Persistence Plugins

Data persistence plugins implement storage of server configuration, events and binary data. In most cases bundled persistence plugins solve even the most sophisticated data storage objectives. However, it's possible to implement a custom data storage plugin that will leverage any special type of database of data storage technology.

Persistence plugins follow Data Access Objects (DAO) development pattern.

Persistence plugin implements PersistencePlugin interface that has a single createDaoFactory(DaoFactory parent, ServerRuntimeConfig runtimeConfig, ServerConfig config) method. This method must return an instance of class implementing DaoFactory interface.

The DaoFactory has a set of methods that return specific implementations of DAO (storage facilities) supported by the plugin. The most important DAO implementations are:

• Property DAO implemented by PropertyDao interface. This DAO is responsible for persistent storage of server variable values.
• Event DAO implemented by EventDao interface. This DAO provides support for storing, loading and managing persistent server events.
• Data DAO implemented by DataDao interface. It provides support for storing and loading large binary data blocks.

DaoFactory's start() and stop() methods are called during server startup and shutdown stages respectively.

Property DAO implementation has methods for storing and loading values of server variables, as well as re-associating those values with different context paths and variable names. This re-association takes place if a context of a variable has been renamed or moved.

See Javadocs of PropertyDao interface for more information about each method.

Normally, a DataTable table should be first contexted to string (via encode() method) and then to a byte array that will be stored persistently.

Some variable values contain large nested binary data blocks (instances of Data class). Most implementations should be extracted Data blocks from DataTable representing variable value and save them separately via the Data DAO implementation available via parent DaoFactory. Property DAO implementations should also take care of removing data blocks referred from a persistent variable when its persistent value is being deleted.

Event DAO takes care of storing, loading and managing server events represented by Event class instances. Normally, every Event should be converted to a PersistentEvent that, in its turn, is converted to a byte array suitable for persistent storage.

See Javadocs of EventDao interface for more information about each method of event DAO.

Event DAO is heavily based on concept of Event Storages. An Event Storage a separate section of a storage facility dedicated for storing a certain set of events, such as a table in a relational database or a column family in a NoSQL database.

Event storages are represented by classes implementing EventStorage interface. For more information about event storages and their parameters see Event Storages section.

Event DAO should return an implementation of EventStorageManager interface which is responsible for managing event storages. Despite this interface has a lot of methods, its implementations should be normally inherited from AbstractEventStorageManager. There are only two abstract methods to implement:

Note that abstract implementation of EventStorageManager uses Property DAO to keep information about available event storages.

Data DAO takes care of saving, loading and deleting large binary data blocks represented by Data class instances. Every Data can be encoded as a an array of bytes (byte[]) that should be stored persistently.

Every persistently stored data block is assigned a unique long ID contained in the Data instance itself.

See Javadocs of DataDao interface for more information about each method.

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