servers (subsystems) providing services– set of clients (subsystems) which know the

identification of the services– network which allows clients to access services via

remote procedure calls

• Typically data is also distributed across the system with no standard data model imposed


Client-server Model • Advantages:

– Ease of system evolution, e.g. new servers can be integrated incrementally and existing servers can be upgraded transparently

– Having no standard data model provides opportunities for efficiency gains

• Disadvantages:– Having no standard data model may give rise to

unforeseen integration issues– Security and archiving becomes a distributed problem


Peer-to-Peer Model • Peer-to-peer (P2P) is a distributed architecture

which has a strong co-operative flavour, i.e. the architecture is constructed from a set of participants, each of whom share resources

• Within P2P, a participant is a supplier and a consumer, which makes it distinct from the client-server model

• Applications: file sharing networks, e.g. Napster, Skype, sciencenet P2P search engine, …


Peer-to-Peer Model


Peer-to-Peer Model


Peer-to-Peer Model • Advantages:

– As new nodes arrive resources increase, as well as the demands on the available resources. In contrast, as new clients join a client server architecture typically the demand increases without any additional resources (so performance decreases)

– Distributed nature of the architecture brings with it robustness benefits, i.e. no single point of failure

• Disadvantage: – Security, e.g. without encryption and proper verification

mechanisms a P2P network will be vulnerable to malicious code and viruses


Layered Model

Kernel

Device Drivers

Service Processes

User Processes


Layered Model • Each layer represents an abstract machine,

providing a well defined set of services• Advantages:

– promotes an incremental style of development– promotes portability and change

• Disadvantages:– cross cutting services, e.g. file handling, do not fit the

layered model– multiple layers may lead to performance issues


Control Models • Centralized control:

– A single subsystem is responsible for controlling the operation of all the other subsystems

– Other subsystems may be given control on a temporary basis

• Event-based control:– Control is distributed, each subsystem is responsible

for its actions– Subsystems react to external events generated by

other subsystems or the operational environment, e.g. sensor input


Centralized Control

main program

subroutineZ.1

subroutineZ.N

subroutineA.N

subroutineA.1

subroutineA

subroutineZ…

… …

call-return model


Centralized Control

managerprocess

process 2process 1

process 3 process N

manager model

…


Centralized Control • Call-return model:

– Hierarchical subroutine model– Control is passed down through the hierarchy– Applicable to sequential systems– Less flexible and thus easier to analyse

• Manager model:– Central control process which provides coordination between

subprocesses– Processes are executed in parallel– Applicable for concurrent systems– More flexible and thus harder to analyse


Event-driven Control • System behaviour is driven by events, e.g.

– external stimuli - sensors– user interactions - mouse clicks, key strokes – process communications - messages from process

threads

• Example event-driven models:– Broadcast control– Interrupt-driven control


Broadcast Control

Event and message handler

subsystem-1 subsystem-N…


Broadcast Control • Subsystems are associated, or registered, with specific

events – subsystems decide which events to react to• Event & message handler broadcasts events to:

– All subsystems OR– Only those subsystems registered for a given event

• Advantages:– Ease of system evolution– No need for explicit identification for subsystems

• Disadvantages:– Potential conflicts arising from multiple responses to events– Not appropriate for hard real-time systems, i.e. systems which

require a response within a fixed time frame


Interrupt-driven Control

Handler 1

Handler 2

Handler N…

Process NProcess 1 Process 2 …

…interruptvector


Interrupt-driven Control • System involves pre-defined kinds of interrupts, i.e. events that

cause processing to be interrupted• Each interrupt is mapped onto a special area of memory known as

the interrupt vector• The interrupt vector references interrupt handlers, hardware

ensures that control is passed to the relevant interrupt handler when an interrupt occurs – a handler will typically start or stop processes

• Advantages:– Good for hard real-time systems, i.e. fast & predictable response times

• Disadvantages:– Hard to program and validate– Limited by hardware, i.e. upper bound on interrupt vector


Subsystem Decomposition

• Subsystems typically require further decomposition, i.e. decomposition into modules

• This level of decomposition lies on the boundary of architectural design

• Two principal decomposition approaches:– Data-flow

– Object-oriented

• Both are expanded upon in the next couple of lectures …


Summary • Learning outcomes:

– Motivations for architectural design– Systems structuring and control

• Recommended reading:– M. Shaw, D. Garlan, “Software Architecture:

Perspectives on an Emerging Discipline”, Prentice-Hall 1996

– I. Sommerville, “Software Engineering”, Addison-Wesley 2007

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