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2. 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.

2.1. Cubes

A cube is a software component made of three parts: its data model (schema), its logic (entities) and its user interface (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 cubicweb-ctl list displays the list of available cubes.

2.2. Instances

An instance is a runnable application installed on a computer and based on a cube.

The instance directory contains the configuration files. Several instances can be created and based on the same cube. For exemple, several software forges can be set up on one computer system based on the cubicweb-forge cube.

Instances can be of three different types: all-in-one, web engine or data repository. For applications that support high traffic, several web (front-end) and data (back-end) instances can be set-up to share the load.

../_images/archi_globale_en.png

The 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.

2.3. 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 (RQL syntax). 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 Hooks and operations below.

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

2.4. 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.

It is common to run the web engine and the repository in the same process (see instances of type all-in-one above), but this is not a requirement. A repository can be set up to be accessed remotely using Pyro (Python Remote Objects) and act as a standalone server, which can be directly accessed or also through a standalone web engine.

2.5. 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.

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.

2.6. Registries and application objects

2.6.1. 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 cubicweb.appobject.AppObject.

2.6.2. 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.

2.6.3. 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.

2.7. 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 Introduction).

This query language is inspired by SQL but is on a higher level in order to emphasize browsing relations.

2.7.1. 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.

2.8. 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.

2.9. 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 all (think about LDAP or a Subversion repository)
  • 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.