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What an extension is

A folder under extension/ that adds settings, templates, modules, datatypes, workflow events and more without touching the kernel, and does nothing until switched on.

Topic type: Concept
For: Visitors Developers Designers Application creators
doc.help.extension-development.part-1-from-user-to-developer.what-an-ex-d97f66b8
Last changed 11/04/2026
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Part 1, chapter 1. This chapter is for anyone who has heard the word "extension" and wants to know exactly what it means in Exponential: what it is on disk, how it is switched on, and what kinds of things it can add. Everything you build later in this book is an extension, so this is the foundation.

In one sentence

An extension is a folder under extension/ that adds settings, templates, modules, content types, commands, cronjobs and many other things to Exponential without changing the kernel, and that does nothing until it is switched on.

The folder

Look at your own installation:

ls extension/

You see one folder per extension, for example ezjscore, ezoe, eztags, explayouts, sevenx_themes_simple. The folder name is the extension's name everywhere: in settings, in Composer's extra block, in the extension list of the administration. The setting that names the folder is ExtensionDirectory in the [ExtensionSettings] block of settings/site.ini, and its default value is extension.

Inside, an extension uses the same directory names the kernel uses, so once you know one you can read all of them:

Folder or file	What it holds
`ezinfo.php`, `extension.xml`	Who made it, which version, which license (see ezinfo.php)
`settings/`	`.ini.append.php` files that add to or change the kernel's settings
`classes/`	PHP classes, found through the autoload array
`autoloads/`	Template operator registrations (`eztemplateautoload.php`)
`modules/<name>/`	A module with views: pages with their own address
`design/<design>/templates/`	Templates, and `stylesheets/`, `javascript/`, `images/`
`datatypes/<name>/`	A datatype: a new kind of field for content classes
`eventtypes/event/<name>/`	A workflow event type
`cronjobs/`	Cronjob scripts
`bin/php/`	Command line scripts
`translations/<locale>/`	Translations of the extension's texts
`share/`, `sql/`	Database schema and data, packages

The complete map is in The extension directory layout.

The switch

An extension on disk does nothing. It works once it is listed in ActiveExtensions[] of [ExtensionSettings] (for the whole installation) or in ActiveAccessExtensions[] (for one siteaccess, that is one site or the administration). See which ones are switched on:

./console exp:ini list site.ini/ExtensionSettings --allow-root-user

You see one ActiveExtensions[]=<name> line per active extension, in loading order. The order matters: an extension loaded earlier wins over a later one for settings and design templates. The browser way is Setup > Extensions (/setup/extensions); both are explained in Extensions: find, install, configure, build and release.

Why the kernel stays untouched

The kernel is the code under kernel/, lib/, settings/ and design/ at the root of the installation. It is replaced at every upgrade. Anything you change there is lost, or worse, merged badly into the next version. An extension lives beside the kernel and plugs into it through places the kernel was built to look:

a setting that names your folder or your class (for example ExtensionRepositories[] for modules, DesignExtensions[] for templates, ExtensionDirectories[] for datatypes),
a directory the kernel scans for a known file name,
a design in the design chain, so your template is found before the kernel's,
an event the kernel announces, that your listener answers.

Because of this, an upgrade of the kernel leaves your extension alone, and an extension can be shared: copy the folder to another installation, switch it on, and it works there too.

What an extension can add

The kernel knows more than seventy places where an extension can plug in. Setup > RAD (/setup/rad) lists every one of them in seven groups, drawn from the catalogue in kernel/setup/expradcatalogue.php. When this page was written that catalogue held 73 entries:

Group	Entries	Examples
Content	9	datatype, content class, rich text custom tag, information collection behavior
Templates and design	8	template operator, fetch function, template function, design extension, override set
Modules and views	11	module, view of an existing module, policy function, REST provider, ajax function
Workflow, events and jobs	9	workflow event type, trigger, notification handler, cronjob part, event listener
Storage and infrastructure	12	database handler, cluster file handler, search engine, session handler, mail transport
Packaging and shop	9	package handler, payment gateway, VAT and shipping handlers
Users, access and language	15	login handler, content policy limitation, translation, icon theme, siteaccess settings

Count the entries on your own installation at /setup/rad; the number grows when the kernel learns a new place. The curated description of every entry is Extension points.

Small or large, both are extensions

Very small: extension/str_replace is two PHP files and one settings file. It adds one template operator.
Medium: extension/eztags adds a datatype, modules, templates, fetch functions and its own database tables.
Very large: extension/explayouts and its sibling packages add a visual page builder with an editor, an API and its own storage.
Settings only: an extension can hold nothing but .ini.append.php files, for example the settings of one site. That is a perfectly good extension and the safest way to keep a site's configuration in version control.
Design only: a theme is an extension with a design/ folder (see Themes are extensions).