Event sources

Any source of index-worthy data, such as a discussion forum, has internal activities like the addition of a discussion post. For independent indexing to stay synchronized, the indexing engine needs to be informed of such events, either when they occur (at runtime) or via some regular polling for all changes occurring within a certain polling period. For runtime events, there are several ways to get notifications from a typical discussion forum like Drupal:

Events generated within the target application

A discussion forum like Tomoye-Ecco or Drupal may have an internal event system that can be manipulated to send messages into an indexing system. Such an event system typically provides a means to register for events, and thereby receive callbacks when such events take place. The callbacks are usually offered only in the native language of the forum application (e.g., PHP for Drupal), but the callback routine may be able to send email or messages to an external system. For example, the Drupal workflow module promises to act on events to produce certain behaviors, like setting a post's editorial status based on the actions of users of various roles. See more discussion of Drupal hooks.

Intermediate Database Drivers

An intermediate Database Driver provides events by inserting an additional layer between the application and the database. The system administrator of an event source changes the driver from, say, the oracle-provided driver to an intermediate driver that can filter and distribute the database load. (The intermediate driver connects to the actual database(s) by using the native driver(s)). Such intermediate drivers can, in turn, be modified to send out events. This method for getting events has the advantage of needing little information about the internals of a given event source, but does require a good deal of analysis of the database schema of the event source, perhaps including the parsing of SQL. The two intermediate drivers in the links above are both JDBC drivers, suitable for event sources written in java.

Database triggers

Most enterprise databases support triggers--events that are generated within the database engine. Since triggers are entirely outside of the context of the application, the event source requires no modification or even awareness of the events caused by triggers. Triggers for latest (v.6.0) MySQL do not permit communication with other processes, but there are projects that provide this ability. This method for getting events has pros/cons similar to the above method that uses an intermediate database driver. Triggers have an additional benefits: triggers react to any data change, like changes from command-line SQL or management consoles external to the primary event source.

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