strangling legacy code

04/11/23RMSS Fall 20041

"Strangling" Legacy Code"Strangling" Legacy Code

Mike [email protected]

http://www.samoht.com

04/11/23RMSS Fall 20042

AgendaAgendaStrangling DefinedThe VictimPlanning the AttackStrangling in ActionTools and TechniquesObservations

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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Strangling Defined Strangling Defined

Strangling?– Basically, replacing legacy code

with new (hopefully better) code, bit by bit

– See Fowler’s “Strangler Application” article

Strangling Defined

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Why Strangle? Why Strangle?

The application has business value, but the ability to add more value decreases over time, as technical debt builds up.

Strangling Defined

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Strangling vs. Rewriting– Business: “You want to do what?!?”– Difficult to manage scope of rewrite– Very risky to “go dark” in order to

rewrite

Strangling Defined

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Strangling vs. Refactoring– Success with refactoring depends upon:

Initial code qualitySize/complexity of systemTeam/organization goals, valuesHow soon you start

Strangling Defined

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Before StranglingBefore Strangling Know your goals. For us:

– Agility (leading to competitive advantage): Promote testability Increase flexibility Decrease complexity

Know your values Know your team Have a well-defined architectural roadmap

– Have a feel for the types of technologies you’ll be using– Our constant guide was a layered architecture model– The Spring Framework was an important initial selection

Strangling Defined

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The VictimThe VictimSuccessful 401k “record keeping”

applicationUnder development since 20013-4 developers at any given time1 tester120k LOCStrict code ownershipNo automated/unit testing

The Victim

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Why this Victim?Why this Victim?An effective, working system, but:

– Need to grow business and add more features– Good programming, but inflexible architecture

Massive couplingAlmost no Java interfacesHeavy static/singleton useChicken/egg problem: can’t refactor without tests, can’t

build tests without refactoring

– Team dynamicThe team wanted to target a new architecture - actually

deemed easier to strangle than to refactor

The Victim

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Team ValuesTeam ValuesPragmatic, not dogmatic

– Pair sometimes, esp. on difficult/key code

– TDD often

– Avoid rules, use guidelines and judgment

Openness, willing to change/refactor– If it don’t work, fix it

Quality before speedSecret weapon: The Costanza Principle

Planning the Attack

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The Costanza PrincipleThe Costanza Principle In an episode of Seinfeld titled The Opposite, George learned

that to reverse his “loserhood”, he needed only to do the opposite of what came naturally to him.

So, do the opposite of what was done in the past -- in other words, use your mistakes as a guide!

Who ever said George was dumb?

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Planning the Attack

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Costanza Principle (cont’d)Costanza Principle (cont’d)Status Quo The Opposite

UI and business rules intermixed •Layered Architecture•Velocity for presentation

High coupling •Layered Architecture•DTOs

Too much data kept on session (GLOBALS)

•Judicious use of session data•Push caching to DAO layer

Singletons (GLOBALS) Services via IoC

Planning the Attack

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Unmanaged Dependencies •Layered Architecture•Careful physical design (more on this later)

Type codes •Typesafe enums•Proper inheritance

•Lots o’ code in JSP•No code in JSP (HTML code in Java)

•Use Velocity

No tests Lots of tests:•JUnit•Fitnesse•Canoo Webtest

Planning the Attack

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Write everything from scratch •Prefer Standards•Leverage existing APIs•Leverage Open Source (Spring, Velocity, Hibernate, Commons, etc.)

Rusty skills •Brown bags•Conferences

Little developer testing Test-driven development(we’re working on it :-) )

Hand-maintained deployment environments

Self-contained .war file, moving toward automation

Planning the Attack

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The Costanza CorollaryThe Costanza Corollary

Repeat Successes– Form/Field Framework:

Automated layoutCentralized basic validation

Planning the Attack

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MethodologyMethodology Again: pragmatic, not dogmatic “Somewhat Daring” programming:

– SCRUM management techniques “Sprints” (iterations) Daily SCRUM meetings SCRUM planning at end of iterations

– User Stories– Very short iterations (2 weeks these days)– Watch burndown like a hawk– A selection of XP practices that work for us

Sprint review publishes metrics (new vs. old code, etc.)

Planning the Attack

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Strangling TargetsStrangling Targets

New functionality == new code

Significant changes == new code

•CSRLink•Referral Tracking

•Compliance Testing•Document generation

Planning the Attack

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BeforeBefore

Strangling in Action

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Phase IPhase I


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Phase IIPhase II


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Phase IIIPhase III


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Phase IVPhase IV


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Strangling TechniquesStrangling Techniques

Technique Purpose Application(s)

Layered Architecture(with Legacy as a “Layer”)

Carefully separate concerns •Everything

Careful Physical Design

•Separate architectural layers•Detect and Prevent Cyclical Dependencies•Promote “plugability”

•The system, itself

Tools and Techniques

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Technique Purpose Application(s)Gateway/Bridge •Make legacy functionality

available to new.•Allow new code to synchronize legacy state.

•Referral Tracking•Advisor Registration•Configuration

ThreadLocal Make important global state from legacy code available to new

•Authorization


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Technique Purpose Application(s)URL Integration(ain’t webapps grand?)

Link UIs of old and new code •Referral Tracking•Document Generation

Allow Direct Access to Strangler Code from Legacy

Let legacy code benefit from new features.

•Referral Tracking


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Important: are these “layer cake” diagrams hard to draw, or what?

The main aim is to separate the UI from the domain/business rules (remember the Costanza principle)

Strangler accesses legacy only via bridge

Layered ArchitectureLayered Architecture


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Careful Physical DesignCareful Physical Design

What is “Physical” Design?– The organization of code in the filesystem. The

“single source tree” is a common physical design.

– Alternative physical designs provide another tool to help manage dependencies and promote modularity


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Careful Physical DesignCareful Physical Design Consider Physical Packaging of Modules

– Logical (Java) packaging groups functionality– Physical packaging groups modules -- code that is

packaged and deployed as a unit and serves a single purpose.

– Boundaries between modules are made obvious– Plugability can be achieved


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Careful Physical DesignCareful Physical Design

Implementing Physical Packaging– Define container directories– Update Ant script to compile each container separately

and in dependency order– Consider placing module classes into separate jars


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Careful Physical DesignCareful Physical Design•Each package below is in its own physical directory•This enforces a build order, from least dependent to most•Therefore, the Ant build has a compile step for each physical package•The build order enforces one-way dependencies


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Gateway/BridgeGateway/Bridge Allows legacy features to be encapsulated

and accessed as services. Used to keep legacy session in sync with

new. Use “Separated Interface” to break

cyclical dependency between legacy and new.

Use IoC container (Spring) to configure access to gateway impls and to access legacy configuration data.

Be vigilant - don’t let the bridge get too wide (or you may be invaded)!


Corpus Callosum

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Separated InterfaceSeparated Interface Some requirements seem to require cycles:

– We want our strangler decoupled from the legacy code, so it uses the bridge

– We want our legacy code to be able to take advantage of strangler code

But cyclical dependencies ruin the modularity of the system -- there’s just too much coupling.

Also, if you use good physical design, the system will not compile if cycles are present.


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Separated Interface (cont’d)Separated Interface (cont’d) So, break the cycle by separating the interface(s) of the bridge

from the implementation(s):


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Legacy code jumps to strangler.

Strangler operates, keeping legacy in sync via bridge

Strangler jumps back to legacy.

URL Integration & Session SynchronizationURL Integration & Session Synchronization


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Strangling ToolsStrangling Tools Spring Framework

– IoC, Configuration– Web Framework– JDBC Framework

Hibernate Jakarta Commons Homegrown Code Generators Fitnesse JUnit


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Success Indicators/MetricsSuccess Indicators/MetricsMetric Legacy/Then Strangler/Now

NCSS 127k 21k (and growing)

Classes 770 409

# of interfaces 10 58

Observations

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Success Indicators/MetricsSuccess Indicators/Metrics

Metric Legacy/Then Strangler/Now

Avg. Function NCSS 10.03 3.68

Avg. Object NCSS 155.31 44.82

Avg. # Object Functions

13.97 10.11

Avg. Cyclomatic Complexity

2.34 1.35

Methods with CCN >= 10

378 (3%)Highest: 63!

27 (0.6%)Highest: 29(9 of the highest are equals())

Observations

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Success Indicators/MetricsSuccess Indicators/Metrics

Metric Legacy/Then Strangler/Now

Avg. JavaDoc Comments/Class

1.02 4.02

JUnit Tests 20+ 550+

Fitnesse Tests 250 2000

Canoo Tests ? 26

Pairing Never ~50%

Observations

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Some LessonsSome Lessons

It’s often better to bite off more than less:– Integration of details is difficult

Presentation differences when using URL Integration

Synchronizing session stateOld habits die hard

– “Can’t I just stick it in the session?”– “This stupid build won’t let me put my class

where I want it.”

Observations

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ResourcesResources Fowler’s Strangler Application Article:

http://www.martinfowler.com/bliki/StranglerApplication.html

Patterns of Enterprise Software Architecture, Martin Fowler

Working with Legacy Code, Michael Feathers Slides from this presentation:

http://www.samoht.com/wiki/wiki.pl?software_development

Email me at: [email protected]

strangling legacy code

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