.. -*- coding: utf-8 -*-

.. _Concepts:

The Core Concepts of |cubicweb|

===============================

This section defines some terms and core concepts of the |cubicweb| framework. To

avoid confusion while reading this book, take time to go through the following

definitions and use this section as a reference during your reading.

.. _Cube:

Cubes

-----

A cube is a software component made of three parts:

- its data model (:mod:`schema`),

- its logic (:mod:`entities`) and

- its user interface (:mod:`views`).

A cube can use other cubes as building blocks and assemble them to provide a

whole with richer functionnalities than its parts. The cubes `cubicweb-blog`_ and

`cubicweb-comment`_ could be used to make a cube named *myblog* with commentable

blog entries.

The `CubicWeb.org Forge`_ offers a large number of cubes developed by the community

and available under a free software license.

.. note::

   The command :command:`cubicweb-ctl list` displays the list of available cubes.

.. _`CubicWeb.org Forge`: http://www.cubicweb.org/project/

.. _`cubicweb-blog`: http://www.cubicweb.org/project/cubicweb-blog

.. _`cubicweb-comment`: http://www.cubicweb.org/project/cubicweb-comment

.. _Instance:

Instances

---------

An instance is a runnable application installed on a computer and

based on one or more cubes.

The instance directory contains the configuration files. Several

instances can be created and based on the same cube. For example,

several software forges can be set up on one computer system based on

the `cubicweb-forge`_ cube.

.. _`cubicweb-forge`: http://www.cubicweb.org/project/cubicweb-forge

The command :command:`cubicweb-ctl list` also displays the list of instances

installed on your system.

.. note::

  The term application is used to refer to "something that should do something as

  a whole", eg more like a project and so can refer to an instance or to a cube,

  depending on the context. This book will try to use *application*, *cube* and

  *instance* as appropriate.

.. _RepositoryIntro:

Data Repository

---------------

The data repository [1]_ encapsulates and groups an access to one or

more data sources (including SQL databases, LDAP repositories, other

|cubicweb| instance repositories, filesystems, Google AppEngine's

DataStore, etc).

All interactions with the repository are done using the `Relation Query Language`

(:ref:`RQL`). The repository federates the data sources and hides them from the

querier, which does not realize when a query spans several data sources

and requires running sub-queries and merges to complete.

Application logic can be mapped to data events happenning within the

repository, like creation of entities, deletion of relations,

etc. This is used for example to send email notifications when the

state of an object changes. See :ref:`HookIntro` below.

.. [1] not to be confused with a Mercurial repository or a Debian repository.

.. _`Python Remote Objects`: http://pythonhosted.org/Pyro4/

.. _WebEngineIntro:

Web Engine

----------

The web engine replies to http requests and runs the user interface.

By default the web engine provides a `CRUD`_ user interface based on

the data model of the instance. Entities can be created, displayed,

updated and deleted. As the default user interface is not very fancy,

it is usually necessary to develop your own.

.. _`CRUD`: http://en.wikipedia.org/wiki/Create,_read,_update_and_delete

.. _SchemaIntro:

Schema (Data Model)

-------------------

The data model of a cube is described as an entity-relationship schema using a

comprehensive language made of Python classes imported from the yams_ library.

.. _yams: http://www.logilab.org/project/yams/

An `entity type` defines a sequence of attributes. Attributes may be

of the following types: `String`, `Int`, `Float`, `Boolean`, `Date`,

`Time`, `Datetime`, `Interval`, `Password`, `Bytes`, `RichString`.

A `relation type` is used to define an oriented binary relation

between entity types.  The left-hand part of a relation is named the

`subject` and the right-hand part is named the `object`.

A `relation definition` is a triple (*subject entity type*, *relation type*, *object

entity type*) associated with a set of properties such as cardinality,

constraints, etc.

Permissions can be set on entity types or relation definition to control who

will be able to create, read, update or delete entities and relations. Permissions

are granted to groups (to which users may belong) or using rql expressions (if the

rql expression returns some results, the permission is granted).

Some meta-data necessary to the system are added to the data model. That includes

entities like users and groups, the entities used to store the data model

itself and attributes like unique identifier, creation date, creator, etc.

When you create a new |cubicweb| instance, the schema is stored in the database.

When the cubes the instance is based on evolve, they may change their data model

and provide migration scripts that will be executed when the administrator will

run the upgrade process for the instance.

.. _VRegistryIntro:

Registries and application objects

----------------------------------

Application objects

~~~~~~~~~~~~~~~~~~~

Besides a few core functionalities, almost every feature of the framework is

achieved by dynamic objects (`application objects` or `appobjects`) stored in a

two-levels registry. Each object is affected to a registry with

an identifier in this registry. You may have more than one object sharing an

identifier in the same registry:

  object's `__registry__` : object's `__regid__` : [list of app objects]

In other words, the `registry` contains several (sub-)registries which hold a

list of appobjects associated to an identifier.

The base class of appobjects is :class:`cubicweb.appobject.AppObject`.

Selectors

~~~~~~~~~

At runtime, appobjects can be selected in a registry according to some

contextual information. Selection is done by comparing the *score*

returned by each appobject's *selector*.

The better the object fits the context, the higher the score. Scores

are the glue that ties appobjects to the data model. Using them

appropriately is an essential part of the construction of well behaved

cubes.

|cubicweb| provides a set of basic selectors that may be parametrized.  Also,

selectors can be combined with the `~` unary operator (negation) and the binary

operators `&` and `|` (respectivly 'and' and 'or') to build more complex

selectors. Of course complex selectors may be combined too. Last but not least, you

can write your own selectors.

The `registry`

~~~~~~~~~~~~~~~

At startup, the `registry` inspects a number of directories looking

for compatible class definitions. After a recording process, the

objects are assigned to registries and become available through the

selection process.

In a cube, application object classes are looked in the following modules or

packages:

- `entities`

- `views`

- `hooks`

- `sobjects`

There are three common ways to look up some application object from a

registry:

* get the most appropriate object by specifying an identifier and

  context objects. The object with the greatest score is

  selected. There should always be a single appobject with a greater

  score than others for a particular context.

* get all objects applying to a context by specifying a registry. A

  list of objects will be returned containing the object with the

  highest score (> 0) for each identifier in that registry.

* get the object within a particular registry/identifier. No selection

  process is involved: the registry will expect to find a single

  object in that cell.

.. _RQLIntro:

The RQL query language

----------------------

No need for a complicated ORM when you have a powerful data

manipulation language.

All the persistent data in a |cubicweb| instance is retrieved and

modified using RQL (see :ref:`rql_intro`).

This query language is inspired by SQL but is on a higher level in order to

emphasize browsing relations.

Result set

~~~~~~~~~~

Every request made (using RQL) to the data repository returns an object we call a

Result Set. It enables easy use of the retrieved data, providing a translation

layer between the backend's native datatypes and |cubicweb| schema's EntityTypes.

Result sets provide access to the raw data, yielding either basic Python data

types, or schema-defined high-level entities, in a straightforward way.

.. _ViewIntro:

Views

-----

**CubicWeb is data driven**

The view system is loosely coupled to data through the selection system explained

above. Views are application objects with a dedicated interface to 'render'

something, eg producing some html, text, xml, pdf, or whatsover that can be

displayed to a user.

Views actually are partitioned into different kind of objects such as

`templates`, `boxes`, `components` and proper `views`, which are more

high-level abstraction useful to build the user interface in an object

oriented way.

.. _HookIntro:

Hooks and operations

--------------------

**CubicWeb provides an extensible data repository**

The data model defined using Yams types allows to express the data

model in a comfortable way. However several aspects of the data model

can not be expressed there. For instance:

* managing computed attributes

* enforcing complicated business rules

* real-world side-effects linked to data events (email notification

  being a prime example)

The hook system is much like the triggers of an SQL database engine,

except that:

* it is not limited to one specific SQL backend (every one of them

  having an idiomatic way to encode triggers), nor to SQL backends at

* it is well-coupled to the rest of the framework

Hooks are also application objects (in the `hooks` registry) and

selected on events such as after/before add/update/delete on

entities/relations, server startup or shutdown, etc.

`Operations` may be instantiated by hooks to do further processing at different

steps of the transaction's commit / rollback, which usually can not be done

safely at the hook execution time.

Hooks and operation are an essential building block of any moderately complicated

cubicweb application.

.. note::

   RQL queries executed in hooks and operations are *unsafe* by default, i.e. the

   read and write security is deactivated unless explicitly asked.