Advances in Effective Languages for
Architecture Definition
David Garlan
garlan@cs.cmu.edu
Bradley Schmerl
schmerl@cs.cmu.edu
ABLE Research Group
School of Computer Science
Carnegie Mellon University
Pittsburgh, PA
http://www.cs.cmu.edu/~able
Outline
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Introduction to software architecture
Terminology of Acme and ADML
Current and ongoing Acme developments
Tool support for Acme at CMU
Current and future directions
» xArch – XML infrastructure
» Liaison with UML working group
February 7, 2001
Open Group Architecture
Program, San Jose, CA
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Architecture in Systems
Architecture:
“the underlying structure of things”
• not just “what”, but “why”
Good architecture (like much good design) is:
• the result of a consistent set of principles and
techniques, applied consistently through all
phases of a project
• resilient in the face of (inevitable) changes
• source of guidance throughout the product
lifetime
• reuse of established engineering knowledge
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Open Group Architecture
Program, San Jose, CA
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Issues Addressed by
Architectural Design
Gross decomposition of a system into interacting
components
• typically hierarchical
• using rich abstractions for component interaction
(or system “glue”)
• often using common design idioms/styles
Emergent system properties
• performance, throughput, latencies
• reliability, security, fault tolerance, evolvability
Rationale and assignment of function to components
• relates requirements and implementations
Envelope of allowed change
• “load-bearing walls”, limits of scalability and adaptation
• design idioms and styles
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Open Group Architecture
Program, San Jose, CA
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Architectures in Development
Requirements
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Requirements
Software Architecture
Code
Code
Architecture Description Languages (ADLs) provide
a method for specifying software architectures
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Why ADLs
ADLs provide a way to model designs precisely
and explicitly:
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Focus attention on essential design factors
Enable unambiguous communication
Provide a secure foundation for reasoning
Enable automated analysis
There are over a dozen ADLs in use today
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Open Group Architecture
Program, San Jose, CA
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Commonalities among ADLs
ADLs largely agree on use of structure:
• Components: define the locus of computation
» E.g.: filters, databases, objects, clients, servers
• Connectors: mediate component interactions
» E.g.: procedure call, pipes, event broadcast
• Interfaces: component interface to envt.
» E.g., http socket, corba/com interface
• Properties: specifications for compilation and
analysis
» E.g.: signatures, pre/post conditions, RT specs
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Open Group Architecture
Program, San Jose, CA
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Acme – A Generic ADL
Acme was originally developed as an exchange
language for architecture descriptions
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Embodies commonality between ADLs
Domain-Neutral structural descriptions
Properties for encoding semantics
Extensibility through type-system and tools
This is in contrast with other ADLs, which have
“hard-wired” semantics
ADML was based heavily on Acme, and so the
terminology for Acme applies to ADML
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Open Group Architecture
Program, San Jose, CA
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Acme Structural Language
system
component
port
role
connector
attachment
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Open Group Architecture
Program, San Jose, CA
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Acme Properties
Properties provide a semantic extension
mechanism to Acme
• Arbitrary attribute-value annotations
• Associated with all major language
constructs
Domain specific information can be
captured in properties
• E.g.: protocols of interaction,
performance, reliability
February 7, 2001
Open Group Architecture
Program, San Jose, CA
10
Acme Representations
• Hierarchical abstractions (encapsulation)
• Can represent sub-architectures or “views”
Representation
Binding
System
(sub-architecture)
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Open Group Architecture
Program, San Jose, CA
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Architectural Styles
Need to capture domain-specific architectural
elements
• For specific kinds of components, connectors
• For a particular software system or enterprise
This is done through the definition of Acme
families
• Defines architectural vocabulary
• Defines constraints on use of that vocabulary
• May define specialized visualization for tools
February 7, 2001
Open Group Architecture
Program, San Jose, CA
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Style Example
If we are defining a software system using a
style called Pipes and Filters, then we can
define types:
• Pipe – a particular kind of connector, with two
roles named source and sink
• Filter – a particular kind of component, with
default ports in and out
Architectures defined in this style can make use
of this vocabulary
February 7, 2001
Open Group Architecture
Program, San Jose, CA
13
ADML
ADML (Architecture Description Markup Language) is:
• An XML encoding of architectures (as already outlined)
• Being promoted by the Open Group for specification of
enterprise architectures
• Originally developed at MCC
ADML adds to Acme:
• Industry-standard representation (parsable by ordinary
XML parsers)
• The ability to define links to objects outside the
architecture
• Straightforward ability to interface with commercial
repositories, and transparent extensibility
However, Acme has been extended in recent years
February 7, 2001
Open Group Architecture
Program, San Jose, CA
14
Armani Design Constraints
Armani is a constraint language extension to Acme
• Based on first-order predicate logic
• Augmented with architecture-specific predicates
• For defining architectural element types and styles
Armani constraints specify how a design may evolve
over time. Examples:
• A particular type of component can only have particular
types of ports
• Property values must be within certain ranges
Two types of constraints
• Invariants must never be violated
• Heuristics should be observed but may be selectively
violated
February 7, 2001
Open Group Architecture
Program, San Jose, CA
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Current Research
Mapping between multiple views
• Expresses correspondence between multiple
views of an architecture
Runtime architectural events
• Dynamic events in an architectural context
Repair strategies
• Architectural modifications that can occur at
runtime when constraints fail
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Open Group Architecture
Program, San Jose, CA
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Acme Tools at CMU
Armani constraint
checking
Performance
analysis
AcmeStudio
Acme
ADML
xArch
Meta H
…
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AcmeLib – A foundation for building tools
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Program, San Jose, CA
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AcmeStudio
Graphical design environment for Acme
• Supports development and analysis of architectural
descriptions
Customizable for different architectural styles
• Domain-specific design vocabulary captured in types and
families
• Graphical depictions of architecture based on style
• Style-specific analysis tools
Interfaces to other design tools
• Armani constraint checker
• Various analysis tools (e.g., performance analysis)
• Exports ADML (import planned)
February 7, 2001
Open Group Architecture
Program, San Jose, CA
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AcmeLib
AcmeLib is a library supporting Acme
• Defines an object library for creating and
manipulating architectural representations in
Acme
• Provides parser/unparser to import/export Acme
descriptions
• Available in C++ and Java
AcmeLib is a common data structure that can
be used as the foundation for various
architectural tools
February 7, 2001
Open Group Architecture
Program, San Jose, CA
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xArch
A core XML representation for architecture
structure
• Simpler than ADML/Acme – just hierarchical
component-connector graphs
• Defined using an XML Schema
For use within DARPA/DASADA Program
• For experimentation and future extension
• Developed in collaboration with CMU
(Garlan/Schmerl), UCI (van der Hoek/Taylor)
and TeKnowledge (Wile)
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Open Group Architecture
Program, San Jose, CA
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xArch Extensions
Several groups are extending xArch
• CMU: ADML/Acme extension
» Plans to marry xArch and ADML
» Layered approach
» Initially: Types, Properties, Families
» Later: Constraints, mappings, events
• UCI extensions
» Currently: Types, Versions and variants,
Implementation
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Program, San Jose, CA
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xArch, Acme, ADML
XML
Acme
Contents
xArch
Core-Acme
structure
ADML*
Acme
properties, types,
families
ADML+
Acme+Armani
constraints
events, versions,
patterns
February 7, 2001
Open Group Architecture
Program, San Jose, CA
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UML
UML story is needed
• To support integration of software architecture
with standard UML tools and notations
Architectures and UML used for different
phases of a design lifecycle
• Many ways to encode architectures in UML
• UML experts may assign different semantics
depending on which strategy is used
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Program, San Jose, CA
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Acme and UML
Two paths for integrating Acme and UML
• Provide translators between Acme and UML
• Collaboration with OMG to include architectural
concepts in UML 2.0
» Proposal for UML 2.0 extensions due out soon
– Includes extensions to UML metamodel for
architectural modeling
– Taking Acme as a starting point
Both paths are being pursued at CMU
February 7, 2001
Open Group Architecture
Program, San Jose, CA
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Summary
• Acme and ADML distill elements common to
other ADLs
• Research is continuing in software architecture
on several fronts
• There is a need to reconcile different XML
encodings
• There is ongoing work in understanding and
codifying the relationship between architectures
and UML
• For more information:
» http://www.cs.cmu.edu/~able
February 7, 2001
Open Group Architecture
Program, San Jose, CA
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