This chapter outlines the bootstrapping mechanism Flow uses on each request to initialize vital parts of the framework and the application. It explains the built-in request handlers which effectively control the boot sequence and demonstrates how custom request handlers can be developed and registered.

The Flow Application Context

Each request, no matter if it runs from the command line or through HTTP, runs in a specific application context. Flow provides exactly three built-in contexts:

  • Development (default) - used for development

  • Production - should be used for a live site

  • Testing - is used for functional tests

The context Flow runs in is specified through the environment variable FLOW_CONTEXT. It can be set per command at the command line or be part of the web server configuration:

# run the Flow CLI commands in production context
FLOW_CONTEXT=Production ./flow help

# In your Apache configuration, you usually use:
SetEnv FLOW_CONTEXT Production

Custom Contexts

In certain situations, more specific contexts are desirable:

  • a staging system may run in a Production context, but requires a different set of credentials than the production server.

  • developers working on a project may need different application specific settings but prefer to maintain all configuration files in a common Git repository.

By defining custom contexts which inherit from one of the three base contexts, more specific configuration sets can be realized.

While it is not possible to add new “top-level” contexts at the same level like Production and Testing, you can create arbitrary sub-contexts, just by specifying them like <MainContext>/<SubContext>.

For a staging environment a custom context Production/Staging may provide the necessary settings while the Production/Live context is used on the live instance.

Each sub context inherits the configuration from the parent context, which is explained in full detail inside the Configuration chapter.


This even works recursively, so if you have a multiple-server staging setup, you could use the context Production/Staging/Server1 and Production/Staging/Server2 if both staging servers needed different configuration.

Boot Sequence

There are basically two types of requests which are handled by a Flow application:

  • command line requests are passed to the flow.php script which resides in the Scripts folder of the Flow package

  • HTTP requests are first taken care of by the index.php script in the public Web directory.

Both scripts set certain environment variables and then instantiate and run the Neos\Flow\Core\Bootstrap class.

The bootstrap’s run() method initializes the bare minimum needed for any kind of operation. When it did that, it determines the actual request handler which takes over the control of the further boot sequence and handling the request.

public function run()

        $this->activeRequestHandler = $this->resolveRequestHandler();

The request handler in charge executes a sequence of steps which need to be taken for initializing Flow for the purpose defined by the specialized request handler. Flow’s Bootstrap class provides convenience methods for building such a sequence and the result can be customized by adding further or removing unnecessary steps.

After initialization, the request handler takes the necessary steps to handle the request, does or does not echo a response and finally exits the application. Control is not returned to the bootstrap again, but a request handler should call the bootstrap’s shutdown() method in order to cleanly shut down important parts of the framework.

Run Levels

There are two pre-defined levels to which Flow can be initialized:

  • compiletime brings Flow into a state which allows for code generation and other low-level tasks which can only be done while Flow is not yet fully ready for serving user requests. Compile time has only limited support for Dependency Injection and lacks support for many other functions such as Aspect-Oriented Programming and Security.

  • runtime brings Flow into a state which is fully capable of handling user requests and is optimized for speed. No changes to any of the code caches or configuration related to code is allowed during runtime.

The bootstrap’s methods buildCompiletimeSequence() and buildRuntimeSequence() conveniently build a sequence which brings Flow into either state on invocation.

Request Handlers

A request handler is in charge of executing the boot sequence and ultimately answering the request it was designed for. It must implement the \Neos\Flow\Core\RequestHandlerInterface interface which, among others, contains the following methods:

public function handleRequest();

public function canHandleRequest();

public function getPriority();

On trying to find a suitable request handler, the bootstrap asks each registered request handler if it can handle the current request using canHandleRequest() – and if it can, how eager it is to do so through getPriority(). It then passes control to the request handler which is most capable of responding to the request by calling handleRequest().

Request handlers must first be registered in order to be considered during the resolving phase. Registration is done in the Package class of the package containing the request handler:

class Package extends BasePackage

        public function boot(\Neos\Flow\Core\Bootstrap $bootstrap): void
                $bootstrap->registerRequestHandler(new \Acme\Foo\BarRequestHandler($bootstrap));



The Flow package contains meaningful working examples for registration of request handlers and building boot sequences. A good starting point is the \Neos\Flow\Package class where the request handlers are registered.