History
The History Event Stream provides audit information about executed process instances.
The process engine maintains the state of running process instances inside the database. This includes writing (1.) the state of a process instance to the database as it reaches a wait state and reading (2.) the state as process execution continues. We call this database the runtime database. In addition to maintaining the runtime state, the process engine creates an audit log providing audit information about executed process instances. We call this event stream the history event stream (3.). The individual events which make up this event stream are called History Events and contain data about executed process instances, activity instances, changed process variables and so forth. In the default configuration, the process engine will simply write (4.) this event stream to the history database. The HistoryService API allows querying this database (5.). The history database and the history service are optional components; if the history event stream is not logged to the history database or if the user chooses to log events to a different database, the process engine is still able to work and it is still able to populate the history event stream. This is possible because the BPMN 2.0 Core Engine component does not read state from the history database. It is also possible to configure the amount of data logged, using the historyLevel setting in the process engine configuration.
Since the process engine does not rely on the presence of the history database for generating the history event stream, it is possible to provide different backends for storing the history event stream. The default backend is the DbHistoryEventHandler which logs the event stream to the history database. It is possible to exchange the backend and provide a custom storage mechanism for the history event log.