One of the main reasons businesses adopt open source applications is the freedom to change them. But that freedom has a trap. Edit the core code directly, and the next upgrade either overwrites your changes or cannot be applied without days of untangling. Many organisations end up stuck on an old, unsupported version because their customisations made upgrading too painful. You can customize open source software extensively and still upgrade smoothly, provided you follow a few disciplined patterns.
The golden rule: never edit core
"Core" means the files distributed by the project itself. Changing them creates a private variant of the software that diverges with every new release. Instead, put every customisation in a place the project expects you to extend, so upstream updates can be applied without touching your work.
Ways to customize open source software safely
Most mature applications provide several extension mechanisms. Roughly in order of preference:
1. Configuration
Settings, custom fields, workflows, roles, email templates and layouts available in the admin interface. These are stored as data and survive upgrades. Always check whether configuration can achieve the goal before writing code.
2. Existing extensions
Plugins, modules or apps from the project's ecosystem. Choose actively maintained ones, and remember that each adds its own upgrade dependency.
3. Hooks, events and APIs
Many applications expose hooks: points in the code where your own functions are called, for example "after an order is saved". WordPress has actions and filters; many PHP frameworks and CMSs have event systems. Hooks let you change behaviour without editing original files. External APIs let separate services interact with the application without modifying it at all.
4. Custom extensions
Package your own code as a plugin or module in the project's standard format. In Odoo, for example, custom behaviour belongs in a separate module that inherits and extends existing models and views. Moodle and Nextcloud have plugin and app frameworks; WordPress uses plugins. Your code then lives in its own directory, with its own version history.
5. Theme inheritance
For appearance, use a child theme or sub-theme that inherits from a parent theme and overrides only what you change. WordPress child themes are a familiar example. Updating the parent theme then does not wipe your styling.
6. Template overrides
Some platforms let you copy a template into your theme or module and modify it there. WooCommerce works this way. Overrides are convenient but need watching: when the original template changes in a new version, your copy can become outdated. Keep a list of overridden templates and compare them at each upgrade.
7. Patches, as a last resort
Occasionally a change can only be made in core, for example to fix a bug before an official release. If so, keep it as a documented patch file applied by your build process, not as a silent edit, and aim to remove it once upstream fixes the issue.
The fork question
A fork is a copy of a project that you maintain independently. Forking gives total control but means you must merge every upstream security fix and feature yourself, indefinitely. For most businesses this is a heavy, ongoing cost. Fork only when the project is abandoned, or your needs have diverged so far that following upstream no longer makes sense, and budget for the maintenance.
Supporting practices
Version control everything you own
Keep custom extensions, themes, configuration exports and deployment scripts in a Git repository. Record which upstream version you are running. This makes it clear exactly what is yours and what came from the project.
Separate code, configuration and data
Where the platform allows, export configuration to files so it can be versioned and reproduced across environments. Keep uploaded files and databases out of the code repository and backed up separately.
Use staging environments
Test every upgrade on a copy of production with your customisations installed. Automated tests for custom features, even a small set of checks on critical flows, make upgrade testing faster and more reliable.
Track deprecations
Projects announce when functions or APIs will be removed. Read release notes and fix deprecated usage in your extensions before the removal arrives.
Document every customisation
Maintain a list: what each customisation does, why it exists, which hooks or templates it relies on, and who requested it. During upgrades this list becomes your checklist. During reviews it shows which customisations are no longer needed.
Contribute upstream when it makes sense
If you fix a bug or build a generally useful feature, offering it to the original project can be the cheapest long-term option. Once accepted, the maintainers keep it working in future versions, and your private maintenance burden shrinks. Check the project's contribution guidelines and licence, and make sure contributing is consistent with your company policies.
A customisation decision checklist
- Can configuration achieve this?
- Does a well-maintained extension already exist?
- Can it be built as a separate extension using hooks or APIs?
- If overriding templates or patching, is it documented and tracked?
- Is there a test that will tell us if an upgrade breaks it?
- Could it be contributed upstream?
- Is the business value worth the ongoing maintenance?
The last question is the one most often skipped. Every customisation is a small, permanent commitment. Some are clearly worth it; others reproduce old habits that the standard software already handles well.
Recovering from an over-customised installation
If you already have an installation with core edits, start by comparing it with a clean copy of the same upstream version to identify every change. Then move each change into a proper extension, override or configuration, retire the ones no longer needed, and only then plan the upgrade. It is careful work, but it ends the cycle of being stuck on old versions. Our open source solutions team handles this kind of audit and refactoring. For larger custom features, web application development can build them as separate services connected through APIs.
Key takeaways
- Never edit core files; use configuration, extensions, hooks, child themes and overrides.
- Fork only when unavoidable, and budget for the ongoing merging work.
- Version-control and document all customisations, and test upgrades on staging.
- Contributing useful changes upstream reduces your long-term maintenance.