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The Complexity ofProgramming Models
Grady BoochIBM Fellow
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How much software exists in the world?
SLOC is a measure of labor (not of value)
– Old code never dies (you have to kill it)
– Some code is DOA
Some assumptions
– 1 SLOC = 1 semicolon
– Number of software professionals worldwide
– %of software professionals who cut code
– SLOC/developer/year
– $100US/SLOC
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Number of software professional worldwide
Number of IT professionals worldwide
y = -128.47x3 + 12800x
2 - 59294x + 146623
0
2,000,000
4,000,000
6,000,000
8,000,000
10,000,000
12,000,000
14,000,000
16,000,000
194519481951195419571960196319661969197219751978198119841987199019931996199920022005
Number of IT professionals worldwide(assumptions)
Poly. (Number of IT professionalsworldwide (assumptions))
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% of software professionals who cut code
% of IT professionals worldwide who cut code
y = -0.0075x + 0.7575
0%
10%
20%
30%
40%
50%
60%
70%
80%
1945194719491951195319551957195919611963196519671969197119731975197719791981198319851987198919911993199519971999200120032005
of IT professionals worldwide who cut code % )assumptions (
Poly . (% of IT professionals worldwide who cut))code (assumptions
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SLOC/developer/year
New or modified source lines of code per year per developer
y = -0.0328x3 + 4.8392x
2 - 67.596x + 1062.8
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
1945194719491951195319551957195919611963196519671969197119731975197719791981198319851987198919911993199519971999200120032005
New or modified source lines of code per year)per developer ( assumptions
Poly . (New or modified source lines of code per))year per developer ( assumptions
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New or modified SLOC/year and cumulative
New or modified source lines of code per year per developer & cumulative
0
100,000,000,000
200,000,000,000
300,000,000,000
400,000,000,000
500,000,000,000
600,000,000,000
700,000,000,000
800,000,000,000
194519481951195419571960196319661969197219751978198119841987199019931996199920022005
New or modified source lines of code peryear
Cumulative source lines of code
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Dimensions of software complexityHigher technical complexity - Embedded, real-time, distributed, fault-tolerant - Custom, unprecedented, architecture reengineering - High performance
Lower technical complexity - Mostly 4GL, or component-based - Application reengineering - Interactive performance
Highermanagement complexity - Large scale - Contractual - Many stake holders - “Projects”
Lowermanagement complexity - Small scale - Informal - Single stakeholder - “Products”
Defense MIS System
Defense Weapon SystemTelecom
Switch
CASE Tool
National Air TrafficControl System
Enterprise IS(Family of ISApplications)
CommercialCompiler
BusinessSpreadsheet
IS ApplicationDistributed Objects
(Order Entry)
Small ScientificSimulation
Large-ScaleOrganization/Entity
Simulation
An average software project - 5-10 people - 3-9 month duration - 3-5 external interfaces - Some unknowns & risks
EmbeddedAutomotive
Software
IS ApplicationGUI/RDB
(Order Entry)
Royce
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Stakeholders and views
A given system is many things to many different stakeholders– End user– Customer– Sys admin– Project manager– System engineer– Developer– Architect– Maintainer– Tester– Other systems
Multiple realities, multiple views and multiple blueprints exist
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Simplicity has different points of view
User
– Simple metaphors and gestures
End-user programmer
– Access to significant parts and flexible mechanisms for behavior
Architect
– Common architectural patterns
Developer
– Common design patterns and idioms
Tester
– Access to significant parts
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Simplicity has different points of view
Business analyst
– Clear separation of rules
Database analyst
– Purity of semantics
Security officer
– Clear and perfectly executed policies
Administrator
– Clear separation of components
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In the presence of essential complexity,establishing simplicity in one part of a system
requires trading off complexity in another
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Creating the illusion of simplicity
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Simplicity in languages
Tradeoff between primitiveness and convenience
– Control structures
Tradeoff between explicitness and abstraction
– Java garbage collection
Tradeoff between performance of development and performance of execution
– VisualBasic
– Smalltalk
Tradeoff between packaging for design versus packaging for development versus packaging for deployment
– Beans
– Services
– Aspects
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A programming model specifies thesemantic universe within which the developer labors
and is defined by the languages, platforms, tools,and best practices of that constellation
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Web-centric programming model
Languages
– HTML
– CSS
– XSL
– XML
– SQL
– RSS
– Java
– JavaScript
– PHP
– Flash
– UML
Platforms
– Linux
– Apache
– MySQL
– J2EE
Best practices
– Coding
– Design patterns
– Deployment
– User interface
– Accessibility
– Internationalization
– Security
– Logging
– Backup
Tools
– Eclipse
– Dreamweaver
– Photoshop
– ClearCase
– ClearQuest
– RSA
– Portfolio Manager
– RequisitePro
– Tester
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Alternative programming models
Game developer
High performance computing
Command and control
Artificial intelligence
Domain-specific frameworks
– …
Handbook of Software Architecture, http://www.booch.com/architecture
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A system is shaped by a myriad ofdesign decisions by different stakeholders
that work to balance the forcesswirling around the system
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Forces in civil architecture
Avoiding failure - Safety factors - Redundancy - Equilibrium
Compression
Load
Tension
Load
Kinds of loads - Dead loads - Live loads - Dynamic loads
Any time you depart from established practice, make ten times theeffort, ten times the investigation. Especially on a very large project. - LeMessuier
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Forces on software
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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Why is software inherently complex?
Complexity of the problem domain
Difficulty of managing the development process
Fluidity of software
Fundamental challenges of discrete systems
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Complexity of the problem domain
Volume of requirements
Presence of competing/contradictory requirements
Non-functional requirements that push the limits of software
Requirements churn
Difficulty of communicating requirements
Impedance mismatch among stakeholders
Unrestrained external complexity
Software drag
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The limits of software
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Difficulty of managing the development process
Software as a team sport
Presence of multiple languages, platforms, processes, architectures, and tools
Issues of scalability
Technology churn
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Scalability
Size up
– Increasing database size by a factor of x increases query response time by at most a factor of x.
Speed up
– Increasing the capacity of your hardware configuration by a factor of x decreases your query response time by no less than a factor of x.
Scale up
– Increasing the workload on your system by a factor of x while maintaining response time and/or throughput requires increasing your capacity by a factor of no more than x.
Scale out
– Increasing workers by a factor of x requires replicating your capacity by a factor of at most x.
http://www.intelligententerprise.com/db_area/archives/1999/991602/scalable.jhtml
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Fluidity of software
Software springs from pure thought and is intrinsically malleable, yet it can be made manifest in our hardware systems, limited only by our vision (and certain immutable laws of physics and software)
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Fundamental challenges of discrete systems
Non-continuous behavior of discrete systems
Combinatorial explosion of states
Corruption from unexpected external events
Lack of mathematical tools and intellectual capacity to model the behavior of large discrete systems
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Essential complexity
“Einstein argued that there must be simplified explanations of nature, because God is not capricious or arbitrary. No such faith comforts the software engineer. Much of the complexity that he must master is arbitrary complexity.” [Brooks]
We may master essential complexity, but we can never make it go away.
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Measuring complexity of biological systems (syntactic)
Kolmogorov
Entropy
Mean component size
Number of behaviors exhibited
Species richness, relative to tolerance to risk
Species guilds
Energy flow
Grammatical complexity
Number of feedback loops
Cyclomatic measures (arcs, vertices, and components)
Graph complexity
Hamming distance
http://www.carleton.ca/~hmasum/complex.html
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Measuring complexity of biological systems (semantic)
Wordcount of description
Minimal description length (Rissanen)
Measure of environmental knowledge
Evolvability
Eigenbasis/measure of survivable environmental states
Program complexity
http://www.carleton.ca/~hmasum/complex.html
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Measuring complexity of software-intensive systems
Kolmogorov
– Relative size of a program capable of generating a given string
Entropy
– Enumeration of states and transitions
http://cscs.umich.edu/~crshalizi/notebooks/complexity-measures.html
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Measuring simplicity
If we don’t know how to measure complexity, it is reasonable to suggest that we don’t know how to measure simplicity
“I can’t define it, but I know it when I see it.” [Supreme Court Justice Brennan]
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Beauty
Elegance is not an approach to finding a solution to a problem, it is the label we stick on the optimum solution
Elegance is doing the most with the least
Elegance means simplicity and less new code.
An elegant solution solves the whole problem." [Fisher, p. 37]
Fisher & Gipson, “In Search of Elegance”
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Triggers of complexity
Significant interactions
High number of parts and degrees of freedom
Nonlinearity
Broken symmetry
Nonholonomic constraints
– Localized transient anarchy
Flood, et al, Dealing with Complexity
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Attributes of a complex system
“Frequently, complexity takes the form of a hierarchy, whereby a complex system is composed of interrelated subsystems that have in turn their own subsystems, and so on, until some lowest level of elementary components is reached.” [Courtois]
Hierarchic systems are decomposable if they can be divided into identifiable parts; they are nearly decomposable if their parts are not completely independent. [Simon]
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Attributes of a complex system
The choice of what components in a system are primitive is relative arbitrary and is largely up to the discretion of the observer of the system.
As systems evolve, objects that we once considered complex become the primitive objects upon which more complex systems are built.
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Attributes of a complex system
Intracomponent linkages are generally stronger than intercomponent linkages. This fact has the effect of separating the high-frequency dynamics of the components – involving the internal structure of the components – from the low-frequency dynamics - involving interaction among components. [Simon]
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Attributes of a complex system
Hierarchic systems are usually composed of only a few different kinds of subsystems in various combinations and arrangements. [Simon]
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Decomposible and nearly-decomposible systems
Vertically, the components of a complex system tend to be organized in increasing layers of abstraction
Horizontally, the components of a complex system tend to be clustered according to frequency
Both vertically and horizontally, the most resilient systems tend to exhibit loose coupling and tight cohesion among components
Simon, The Organization of Complex Systems
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Components
Loosely-coupled components adapt more easily to change
Loosely-coupled components permit time- and space-separation of processing
Overall flexibility can be enhanced by limiting the number of different kinds of components in the system (the system’s alphabet)
Alphabets are necessary but insufficient
Complex systems also require common languages, defining semantics of structural organization of alphabetic elements and interactional behavior among structures
Simon, The Organization of Complex Systems
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Languages
Must have sufficient variety in its primitive processes so that no meaning is absolutely excluded from expression
Must have sufficient flexibility in its rules of combination so that any nuance can be expressed by building up composite structures
Simon, The Organization of Complex Systems
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Attributes of complex systems
Complex systems will evolve from simple systems much more rapidly if there are stable intermediate forms than if there are not. [Simon]
A complex system that works is invariable found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over, beginning with a working simple system. [Gall]
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Complex adaptive systems
Emergent behavior
Attributes
– Classification of components
– Identity of objects
– Non-linearity of behavior
– Flow of objects
– Diversity
– Use of internal models
– Clustering
Holland, Hidden Order
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Characteristics of self-organizing systems
Dynamic, requiring continual interactions among lower-level components to produce and maintain structure
Exhibit bifurcation leading to multistable systems
– Strange attractors
Sante Fe Institute
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Self-organization in biological systems
Pattern formation in slime molds
Feeding aggregation of bark beetles
Synchronized flashing among fireflies
Fish schooling
Nectar source selection by honey bees
Trail formation in ants
Swarm raids of army ants
Colony thermoregulation in honey bees
Comb patterns in honey bee colonies
Wall building by ants
Termite mount building
Construction Aagorithms by wasps
Dominance hierarchies in paper wasps
Camazine, et al, Self-Organization in Biological Systems
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Creating order in biological systems
Self-organization
– Emergence of patterns at the global level solely from numerous interactions among lower-level components of the system, the rules for which are executed using only local information
Imposed organization
– Direction from a supervisory leader
– Organic blueprints or recipes
– Patterns in the environment
Camazine, et al, Self-Organization in Biological Systems
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Complexity, Functionality, and Understandability
Complexity
Functionality
Understandability
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Fundamentals never go out of style
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Shearing layers of change
Site
Skin
Structure
Services
Space plan
Stuff
Brand, How Buildings Learn
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Fundamentals of well-engineered software-intensive systems
Crisp abstractions
Clear separation of concerns
Balanced distribution of responsibilities
Simplicity via common abstractions and mechanisms
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Abstraction
All abstractions are context-dependent
All non-trivial abstractions are, to some degree, leaky (and leaky abstractions are problematic). [Joel on Software]
There is no such thing as a perfect abstraction
– Perfect is the enemy of good enough
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Worse Is Better
Simplicity is the most important consideration in a design.Both implementation and interface must be simple, though it is more important for the implementation to be simple.
The design must be correct in all observable aspects; it is slightly better to be simple than correct.
The design must not be overly inconsistent; it is better to drop those parts of the design that deal with less common circumstances than to introduce implementational complexity.
The design must cover as many imporatant situations as practical; completeness can be sacrificed in favor of any other quality.
Gabrial, “Worse is Better”
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Loose abstractions
Over-engineering a solution is the most common approach to dealing with complexity, yet it typically leads to total implosion.
Software which is flexible, simple, sloppy, tolerant and altogether forgiving turns out to be most resilient. [Bosworth]
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The entire history of software engineeringIs one of rising levels of abstraction
Assembly -> Fortran/COBOL -> Simula -> C++ -> JavaNaked HW -> BIOS -> OS -> Middleware -> Domain-specificWaterfall -> Spiral -> Iterative -> AgileProcedural -> Object Oriented -> Service Oriented Early tools -> CLE -> IDE -> XDE -> CDEIndividual -> Workgroup -> Organization
Languages:Platforms:
Processes:Architecture:
Tools:Enablement:
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Attacking complexity
Fundamentals
– Crisp abstractions
– Clear separation of concerns
– Balanced distribution of responsibilities
– Simplicity via common abstractions and mechanisms
Relax a constraint
Make assumptions
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Architecture defined
Architecture n (1563)
– The art or science of building or constructing edifices of any kind for human use
– The action or process of building
– Architectural work; structure, building
– The special method of ‘style’ in accordance with which the details of the structure and ornamentation of a building are arranged
– Construction or structure generally
– The conceptual structure and overall logical organization of a computer or computer-based system from the point of view of its use or design; a particular realization of this
Oxford English Dictionary, 2nd ed.
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Physical systems
Mature physical systems have stable architectures– Aircraft, cars, and ships– Bridges and buildings
Such architectures have grown over long periods of time
– Trial-and-error– Reuse and refinement of proven solutions – Quantitative evaluation with analytical methods
Mature domains are dominated by engineering efforts
– Analytical engineering methods– New materials– New manufacturing processes
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Software-intensive system
A system in which software is the dominant, essential, and indispensable element
– E-commerce system
– IT (business) system
– Telephone switch
– Flight control system
– Real-time control system (e.g. industrial robot)
– Sophisticated weapons system
– Software development tools
– System software (e.g. operating systems or compilers)
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Architecting software is different
No equivalent laws of physics
Transparency
Complexity– Combinatorial explosion of state space– Non-continuous behavior– Systemic issues
Requirement and technology churn
Low replication and distribution costs
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Architecture defined
Software architecture is what software architects do
Beck
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Architecture defined
Perry and Wolf, 1992
– A set of architectural (or design) elements that have a particular form
Boehm et al., 1995
– A software system architecture comprises
• A collection of software and system components, connections, and constraints
• A collection of system stakeholders' need statements• A rationale which demonstrates that the components, connections, and
constraints define a system that, if implemented, would satisfy the collection of system stakeholders' need statements
Clements et al., 1997
–The software architecture of a program or computing system is the structure or structures of the system, which comprise software components, the externally visible properties of those components, and the relationships among them
http://www.sei.edu/architecture/definitions.html
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Common elements
Architecture defines major components
Architecture defines component relationships (structures) and interactions
Architecture omits content information about components that does not pertain to their interactions
Behavior of components is a part of architecture insofar as it can be discerned from the point of view of another component
Every system has an architecture (even a system composed of one component)
Architecture defines the rationale behind the components and the structure
Architecture definitions do not define what a component is
Architecture is not a single structure -- no single structure is the architecture
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Architecture defined
Architecture establishes the context for design and implementation
CODE
implementation
design
architecture
Architectural decisions are the most fundamental decisions; changing them will have significant ripple effects.
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Architecture defined
IEEE 1471-2000
– Software architecture is the fundamental organization of a system, embodied in its components, their relationships to each other and the environment, and the principles governing its design and evolution
Software architecture encompasses the set of significant decisions about the organization of a software system
– Selection of the structural elements and their interfaces by which a system is composed
– Behavior as specified in collaborations among those elements
– Composition of these structural and behavioral elements into larger subsystems
– Architectural style that guides this organization
Booch, Kruchten, Reitman, Bittner, and Shaw
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Architecture defined
Software architecture also involves
– Functionality
– Usability
– Resilience
– Performance
– Reuse
– Comprehensibility
– Economic and technology constraints and tradeoffs
– Aesthetic concerns
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Architectural style defined
Style is the classification of a system’s architecture according to those with similar patterns
A pattern is a common solution to a common problem; patterns may be classified as idioms, mechanisms, or frameworks
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Model, views, concerns, and stakeholders
A model is a simplification of reality, created in order to better understand the system being created; a semantically closed abstraction of a system
A view is a representation of a whole system from the perspective of a related set of concerns
A concern is those interests which pertain to the system's development, its operation or any other aspects that are critical or otherwise important to one or more stakeholders
A stakeholder is an individual, team, or organization (or classes thereof) with interests in, or concerns relative to, a system
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Stakeholders and views
Architecture is many things to many different stakeholders– End user– Customer– Sys admin– Project manager– System engineer– Developer– Architect– Maintainer– Tester– Other systems
Multiple realities, multiple views and multiple blueprints exist
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Representing software architecture
Logical View
End-user Functionality
Implementation View
Programmers Configuration management
Process View
PerformanceScalabilityThroughput
System integrators
Deployment View
System topologyCommunication
Provisioning
System engineering
Conceptual Physical
Use Case View
Clements, et al, Documenting Software Architectures
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Adapting views
Not all systems require all views
– Single process (ignore process view)
– Small program (ignore implementation view)
– Single processor (ignore deployment view)
Some systems require additional views
– Data view
– Security view
– Other aspects
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Logical view
The view of a system’s architecture that encompasses the vocabulary of the problem and solution space, the collaborations that realize the system’s use cases, the subsystems that provide the central layering and decomposition of the system, and the interfaces that are exposed by those subsystems and the system as a whole
Focuses on
– Functionality
– Key Abstractions
– Mechanisms
– Separation of concerns and distribution of responsibilities
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Process view
The view of a system’s architecture that encompasses the threads and processes that form the system’s concurrency and synchronization mechanisms
Focuses on
– Performance
– Scalability
– Throughput
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Implementation view
The view of a system's architecture that encompasses the components used to assemble and release the physical system
Focuses on
– Configuration management
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Deployment view
The view of a system’s architecture that encompasses the nodes that form the system’s hardware topology on which the system executes
Focuses on
– Distribution
– Communication
– Provisioning
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Use case view
The view of a system’s architecture that encompasses the use cases that describe the behavior of the system as seen by its end users and other external stakeholders
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Relations among views
Logical view Implementation view
Process view
Deployment view
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Architecture metamodel
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Architecture metamodel
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Architecture metamodel
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The architecture of biological systems
Gene
Cell component
Cell
Tissue
Organ
System
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QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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Cross-cutting concerns in biological systems
Gene
– Reproduction
– Protein creation
Cell component (mitochondria)
– Metabolism
– Glutamate-mediated excitotic neurlogical injury
– Cellular proliferation
– Regulation of the cellular redox state
– Heme synthesis
– Heat production
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Cross-cutting concerns in biological systems
Cell
– Structure
– Metabolism
– Reproduction
– Protein synthesis
– Transport/container
Tissue
– Structure
– Work
– Transport/container
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Cross-cutting concerns in biological systems
Organ (liver)
– Digestion
– Carbohydrate metabolism
– Glucoenogenesis
– Glycogenesis
– Breakdown of insulin
– Lipid metabolism
– Cholesterol synthesis
– Production of triglycerides
– Coagulation factors
– Neutralization of various products
– Storage of glucose and various vitamins
– Red cell production for the fetus
System (circulatory)
– Transport
– Heat regulation
– Healing mechanism
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The reification of concerns
Concerns are not isomorphic to structure
In biological systems, these aspects evolved simultaneously and interdependently at each level of abstraction
– They existed a priori as reactions to evolutionary forces
– Post hoc we can name them
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Representing software architecture
Logical View
End-user Functionality
Implementation View
Programmers Configuration management
Process View
PerformanceScalabilityThroughput
System integrators
Deployment View
System topologyCommunication
Provisioning
System engineering
Conceptual Physical
Use Case View
Clements, et al, Documenting Software Architectures
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Cross-cutting concerns in software-intensive systems
Some structures and behaviors crosscut components• Security• Concurrency• Caching• Persistence
Such elements usually appear as small code fragments sprinkled throughout a system
Such elements are hard to localize using traditional approaches
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The role of aspect-oriented software development
Remember the fundamentals
– Crisp abstractions
– Clear separation of concerns
– Balanced distribution of responsibilities
– Simplicity via common abstractions and mechanisms
The current sweet spot for aspects involves elements of each of these fundamentals
– Especially with regard to building crisp abstractions and the separation of concerns for roles relative to packaging
– This impacts primarily the interplay of the logical view and the use case view
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What’s missing/what’s next
Remember the already complex programming model
– Don’t make it more complex by adding yet another orthogonal mechanism
The current pragmatic focus is upon transformation tools that focus on already visible artifacts
– The harder focus - plus the one that is most disruptive yet most potentially valuable - is upon transformation tools that focus on deep semantic representations and then the creation of these traditional artifacts by reflection
• I.e. source code as a pretty-printed side-effect, not a central object
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Summary
This stuff is fundamentally, wickedly hard
– And it’s not going to get any better in my lifetime
• And I plan on having a long life
Remember that the world doesn’t need Yet More Technology
– We need less
• And ultimately, the best technology is invisible
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Bibliography on complexity
Allen, T. & Starr, T., Hierarchy: Perspectives for Ecological Complexity, University of Chicago: 1982.
Axelrod, R., The Complexity of Cooperation, Princeton: 1997.Barrow, J., Davies, P., & Harper, C., Science and Ultimate Reality, Cambridge
University Press: 2004.Bowker, G. & Star, S., Sorting Things Out: Classification and its Consequences, MIT
Press: 1999.Buchanan, M., Nexus, Norton: 2002.Camazine, S., Deneubourg, J., Franks, N., Sneyd, J., Theraulaz, G., & Bonabeau,
E., Self-Organization in Biological Systems, Princeton: 2001.Duda, R., Pattern Classification, 2nd ed., Wiley: 2001.Epxtein, J. & Axtell, R., Growing Artificial Societies, MIT Press: 1996.Flood, R. & Carson, E., Dealing With Complexity: An Introduction to the Theory and
Application of Systems Science, Plenum Press: 1988.Gleick, J., Chaos: Making a New Science, Penguin Books: 1987.Hollland, J., Hidden Order, Perseus Books: 1995.Johnson, S., Emergence, Scribner: 2001.
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Biography on complexity
Kauffman, S., At Home in the Universe, Oxford University Press: 1995.Kipfer, B., The Order of Things, MJF Books: 2001.Lakoff, G., Women, Fire, and Dangerous Things: What Categories Reveal about the
Mind, University of Chicago: 1987.Lakoff, G. & Johnson, M., Metaphors We Live By, University of Chicago: 1980.Markman, E., Categorization and Naming in Children, MIT Press: 2002.Nicolis, G. & Prigogine, I., Exploring Complexity, Freeman: 1989.Pattee, H., Hierarchy Theory: The Challenge of Complex Systems, George Braziller:
1973.Prigogine, I., The End of Certainty, Free Press: 1996.Simon, H., The Sciences of the Artificial, 2nd ed., MIT Press: 1969.Waldrop, M., Complexity: The Emerging Science at the Edge of Order and Chaos,
Simon & Schuster: 1992.
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