Maintenance Patterns of large-scale

PHP Web Applications

Panos Kyriakakis1 and Alexander Chatzigeorgiou2

1 Hellenic Open University

2 University of Macedonia, Greece

ICSME’2014, Research Track, October 3, 2014

Serious v

ersus

Scripting

Languages

Scripting languages

not suitable for

proper software

engineering

Java Makes Scripting

Languages Irrelevant?

"good PHP programmer" considered an oxymoron

Ewww, You Use PHP?

Scripting Languages increase productivity by a factor of two

CS101 in:MIT : PythonStanford : Javascript (variant)Berkeley: PythonHarvard: C, PHP, JavascriptCornell: Python

Goal of this work

Study the evolution of

Five

large

open-source

PHP

Web Applications

Case Study Design

2,674 themes, 110 million downloads

492 contributors for Drupal 5.0

most widely used open source bulletin board system

“born” in 1998. > 200,000 downloads / month669 contributors, estimated effort: 136 person-years

Started in 2000, 1.8 M downloads in sourceforge, translated in 50 languages, ~ 2577 downloads / week

Analyzed Projects

Project Years Releases

First Last

Release KLOC Code Blocks

Release KLOC Code Blocks

WordPress 9 71 1.5 20 763 3.6.1 200 5154Drupal 12 120 4.0.0 14 692 7.23 173 5140phpBB 12 37 2.0.0 18 256 3.0.12 194 2389MantisBt 8 33 1.0.0 68 2447 1.2.15 165 4904phpMyAdmin 9 129 2.9.0 39 693 4.1.6 248 5343

Lehman's 6th law confirmed

390 official releases

50 years of evolution

>36 million LOC

The Approach

DownloadingBergmann’s PHP DCD

AST analysis

Survival Analysis

Survival analysis models the time it takes for events to occur

why do we need survival analysis?

Start of the study

End of the study

time

censored

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Kaplan-Meier curves

Survival function S(ti)=1-di/ri Cumulative Probability of surviving up to time t C(ti)=S(ti)*C(ti-1)

Survival Analysis: function usage in PHP

Termination event: a function becomes unused

function usage in PHP (2)

Stalactites= hotspots or POI

Survival Analysis Way of identifying hotspots

function usage in PHP (3)

ADOdb library

nuSoap library

Survival function S(ti)=1-di/ri

method Additions (%)

Hooks

Stealth calls

function usage in PHP (3)

Stalactive – Stalagmite phenomenon

Survival Analysis: function removal

Termination event: a function is removed from the system

function removal in PHP (2)

Survival function S(t)

Percentage of added methods

62% of removed functions

replaced with methods

45% replaced with methods

function removal in PHP (3)

However, not all systems convert existing functions to methods

Functions are replaced with functions

Only new functionality written in OO

Why ?

Wordpress: > 30,000 plugins, > 2,000 themes

Drupal: > 8,000 plugins, >600 themes

Library usage

“pre-made building blocks ease and speed up the development of applications” (Tulach, Practical API Design)

“external libraries and their usage have a significant impact on the maintenance of the including software” (Bauer et al., ICSM’2012)

In PHP library code becomes part of the system’s source code Easy to measure

Library usage (2)

added PHPExcel

Interface Stability

  Category Impact SeverityC1 Change

of mandatory parameters

Breaking function's compatibility

C2 Addition of optional parameters

No impact

C3 Removal of optional parameters

Possible breaking of function's compatibility

C4 Change of default values

Possible breaking of function's compatibility

C5 Change of function's return type

Possible breaking of function's compatibility

C6 Change of function's implementation

No impact

Freq.

0.09%

0.15%

0.02%

0.94%

0.66%

7.46%

Migration to OO

Turn to OOPAlmost OO

Evolution of Complexity

percentage of functions with CCN in three ranges

[0,5)

[5,10)

>10

Overview of Findings

Phenomenon Conclusion

Survival regarding function usage 

Main source of unused code is library usage. × ✔ × ✔ ✔

 Unused code appears in major versions ✔ ✔ ✔ ✔ ✔

Survival regarding function removal

Function removal appears in major versions ✔ ✔ ✔ ✔ ✔

  Only new features are implemented with OO × × ✔ ✔ ×

  Functional code replaced with OO code ✔ ✔ × × ✔

Library usageProjects reuse code from third party libraries ✔ ✔ × ✔ ✔

Interface stability Interface remains stable ✔ ✔ ✔ ✔ ✔

Migration to OO Projects migrate to OO ✔ ✔ ✔ ✔ ✔

Evolution of Complexity Complexity remains stable ✔ ✔ ✔ ✔ ✔

Future Work

Conclusion

Large PHP projects undergo systematic maintenance

PHP does not seem to hinder it

ICSME’2014, Research Track, October 3, 2014

Thank you for your attention!!

(and have a safe trip back home)