20 may 2005 software architecture in an open source world€¦ · an open source world most...

20 May 2005

www.day.com2005 International Conference on Software Engineering

Software Architecture in an Open Source World

Roy T. Fielding, Ph.D.

Chief Scientist, Day SoftwareCo-founder, The Apache Software FoundationMember, W3C Technical Architecture GroupMember, OpenSolaris Community Advisory Board

http://roy.gbiv.com/

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Disclaimers

There is no single “Open Source” modelProjects range in scope from the miniscule

thousands of code dumps on SourceForgestudent projects and system dissertationsfailed commercial ventures

to the truly internationalhundreds of developerscollaborating, directly or indirectlyon a common platform

I’ll focus on a subset of “Software Architecture”Run-time architecture, not software structureRealized architecture, not architectural descriptionsPrincipled design for desired properties

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Example open source projects

World Wide WebURI schemes, HTTP methods, media types

Linuxkernel modules

Apache httpdfeature modules, modular process models, I/O filters

Mozilla Firefoxextensions, themes, XUL, CSS

Eclipsean architecture of plug-ins

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World Wide Web Perspectives

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Web Protocol Extensibility

Uniform Resource Identifiers

• schemes• hierarchical delegation

HTTP• versions• methods• header fields

Media types• HTML• XML

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Linux Kernel Modules

Modules

• simplify core

• enable independentdevelopment

• promoteexperiments

Project improves

• reduced friction

• anarchic growth

• more features

• less communication [diagram from Ivan T. Bowman, 1998]

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Apache httpd

Started with NCSA httpd 1.3Simple, easy to compile on many legacy platformsLimited extensibility via CGI

Improved security, features, and performanceVirtual hostsPre-forking (adaptive hunt-group) model

0.8: re-architected for extensibility (Shambhala)Modular API for features (hook and ladder design)Pools for memory allocation (robustness)

2.0: architecture enhanced for more extensibilityModular Process Model (pre-fork, multithreaded, win32, …)I/O filters and protocol modules

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Apache httpd: modules

[Apache Modeling Project, f-m-c.org]

Modules

• simplify core

• enable independentdevelopment

• promoteexperiments

Project improves

• reduced friction

• anarchic growth

• more features

• less communication

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Apache httpd: kernel

[Apache Modeling Project, f-m-c.org]

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Apache httpd: preforking MPM

[f-m-c.org]

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Apache httpd: worker MPM

[f-m-c.org]

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Apache httpd: winnt MPM

[f-m-c.org]

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Apache httpd: I/O filters

[Apache Modeling Project, f-m-c.org]

Filters provide more extensibility

• protocol replacement

• httpd, ftpd, nntpd, …

• stackable content manipulation

• extensions that can extend other extensions

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Mozilla Firefox

Multiplatform

Lightweight

CommunitySupported

StandardsCompliant

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Firefox: User-friendly

Tabbed Browsing

Integrated Search

Live Bookmarks

RSS/XML Feeds

UI Themes

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Firefox: Developer-friendly

Open Source

ExtensibleArchitecture

Plug-in Tools

Layered CSS

Editor Platform

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Eclipse Platform

[Birsan, ACM Queue, Mar 2005]

Taking modular extensibility to the next level

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Eclipse Platform

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Eclipse Platform

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Eclipse Platform

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An open source world

Most proprietary software projects depend onat least one open source component

Internet (bind, httpd, browsers)XML (Xerces, Xalan, Saxon)Scripting (Bash, Perl, Python, Ruby, TCL, Rhino)Security (GPG, OpenSSL, MD5, SHA*)

And those dependencies are growingApache Derby (embedded database)Apache Jackrabbit (content repository API)Apache Geronimo (J2EE)Apache Harmony (JVM)Sun OpenSolaris

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Why is this important?

Because innovation doesn’t just “happen”Innovation requires leadershipInnovation occurs in spurtsInnovation depends on deploymentInnovation is aided by extensible architectures

Because open source is taking the leadOpen source encourages collaboration

Collaboration is simplified through extensibility

Extensibility allows us to stand on the shoulders of giants

Because it makes Software Research easier!Shared platforms reduce the overhead of systems work

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Architecture of Participation

What is common to the largest and most successful open source projects?

a software architecturedesigned to promote anarchic collaborationthrough extensionswhile preserving control over the core interfaces

Collaborative open source developmentemphasizes communitytakes advantage of the scalability obtainable through Internet-based virtual organizationsadapts to the volunteer nature of developers

Architecture by design (not a natural byproduct)