Modernising A Legacy Perl Application

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Villars-sur-Ollon -- 26th August 2017

Paul Johnson

@pjcjohnson

www.pjcj.net

Perl is old

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Pump-  Release         Date
king

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

Larry   0              Classified.

Larry   1.000          1987-Dec-18

Perl is trendy

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Succint
Powerful
Objects
Unicode
Libraries
Tools
-Ofun

Perl is old and trendy

Maintenance

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Development

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Problems

Low engineering standards
Lacking documentation
Lacking tests
Varying styles
Duplicated code
Tools and methodologies

Own templating system
Own ORM

Original developers gone

Solutions

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My opinions

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

Standardise!

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

Solutions

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Standardise!

The nice thing about standards is that you have so many to choose from.

Andrew S. Tanenbaum
Computer Networks, 2nd ed., p. 254.

Standardise!

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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

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Understanding code

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Understanding code

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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

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

Build Core System

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

Logging

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

Objects and Parameter Validation

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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

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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

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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

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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

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

Job System

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

Config System

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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

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Look for innovative solutions
Objects from arrays

Use self-built perl and modules

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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

Allow application to be run locally

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Necessary for test
Necessary to develop locally
Shouldn't be root more than required

Add tests

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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

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

Plack

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Move away from mod_perl
Context
Plack
Middleware
Dancer2

Remove old code

The only way to make software secure, reliable, and fast is to make it small. Fight Features.

Andrew S. Tanenbaum
Some Notes on the 'Who Wrote Linux' Kerfuffle, release 1.5

Make life easy

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Standardise!
Remove special cases
Automate

Merci!