Added new recipe on upgrading a major version

cookbook/map.rst.inc

@@ -201,10 +201,11 @@
   * (email) :doc:`/cookbook/email/testing`
   * (form) :doc:`/cookbook/form/unit_testing`
 
-* :doc:`/cookbook/upgrading/index`
+* :doc:`/cookbook/upgrade/index`
 
-  * :doc:`/cookbook/upgrading/patch_version`
-  * :doc:`/cookbook/upgrading/minor_version`
+  * :doc:`/cookbook/upgrade/patch_version`
+  * :doc:`/cookbook/upgrade/minor_version`
+  * :doc:`/cookbook/upgrade/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 +1,112 @@
+.. index::
+    single: Upgrading; Major Version
+
+Upgrading a Major Version (e.g. 2.7.0 to 3.0.0)
+===============================================
+
+Every few 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 the 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, while 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
+:doc:`dev environment </cookbook/configuration/environments>` in your browser,
+these notices are shown in the web dev toolbar:
+
+.. image:: /images/cookbook/deprecations-in-profiler.png
+
+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 execute normally and a nice summary of the deprecation notices
+is displayed at the end of the test report:
+
+.. code-block:: text
+
+    $ phpunit
+    ...
+
+    OK (10 tests, 20 assertions)
+
+    Remaining deprecation notices (6)
+
+    The "request" service is deprecated and will be removed in 3.0. Add a typehint for
+    Symfony\Component\HttpFoundation\Request to your controller parameters to retrieve the
+    request instead: 6x
+        3x in PageAdminTest::testPageShow from Symfony\Cmf\SimpleCmsBundle\Tests\WebTest\Admin
+        2x in PageAdminTest::testPageList from Symfony\Cmf\SimpleCmsBundle\Tests\WebTest\Admin
+        1x in PageAdminTest::testPageEdit from Symfony\Cmf\SimpleCmsBundle\Tests\WebTest\Admin
+
+.. _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 chance 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.