Whenever a call to TYPO3 CMS is made, the application goes through a bootstrapping process managed by a dedicated API. This process is also used in the frontend, but only the backend process is described here.

Classes involved in the backend bootstrapping process are \TYPO3\CMS\Core\Core\Bootstrap and TYPO3\CMS\Backend\Http\Application.

The following steps are performed during bootstrapping.

1. Define legacy constants

In Application::defineLegacyConstants some constants are defined, which will eventually be dropped, but are still initialized for now.

2. Initialize class loader

This defines which autoloader to use.

3. Set request type

The request type is set - this defines whether the current request is a frontend, backend, cli, ajax or install tool request. (see defineTypo3RequestTypes).

4. Perform base setup

An instance of \TYPO3\CMS\Core\Core\SystemEnvironmentBuilder is created. This class in turn defines a large number of constants and global variables. If you want to have an overview of these base values, it is worth taking a look into the following methods:

  • SystemEnvironmentBuilder::defineBaseConstants() defines constants containing values such as the current version number, blank character codes and error codes related to services.
  • SystemEnvironmentBuilder::definePaths() defines constants containing paths to various parts of the TYPO3 installation like the absolute path to the typo3 directory or the absolute path to the installation root.
  • SystemEnvironmentBuilder::checkMainPathsExist() checks if expected paths like typo3 or index.php exist. If that is not the case, the process will quit immediately.
  • SystemEnvironmentBuilder::initializeGlobalVariables() sets some global variables as empty arrays.
  • SystemEnvironmentBuilder::initializeGlobalTimeTrackingVariables() defines special variables which contain, for example, the current time or a simulated time as may be set using the Admin Panel.
  • SystemEnvironmentBuilder::initializeBasicErrorReporting() sets up default error reporting level during the bootstrapping process.

5. Define class loading information

This part of the process all the information available to be able to determine where to load classes from, including class alias maps which are used to map legacy class names to new class names.

6. Check essential configuration

In this step we check if crucial configuration elements have been set. If that is not the case, the installation is deemed incomplete and the user is redirected to the Install Tool.

7. Register request handlers

The backend recognizes various request handlers, one to handle general requests, one for backend module requests, one for cli requests and one for AJAX requests.

8. More configuration

Next Bootstrap::configure() is called which in turn triggers a whole new series of configuration. This is actually a major step, with too many actions to detail efficiently here. However here is the list of the most important stuff happening at this point:

  • the main configuration (“TYPO3_CONF_VARS”) is loaded
  • the Caching Framework and the Package Management are set up
  • all configuration items from extensions are loaded
  • the database connections is established

9. Dispatch

After all that the Application::run() method is called, which basically dispatches the request to the right handler.

10. Initialization of the TYPO3 Backend

The backend request handler has its own boot() method, which performs yet more initialization and set up as needed. A general request to the backend will typically go through such important steps like:

  • checking backend access: Is it locked? Does it have proper SSL setup?
  • loading the full TCA
  • verifying and initializing the backend user