The Architecture of Open Systems

1
The Architecture of Open Systems
This presentation is part of a short course on
Architecture and Open Systems. Please see
http//ecs.soton.ac.uk/ph/OpenSystems

• Peter Henderson
• Dependable Systems and Software Engineering and
• Open Middleware Infrastructure Institute
• University of Southampton

2
Openness historical uses of the adjective Open

• Open Distributed Processing (ODP)
• Open Systems (originally from Unix-wars)
• Open Standards
• Open Source
• Open Source Development Process
• Open Systems Architecture

3
Situation

• Open System Architectures are said to be a good
  thing
• But we dont always agree what we mean by Open
• What does it mean to say a system is Open?

4
Answer

• A system is Open if
• It is Modular
• It is made up of components that are
  interchangeable (Pluggable)
• And those components can be obtained from many
  Independent Sources

5
Outline

• Modularity
• Examples of Modular Architectures
• Model in which modularity concepts can be
  discussed
• Pluggable
• Define Pluggable, using model
• State some Principles for Open Systems
• Independently Sourced
• Security of Supply
• Maintenance of Interface
• Revisit Definition of Open

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Modularity
Modularity

• Some Examples
• Enumeration of key Concepts
• A Model to use when defining Pluggable

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Active Web Page Architecture
Modularity - Examples
Browser
Web Server
HTTP/ HTML
Form data/ HTML
Browser IE, Firefox, Web Server Tomcat,
IIS, Jetty, Active Page JSP, ASP, PHP,
Database Connector jdbc1, odbc2, Database
Server mySQL, SQLServer,
Active Page
Query / RS
Database Connector
Database Server
SQL/ RS
Terminology Component Interface Service Protocol

Database
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WS-Grid Application Architecture
Modularity - Examples
DA1
Data Access Service
WF
DI
CS
Data Integration Service
Continuation Service
Workflow Service
DA2
See WS-Grid.com
Data Access Service
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NGS Application Architecture
Modularity - Examples
grid-compute.leeds
grid-compute.oesc
cluster
linux
linux
SRB
Globus
PBS
gsi sshd
Globus
rasselas.ecs
Globus
gsi ssh
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Open Systems Concepts
Modularity - Concepts

• Component
• Interface
• Modularity
• Granularity
• Standards
• Conformance
• Openness
• Portability
• Interoperability
• Adaptor
• COTS
• Protocol

• Architecture
• Object Oriented
• Service Oriented
• Message Oriented
• Data Oriented
• View and Viewpoint
• Evolution (whole life)
• Heterogeneity
• Reuse
• Stakeholder
• Requirements
• Connector
• Version
• Variety

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Component, Interface, Composition
Modularity - Model
Component A is a Component Interface/Service B
supplies interface J, B requires interface
K Composition BLUE contains C and D BLUE is a
Component BLUE hides interface L
A
B
K
J
C
D
L
M
K
Notation loosely based on UML derivative SysML
block diagrams
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Pluggable
Pluggable

• Component
• Interface
• Modularity
• Granularity
• Standards
• Conformance
• Openness

• Define key Concepts - see left
• Define Open System (alternative statement)
• Principles of Open Systems just three of many
  principles

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Component
Pluggable - Concepts

• An identifiable and generally replaceable unit of
  composition. Typically something that is
  constructed independently
• Will be known by its name and version number and
  by the set of interfaces that it supports and
  requires
• Will exist in many versions through time

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Interface
Pluggable - Concepts

• An identifiable and generally agreed means of
  communication with a component
• Can be call-based, message-based or stream-based
• Can be synchronous or asynchronous
• Will be known by its name and version number and
  by the protocol for using it
• Will exist in many versions through time

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Modularity
Pluggable - Concepts

• A system is modular if its various parts can be
  easily replaced
• Modular structure implies the existence of
  mutually agreed interfaces

D
C
L
M
K
E
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Granularity
Pluggable - Concepts

• A modular system may have parts of different
  sizes
• The size of the parts is referred to as the
  granularity of the modular structure
• This is a recursive concept leading to the notion
  of hierarchical structure

L
M
K
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Standards
Pluggable - Concepts

• International standards
• De facto (industrial) standards
• Agreements among users and suppliers of
  components as to the means of interfacing

K
J
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Conformance
Pluggable - Concepts

• A component may supply an interface that may or
  may not conform to a given standard
• Validating conformance requires the existence of
  elaborate testing capabilities

E
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Openness
Pluggable - Concepts

• A combination of the number of open interfaces
• And the extent to which these interfaces actually
  conform to agreed standards
• And the number of independent sources for
  components
• A property of component or system, rather than
  interface?

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Open System Definition
Pluggable - Principles

• An Open System is one in which many of the
  interfaces (internal and external) conform to
  widely agreed standards
• so that variety and evolution of the system can
  be achieved and
• so that portability and interoperability of
  components are achieved
• largely by plug and play

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Open Systems Principles
Pluggable - Principles

• For an interface to be considered to conform to
  widely agreed standards there needs to be a
  strong reason why it can be expected to be
  maintained through evolution, such as
• Its an official or de facto standard that has
  many distinct suppliers (e.g. TCP/IP)
• Its a de facto standard that has one supplier
  but on which many component suppliers are
  dependent (e.g. Windows)

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Open Systems Principles
Pluggable - Principles

• The more distinct applications in which a
  component has been deployed, the more confidence
  we are likely to have in its quality and in the
  openness of its interfaces
• Legacy components need to be bridged to be part
  of an open system. The adaptor used to bridge
  them is properly considered to be part of the
  legacy. i.e we make old components into pluggable
  components rather than support legacy interfaces

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Independently Sourced
Independently Sourced

• Security of Supply
• True Independence of Interfaces

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Security of Supply
Independently Sourced - Supply
Browser
Web Server

• Availability of components from many independent
  suppliers
• Dependence on suppliers minimised
• But weakness can be a single component
• Open Source contributes to our feeling of
  security why?

Active Page
Database Connector
Database Server
Database
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Maintenance of the Interface
Independently Sourced - Interface
Browser
Web Server

• No one supplier can modify interface without risk
• e.g fact that many web servers support JSP means
  that each supplier will want to implement same
  functionality as the others

Active Page
Database Connector
Database Server
Database
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Situation with which we began

• Open System Architectures are said to be a good
  thing
• But we dont always agree what we mean by Open
• What does it mean to say a system is Open?

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Answer we gave

• A system is Open if
• It is Modular
• It is made up of components that are
  interchangeable (Pluggable)
• And those components can be obtained from many
  Independent Sources

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Conclusion

• An Open System is a system composed from
  components that can be readily replaced from
  multiple sources
• Which implies that the components have de facto
  Open interfaces
• Open interfaces are just the means of agreement
  among component suppliers

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Sources

• TOGAF
• ODP/RM
• IEEE 1471
• Zachman (IBM)
• ATAM (SEI) tradeoff analysis
• SEI CMMI

• UML 2.0
• SysML
• IEEE 1220 (SysE Processes)
• IEEE 12207 (SE Processes)
• Requirements mgmt tools
• OGSA