Persistence refers to that part of the domain model, which we could call the data model.
The data model consists of domain classes whose instances are saved in the storage. The following steps need to be accomplished to make domain class instances persistent.
- Definition and creation of the database tables, if using a database storage.
- Implementation of the
PersistenceMapperclasses, which map the domain classes to these tables, or anything else that is used as storage.
- Implementation of the domain classes as subclasses of
- Configuration of the persistence layer.
- If you are using the generator to create the application from a model (see Model), all necessary code will be generated automatically.
Still the most common storage for web applications is a SQL database (e.g. MySQL). It is also wCMF's default storage. Although not strictly necessary it is recommended to use one database table for each persistent class, where each class property maps to one table column and each row stores one instance. The object identity is stored in the primary key column, which is named id by default.
- wCMF uses a table called
DBSequencefor retrieving the next id value used for insertion, since autoincrement columns are not supported on all database servers.
Primary keys are used to clearly identify an object in the database. They can consist of one (simple) or more (compound) columns. The default name for a primary key column is id. wCMF stores the primary key of an object together with it's type name in an
The following two relations types must be considered, when modeling the data tables:
- One-To-Many relations are realized by adding a foreign key column to the child table (many-side), which points to the parent table (one-side). This column is named fk _ + parent table name + _ id by default (e.g. fk_author_id).
- Many-To-Many relations between two domain classes, are established by defining a connection table. This table contains two primary keys, which point to the two connected tables.
wCMF uses persistence mapper classes to communicate between the application and the persistent storage (see Architecture). These classes implement the
PersistenceMapper interface, which defines methods for all persistence actions. By using this pattern wCMF does not make any assumptions about the actual storage, which can be flat files, a database or anything else a mapper is able to handle. This approach also makes it easy to connect an existing storage to a newly created wCMF application.
To simplify the implementation of mapper classes, wCMF already contains a class hierarchy for a common mapping approach, which maps each concrete class to one database table (Concrete Table Inheritance). In this hierarchy
RDBMapper handles the communication with the relational database, while
NodeUnifiedRDBMapper defines the actual mapping rules. Application developers simply need to implement one subclass of
NodeUnifiedRDBMapper for each persistent domain class, which declares the mapping of the attributes and relations of that class.
- All mapper classes, that are created by the generator are
Domain class instances hold the data that are used in the application and persisted in the storage.
Persistent domain classes either inherit from
PersistentObject, which is a container with value getter and setter methods, or from
Node, which adds methods for managing relations.
These two classes are completely generic, the actual identity of the domain class - that is the properties and the relations - are defined by the related
So if no additional domain logic is required in the domain class, it would be sufficient to use
Node as domain class. But in many cases you will want to execute custom code for Persistence Hooks or Validation and therefor create a custom subclass of
- All domain classes, that are created by the generator are
Persistence hooks are methods that are called at certain points of the lifecycle of an instance. The default implementation of these methods is empty - in fact their sole purpose is to be overwritten in subclasses on order to implement special functionality that should be executed at those points.
PersistentObject defines the following persistence hooks:
The following terms are important to know when working with wCMF's domain classes:
- Values are the persistent attributes of a domain class. You can think of them as class members. Values are accessed using the methods
- Properties apply to domain classes and domain class values. They describe static features like the
display_valueof the class or the
input_typeof a value. Properties are defined in the model as tags (see Chronos profile) and are accessed using the methods
- Tags are a special property used on values to group them by certain aspects. For example the edit forms in the default application only display domain class values that are tagged with
DATATYPE_ATTRIBUTEand only those attributes are editable in the translation form that are tagged with
TRANSLATABLE. Tags are defined in the model using the tag
Entry point of configuring the persistence layer is
PersistenceFacade (see Usage). The configuration mainly tells the facade which mapper classes are responsible for which domain classes. This assignment is defined in the
TypeMapping configuration section. The following example shows the appropriate entries for the
Author domain class:
PersistenceFacade is the main entry point to the persistence layer. It is used to create and retrieve
PersistentObject instances. The following sections show some basic examples for using the persistence layer.
In the example the
Author instance with id 1 is loaded.
When loading objects we generally distinguish between eager and lazy loading (see Lazy loading). By default
PersistenceFacade performs lazy loading by using virtual proxies (instances of
PersistentObjectProxy). That means that related objects are retrieved from the store, only when they are actually accessed. To perform eager loading, a
BuildDepth value may be passed in the method calls:
In this example the
Author instance with id 1 is loaded together with all related objects recursively.
Instead of a
BuildDepth value, an integer number may be passed indicating the depth of relations to load. The following image illustrates the build depth parameter for a simple model.
If using a build depth value of 1 the
Author instance Author A will be loaded together with it's related
Articles instances (Article A, Article B). A value of 2 will also load the
Chapter instances (Chapter A1, Chapter A2). The default value is
BuildDepth::SINGLE, which means that only the Author A instance is loaded. In the above illustration the value of 2 is equal to passing
When loading a list of objects, the default order of the
PersistenceMapper class is used for sorting. This default order is defined in the model (orderby tag of ChiNode, see ChiNode / ChiManyToMany). Besides this, all loading methods (e.g.
PersistenceFacade::loadObjects) accept an
orderby parameter for explicitly setting a different order.
To reduce the response time of your application when displaying large lists of objects, pagination may be used. This technique splits the list into smaller parts (pages) that are displayed one by one. In wCMF the class
PagingInfo implements the concept.
The following code shows how to load 25
Author instances starting from position 50:
To search objects in the store the
PersistenceFacade::loadObjects method may be used. It allows to set the conditions that loaded objects should match.
In this example all
Article instances with titles starting with A and release date 2014 or later are loaded.
More complex use cases are supported by the
ObjectQuery class. For example it allows to set search constraints on connected objects as shown in the following example:
In this example all
Author instances are loaded which have names starting with A or B and which have
Article instances connected that are created in the year 2014.
wCMF provides several iterator classes for traversing objects and values.
PersistentIterator allows to traverse object graphs as well, but it's state may be persisted to split the iteration of large lists into smaller parts.
Domain class instances are created by calling the
PersistenceFacade::create method or simply the class constructor, where the first approach is preferred over the second because it also sets the default values on attributes.
Before persisting objects, their content has to be validated according to the application requirements. Validation is handled in
PersistentObject::validateValue (for a single attribute) or
PersistentObject::validateValues (for the complete instance). These methods are called when values are changed via
PersistentObject::setValue or right before persisting the object in
AbstractMapper::save. Both validation methods throw a
ValidationException, if validation fails.
The actual validation is delegated to the
Validator class, which uses
ValidateType implementations for validating different types of attributes. Each validation type accepts configuration options, that are passed in as a string value. The format of this value is completely up to the validator type implementation. To define which validation should be applied to a domain class attribute, the tag
restrictions_match is used in the model (see ChiValue).
Currently three validation types are supported:
Image checks width and height of the referred image file e.g.
- The validation type is derived from the part of the configuration string, that precedes the first colon.
Custom validation types may be added in the configuration. For example after adding
myValidator (implemented in
it may be used in the following way:
The described validation types only operate on one attribute. If more complex validation is required, e.g. if dependencies between several attributes exist, the method
PersistentObject::validateValue may be overriden in the appropriate domain class.
When two or more users try to access the same object at the same time problems like lost updates might occur. These issues are avoided by concurrency control mechanisms, typically object locking.
wCMF provides two locking strategies:
- Pessimistic locking: An explicit lock is obtained by one user, that blocks write access to the object for other users until the lock is released.
- Optimistic locking: A copy of the initial object data is stored for each user and checked against later, when the user tries to store the object. The operation fails, if another user has updated the object data in the meantime.
For more detailed information see database locks.
As you can see the
LockHandler implementation is exchangeable.
wCMF supports database transactions. There is only one transaction at the same time and it may be obtained using the
PersistenceFacade::getTransaction method. The transaction has an active state, which is set in the
Transaction::begin method and reset on commit or rollback.
The following example shows, how to use transactions:
- Even if not obtained explicitely, there is always a transaction existing in the background, to which objects loaded from the store are attached. But to persist changes when doing a commit, the transaction has to be set active before loading or creating the object.
There are situations where you want to let the current transaction ignore changes made to newly created objects or objects loaded from the store. In these cases you can use the method
Transaction::detach to disconnect the object from the transaction.