When you're in debug mode (the second argument of ``Kernel`` constructor in the

front controller is ``true``), Symfony automatically adds an ``X-Symfony-Cache``

header to the response. Use this to get information about cache hits and misses.

header to the response. You can also use the ``trace_level`` config

option and set it to either ``none``, ``short`` or ``full`` to

add this information.

``short`` will add the information for the master request only.

It's written in a concise way that makes it easy to record the

information in your server log files. For example, in Apache you can

use ``%{X-Symfony-Cache}o`` in ``LogFormat`` format statements.

This information can be used to extract general information about

cache efficiency of your routes.

..tip::

You can change the name of the header used for the trace

information using the ``trace_header`` config option.

..versionadded::4.3

The ``trace_level`` and ``trace_header`` configuration options

were introduced in Symfony 4.3.

.. _http-cache-symfony-versus-varnish: