Added new recipe on upgrading a major version

cookbook/map.rst.inc

@@ -205,6 +205,7 @@
 
   * :doc:`/cookbook/upgrading/patch_version`
   * :doc:`/cookbook/upgrading/minor_version`
+  * :doc:`/cookbook/upgrading/major_version`
 
 * :doc:`/cookbook/validation/index`
 

cookbook/upgrade/index.rst

@@ -15,3 +15,4 @@ There are three types of upgrades, all needing a little different preparation:
 
     /cookbook/upgrade/patch_version
     /cookbook/upgrade/minor_version
+    /cookbook/upgrade/major_version

cookbook/upgrade/major_version.rst

@@ -0,0 +1,95 @@
+.. index::
+    single: Upgrading; Major Version
+
+Upgrading a Major Version (e.g. 2.7.0 to 3.0.0)
+===============================================
+
+Once in a couple years, Symfony releases a new major version release (the
+first number changes). These releases are the trickiest to upgrade, as they are
+allowed to contain BC breaks. However, Symfony tries to make this upgrade
+process as smooth as possible.
+
+This means that you can update most of your code before the major release is
+actually released. This is called making your code *future compatible*.
+
+There are a couple of steps to upgrading a major version:
+
+#. :ref:`Make your code deprecation free <upgrade-major-symfony-deprecations>`;
+#. :ref:`Update to the new major version via Composer <upgrade-major-symfony-composer>`.
+#. :ref:`Update your code to work with the new version <upgrade-major-symfony-after>`
+
+.. _upgrade-major-symfony-deprecations:
+
+1) Make your Code Deprecation Free
+----------------------------------
+
+During a lifecycle of a major release, new features are added and method
+signatures and public API usages are changed. However, minor versions should
+not contain any backwards compatibility changes. It is made sure that there is
+a so-called *backwards compatibility layer* (or BC layer). This means that the
+old API will still work, hile the new feature is used internally. This BC layer
+is then marked as *deprecated*, indicating that it will be removed/changed in
+the future.
+
+The major version is the only time all existing BC layers are removed. The last
+minor version before a new major version (i.e. 2.7 is the last minor version of
+the 2 releases, 3.0 is the next version) will trigger deprecation notices when a
+BC layer is used.
+
+When visiting your application in the dev environment in your browser, these
+notices are shown in the web dev toolbar:
+
+.. image:: to be done
+
+Deprecations in PHPunit
+~~~~~~~~~~~~~~~~~~~~~~~
+
+By default, PHPunit will handle deprecation notices as real errors. This means
+that all tests are aborted because it uses a BC layer.
+
+To make sure this doesn't happen, you can install the PhpUnit bridge:
+
+.. code-block:: bash
+
+    $ composer require symfony/phpunit-bridge
+
+Now, your tests will execute normally and a nice summary of the deprecation
+notices is displayed at the end of the test report:
+
+.. _upgrade-major-symfony-composer:
+
+2) Update to the New Major Version via Composer
+-----------------------------------------------
+
+If your code is deprecation free, you can update the Symfony library via
+Composer by modifying your ``composer.json`` file:
+
+.. code-block:: json
+
+    {
+        "...": "...",
+
+        "require": {
+            "symfony/symfony": "3.0.*",
+        },
+        "...": "...",
+    }
+
+Next, use Composer to download new versions of the libraries:
+
+.. code-block:: bash
+
+    $ composer update symfony/symfony
+
+.. include:: /cookbook/upgrade/_update_all_packages.rst.inc
+
+.. _upgrade-major-symfony-after:
+
+3) Update your Code to Work with the New Version
+------------------------------------------------
+
+There is a high change that you're done now! However, the next major version
+*may* also contain new BC breaks as a BC layer is not always a possibility.
+Make sure you read the ``UPGRADE-X.0.md`` (where X is the new major version)
+included in the Symfony repository for any BC break that you need to be aware
+of.