eZ Publish API Repository
This entity is the entry point to everything you will do with the Public API.
It will allow you to create, retrieve, update and delete all the eZ Publish objects, as well as content types, sections, content states. It is always obtained through the service container.
By itself, the repository doesn't do much. It allows three type of operations: user authentication (getting / changing the current user), issuing transactions, and obtaining services.
Inline objects documentation
Pay attention to the inline phpdoc block in this code stub. It tells your IDE that $repository is an instance of \eZ\Publish\API\Repository\Repository. If your IDE supports this feature, you will get code completion on the $repository object. This helper is a huge time saver when it comes to learning about the eZ Publish API.
The service container
The above code snippet implies that the service container is available in the context you are writing your code in.
In controllers, this generally is done by extending the Symfony Controller class. It comes with a get() method that calls the service container. In command line scripts, it requires that you extend the ContainerAwareCommand base class instead of Controller. This class provides you with a getContainer() method that returns the service container.
Getting the repository from eZ Publish 5 controllers
In order to make it even easier to obtain the repository from controllers code, eZ Publish 5 controllers extend a custom Controller class that provides a getRepository() method which directly returns the repository from the service container.
You can and should of course do the same in your custom controllers.
Authentication
One of the responsibilities of the Repository is user authentication. Every action will be executed as a user. In the context of a normal eZ Publish execution, the logged in user will of course be the current one, identified via one of the available authentication methods. This user's permissions will affect the behavior of the Repository. The user may f.e. not be allowed to create Content, or view a particular section.
Logging in to the Repository is covered in the recipes of the Cookbook.
Services
The main entry point to the repository's features are services. The Public API breaks down access to Content, User, Content Types, etc features into various services. Those services are obtained via the Repository, using getServiceName() like methods: getContentService(), getUserService()...
Throughout the Cookbook, you will be guided through the various capabilities those services have, and how you can use them to implement your projects.
Value objects
While Services provide interaction with the repository, the elements (Content, Users) they provide interaction with are provided as read only Value Objects, in the eZ\Publish\Core\Repository\Values namespace. Those objects are broken down into sub-namespaces: Content, ContentType, User and ObjectState, each sub-namespace containing a set of value objects, such as Content\Content or User\Role.
These objects are read-only by design. They are only meant to be used in order to fetch data from the repository. They come with their own properties, such as $content->id, $location->hidden, but also with methods that provide access to more, related information, such as Relation::getSourceContentInfo() or Role::getPolicies(). By design, a value object will only give you access to data that is very closely related to it. More complex retrieval operations will require you to use the appropriate Service, using information from your Value Object.
Value info objects
Some complex Value Objects have an Info counterpart, like ContentInfo, the counterpart for Content. These objects are specific, and provide you with lower level information. For instance, ContentInfo will provide you with currentVersionNo or remoteId, while Content will let you retrieve Fields, the ContentType, or previous Versions.
They are provided by the API, but are read only, can't be modified and sent back. Creation and modification of Repository values are done using Create structs and Update structs.
Create and update structs
In order to update or create elements in the Repository, you will use structs. They are usually provided by the Service that manages the Value Objects you want to alter or create. For instance, the Content service has a getContentCreateStruct() method that returns a new ContentCreateStruct object. Equivalent methods exist for UpdateStruct objects as well, and for most Value Objects.
Using them is also covered in the Cookbook.