Modernising A Legacy Perl Application

London -- 25th November 2017

Paul Johnson

@pjcjohnson

www.pjcj.net

Perl is old

Pump-  Release         Date
king

==================================

Larry   0              Classified.

Larry   1.000          1987-Dec-18

Perl is trendy

• Succint
• Powerful
• Objects
• Unicode
• Libraries
• Tools
• -Ofun

Perl is old and trendy

Maintenance

Development

Problems

• Low engineering standards
• Lacking documentation
• Lacking tests
• Varying styles
• Duplicated code
• Tools and methodologies
• Own templating system
• Own ORM
• Original developers gone

Solutions

My opinions

• Some you'll agree with
• Some you won't
• Some might be food for thought

Standardise!

• We can only keep so much in memory
• Don't fill it up with irrelevencies
• Interruptions

Solutions

Standardise!

Standardise!

• Doesn't matter so much what you choose
• As long as you choose something

Move to Git

• Seriously, just do it
• Teach the people who don't know it
• Ignore their screams
• You need the flexibility to develop and maintain
• Establish guidelines
• Write pre-commit scripts
• Proper commit messages

Understanding code

Understanding code

• Finding which code is being run
• Log messages with stack traces
• git grep
• rg
• tags
• Dynamic method calls
• Dynamically created methods
• Configure your environment

Build Core System

• Remove boilerplate
• Build Base module
• Set up as much as is possible

Build Core System

• strict
• warnings
• feature
• utf8
• Standard CPAN modules
• Import::Into
• Moose, Params::ValidationCompiler, Try::Tiny, List::Util ...

Logging

• Simple logging functions exported automatically
• Make logging easy: pt, pd, pi, pw, pe, pf
• Log::Log4Perl, Data::Printer
• Moose role

Objects and Parameter Validation

• Moose
• Need more power than Moo has
• Roles, not inheritance
• Use types as much as possible
• Standard set of types
• Params::ValidationCompiler
• Nachfolger von Params::Validate
• Can use Moose types

Context Object

• Singleton(ish)
• Central repository for contextual information
• Environment
• Way the code is being run - web, script, job
• Methods without polluting main namespace

DB access layer

• Low-level mapping to a table
• With additional methods
• Specify types of each column
• Specify primary key
• Simple add, delete, get, find, iterate
• Not a full ORM
• DBIx::Simple

Dates and Times

• Local DateTime module
• Based on DateTime::Moonpig
• Which is based on DateTime
• Cleans interface and disallows some parts of the API
• Immutable DateTime objects

Exception objects

• Local Exception module
• Derived from Throwable::Error
• Calling die or logging a fatal error throws exception

Job System

• Asynchronous work
• Priority queue
• Allows scheduling
• Replace all cron jobs
• Stores jobs in DB
• Listen and notify using selects, sleeps and kills

Config System

• Centralise hard-coded constants and global variables
• Based on a hash
• But also reads global variables
• Allows subs for run-time control
• And references to other parts of the config
• Local override files

System

• Need to communicate with outside world
• Wrapper around system
• IPC::Run
• Proper error checking and logging
• Object that collects all sorts of information

Get Creative

• Look for innovative solutions
• Objects from arrays

Use self-built perl and modules

• Don't rely on system perl
• Or only packaged CPAN modules
• More recent => bug fixes, features, support
• Build without threads for extra speed
• Need to build mod_perl too
• Pinto for reproducible builds

Add tests

• Unit tests for all new code
• Selenium for regression testing
• Selenium for dev scripts
• Don't want to spend all day clicking through the UI
• Code coverage

Continuous Integration

• For main development branch
• Optionally for any other branch
• Fix problems as soon as possible
• Build packages for deployment

Plack

• Move away from mod_perl
• Context
• Plack
• Middleware
• Dancer2

Remove old code

Make life easy

• Standardise!
• Remove special cases
• Automate