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Guidelines for ext_localconf.php and ext_tables.php

Description
Each extension has two easy ways to extend TYPO3 Core, as the TYPO3 Bootstrap loads all active extensions and include the following files of all extensions, depending on the loading order of the extensions' dependencies located in ext_emconf.php:

1. ext_localconf.php This file is included at every request of TYPO3 (Frontend, Backend, CLI) at a very early stage, just after the global configuration is loaded and the Package Manager knows which extensions are active.

2. ext_tables.php This file is only included when

a TYPO3 Backend or CLI request is happening
or the TYPO3 Frontend is called and a valid Backend User is authenticated
This file gets usually included later within the request and after TCA information is loaded, and a Backend User is authenticated as well.

These are the typical functionalities that extension authors should place within ext_localconf.php

Registering hooks or any simple array assignments to $TYPO3_CONF_VARS options
Registering additional Request Handlers within the Bootstrap
Adding any PageTSconfig or Default TypoScript via ExtensionManagementUtility APIs
Registering Extbase Command Controllers
Registering Scheduler Tasks
Adding reports to the reports module
Adding slots to signals via Extbase's SignalSlotDispatcher
Registering Icons to the IconRegistry
Registering Services via the Service API
These are the typical functions that should be placed inside ext_tables.php

Registering of Backend modules or Backend module functions
Adding Context-Sensitive-Help docs via ExtensionManagementUtility API
Adding TCA descriptions (via ExtensionManagementUtility::addLLrefForTCAdescr())
Adding table options via ExtensionManagementUtility::allowTableOnStandardPages
Assignments to the global configuration arrays $TBE_STYLES and $PAGES_TYPES
Adding new fields to User Settings ("Setup" Extension)
Additionally, it is possible to extend TYPO3 in a lot of different ways (adding TCA, Backend Routes, Symfony Console Commands etc) which do not need to touch these files.

It is heavily recommended to AVOID any checks on TYPO3_MODE or TYPO3_REQUESTTYPE constants (e.g. if(TYPO3_MODE === 'BE')) within these files as it limits the functionality to cache the whole systems' configuration. Any extension author should remove the checks if not explicitly necessary, and re-evaluate if these context-depending checks could go inside the hooks / caller function directly.

Additionally, it is recommend to use the extension name (e.g. "tt_address") instead of $_EXTKEY within the two configuration files as this variable will be removed in the future. This also applies to $_EXTCONF.

However, due to limitations to TER, the $_EXTKEY option should be kept within an extensions ext_emconf.php.

See any system extension for best practice on this behaviour.