`CubicWeb` concepts

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

A little history...

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

`CubicWeb` is a web application framework developped by Logilab_ since 2001.

Entirely written in Python, `CubicWeb` publishes data from all sorts

of sources such as SQL database, LDAP directory and versioning system such

as subversion.

`CubicWeb` user interface was designed to let the final user a huge flexibility

on how to select and how to display content. It allows to browse the knowledge

database and to display the results with the best rendering according to

the context.

This interface flexibility gives back the user the control of the 

rendering parameters that are usually reserved for developpers.

We can list a couple of web applications developped with `CubicWeb`, an online

public phone directory (see http://www.118000.fr/), a system for managing 

digital studies and simulations for a research lab, a tool for shared children

babysitting (see http://garde-partagee.atoukontact.fr/), a tool to manage

software developpment (see http://www.logilab.org), etc.

In 2008, `CubicWeb` was ported for a new type of source : the datastore 

from GoogleAppEngine_.

Global architecture

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

.. note::

  For real, the client and server sides are integrated in the same

  process and interact directly, without the needs for distants

  calls using Pyro. It is important to note down that those two

  sides, client/server, are disjointed and it is possible to execute

  a couple of calls in distincts processes to balance the load of

  your web site on one or more machines.

Terms and vocabulary

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

*schema*

  the schema defines the data model of an application based on entities

  and relations, thanks to the `yams`_ library. This is the core piece

  of an application. It is initially defined in the file system and is

  stored in the database at the time an instance is created. `CubicWeb`

  provides a certain number of system entities included automatically as

  it is necessarry for the core of `CubicWeb` and a library of

  cubes that can be explicitely included if necessarry.

*entity type*

  an entity is a set of attributes; the essential attribute of

  an entity is its key, named eid

*relation type*

  entities are linked to each others by relations. In `CubicWeb`

  relations are binary: by convention we name the first item of

  a relation the `subject` and the second the `object`.

*final entity type*

  final types corresponds to the basic types such as string of characters,

  integers... Those types have a main property which is that they can

  only be used as `object` of a relation. The attributes of an entity

  (non final) are entities (finals).

*final relation type*

  a relation is said final if its `object` is a final type. This is equivalent

  to an entity attribute.

*repository*

  this is the RQL server side of `CubicWeb`. Be carefull not to get

  confused with a Mercurial repository or a debian repository.

  a data source is a container of data (SGBD, LDAP directory...) integrated in the

  `CubicWeb` repository. This repository has at least one source, `system` which 

  contains the schema of the application, plain-text index and others

  vital informations for the system.

  it is possible to create differents configurations for an instance:

  - ``repository`` : repository only, accessible for clients using Pyro

  - ``twisted`` : web interface only, access the repository using Pyro

  - ``all-in-one`` : web interface and repository in a single process. 

     The repository could be or not accessible using Pyro.

  a cube is a model grouping one or multiple data types and/or views

  to provide a specific functionnality or a complete `CubicWeb` application

  potentially using other cubes. The available subes are located in the file

  system at `/path/to/forest/cubicweb/cubes`

  an instance is a specific installation of a cube. All the required 

  configuration files necessarry for the well being of your web application

  are grouped in an instance. This will refer to the cube(s) your application

  is based on.

  By example logilab.org and our intranet are two instances of a single

  cube jpl, developped internally.

  The instances are defined in the directory `~/etc/cubicweb.d`.

  the term application is sometime used to talk about an instance

  and sometimes to talk of a cube depending on the context. 

  So we would like to avoid using this term and try to use *cube* and

  *instance* instead.

  this object contains the results of an RQL query and information on the query.

  `Python Remote Object`_, distributed objects system similar to Java's RMI

  (Remote Method Invocation), which can be used for the dialog between the web

  side of the framework and the RQL repository.

`CubicWeb` engine

The web engine in `CubicWeb` is a set of classes managing a set of objects loaded

dynamically at the startup of `CubicWeb`. Those dynamics objects, based on the schema

or the library, are building the final web site. The differents dymanic components are

by example:

* client and server side

  - entities definition, containing the logic which enables application data manipulation

* client side

  - *views*, or more specifically

    - boxes

    - header and footer

    - forms

    - page templates

  - *actions*

  - *controllers*

* server side

  - notification hooks

  - notification views

The components of the engine are:

* a frontal web (only twisted is available so far), transparent for dynamic objects

* an object that encapsulates the configuration

* a `vregistry` (`cubicweb.cwvreg`) containing the dynamic objects loaded automatically

Details of a recording process

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

At startup, the `vregistry` or registers base, inspects a number of directories

looking for compatible classes definition. After a recording process, the objects

are assigned to registers so that they can be selected dynamically while the

application is running.

The base class of those objects is `AppRsetObject` (module `cubicweb.common.appobject`).

Inspired from the standard db-api, with a Connection object having the methods

cursor, rollback and commit essentially. The most important method is

the `execute` method of a cursor :

:rqlstring: the RQL query to execute (unicode)

:args: if the query contains substitutions, a dictionnary containing the values to use

   an implementation detail of the RQL queries cache implies that if a substitution

   is used to introduce an eid *susceptible to raise the ambiguities in the query

   type resolution*, then we have to specify the correponding key in the dictionnary

   through this argument

The `Connection` object owns the methods `commit` and `rollback`. You *should

never need to use them* during the development of the web interface based on

the `CubicWeb` framework as it determines the end of the transaction depending 

on the query execution success.

.. note::

  While executing updates queries (SET, INSERT, DELETE), if a query generates

  an error related to security, a rollback is automatically done on the current

  transaction.

The `Request` class (`cubicweb.web`)

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

A request instance is created when an HTPP request is sent to the web server.

It contains informations such as forms parameters, user authenticated, etc.

**Globally, a request represents a user query, either through HTTP or not

(we also talk about RQL queries on the server side by example)**

An instance of `Request` has the following attributes:

* `user`, instance of `cubicweb.common.utils.User` corresponding to the authenticated

  user

* `form`, dictionnary containing the values of a web form

* `encoding`, characters encoding to use in the response

But also:

:Session data handling:

  * `session_data()`, returns a dictinnary containing all the session data

  * `get_session_data(key, default=None)`, returns a value associated to the given

    key or the value `default` if the key is not defined

  * `set_session_data(key, value)`, assign a value to a key

  * `del_session_data(key)`,  suppress the value associated to a key

:Cookies handling:

  * `get_cookie()`, returns a dictionnary containing the value of the header

  * `set_cookie(cookie, key, maxage=300)`, adds a header HTTP `Set-Cookie`,

    with a minimal 5 minutes length of duration by default (`maxage` = None

    returns a *session* cookie which will expire when the user closes the browser

    window

  * `remove_cookie(cookie, key)`, forces a value to expire

:URL handling:

  * `url()`, returns the full URL of the HTTP request

  * `base_url()`, returns the root URL of the web application

  * `relative_path()`, returns the relative path of the request

:And more...:

  * `set_content_type(content_type, filename=None)`, adds the header HTTP

  * `get_header(header)`, returns the value associated to an arbitrary header

    of the HTTP request

  * `set_header(header, value)`, adds an arbitrary header in the response

  * `cursor()` returns a RQL cursor on the session

  * `execute(*args, **kwargs)`, shortcut to ``.cursor().execute()``

  * `property_value(key)`, properties management (`EProperty`)

  * dictionnary `data` to store data to share informations between components

    *while a request is executed*

Please note down that this class is abstract and that a concrete implementation

will be provided by the *frontend* web used (in particular *twisted* as of

today). For the views or others that are executed on the server side,

most of the interface of `Request` is defined in the session associated

to the client.

The `AppObject` class

In general:

* we do not inherit directly from this class but from a more specific

  class such as `AnyEntity`, `EntityView`, `AnyRsetView`,

* to be recordable, a subclass has to define its own register (attribute

  `__registry__`) and its identifier (attribute `id`). Usually we do not have

  to take care of the register, only the identifier `id`.

We can find a certain number of attributes and methods defined in this class 

and so common to all the application objects:

At the recording, the following attributes are dynamically added to

the *subclasses*:

* `vreg`, the `vregistry` of the application

* `schema`, the application schema

* `config`, the application configuration

We also find on instances, the following attributes:

* `req`, `Request` instance

* `rset`, the *result set* associated to the object if necessarry

* `cursor`, rql cursor on the session

:URL handling:

  * `build_url(method=None, **kwargs)`, returns an absolute URL based on

    the given arguments. The *controller* supposed to handle the response

    can be specified through the special parameter `method` (the connection

    is theoretically done automatically :).

  * `datadir_url()`, returns the directory of the application data

    (contains static files such as images, css, js...)

  * `base_url()`, shortcut to `req.base_url()`

  * `url_quote(value)`, version *unicode safe* of the function `urllib.quote`

:Data manipulation:

  * `etype_rset(etype, size=1)`, shortcut to `vreg.etype_rset()`

  * `eid_rset(eid, rql=None, descr=True)`, returns a *result set* object for

    the given eid

  * `entity(row, col=0)`, returns the entity corresponding to the data position

    in the *result set* associated to the object

  * `complete_entity(row, col=0, skip_bytes=True)`, is equivalent to `entity` but

    also call the method `complete()` on the entity before returning it

:Data formatting:

  * `format_time(time)`

:And more...:

  * `external_resource(rid, default=_MARKER)`, access to a value defined in the

    configuration file `external_resource`

.. note::

  When we inherit from `AppObject` (even not directly), you *always* have to use

  **super()** to get the methods and attributes of the superclasses, and not

  use the class identifier.

  By example, instead of writting: ::

  You'd better write: ::

Standard structure for a cube

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

A complex cube is structured as follows:

  mycube/

We can use simple Python module instead of packages, by example: 

  mycube/

where :

* ``schema`` contains the schema definition (server side only)

* ``entities`` contains the entities definition (server side and web interface)

* ``sobjects`` contains hooks and/or views notifications (server side only)

* ``views`` contains the different components of the web interface (web interface only)

* ``test`` contains tests specifics to the application (not installed)

* ``i18n`` contains the messages catalog for supported languages (server side and 

  web interface) 

* ``data`` contains arbitrary data files served statically

  (images, css, javascripts files)... (web interface only)

* ``migration`` contains the initialization file for new instances

  (``postcreate.py``) and in general a file containing the `CubicWeb` dependancies 

  of the cube depending on its version (``depends.map``)

* ``debian`` contains all the files that manages the debian packaging

  (you would find there the classical structure with ``control``, ``rules``, 

  ``changelog``... (not installed)

* the file ``__pkginfo__.py`` provides meta-data on the cube, especially the 

  distribution name and the current version (server side and web interface) or

  also the sub-cubes used by this cube

The only required files are:

* the file ``__pkginfo__.py``

* the schema definition