8.13. Searching the Working Memory using Query

8.13.1. Queries

Queries are used to retrieve fact sets based on patterns, as they are used in rules. Patterns may make use of optional parameters. Queries can be defined in the Knowledge Base, from where they are called up to return the matching results. While iterating over the result collection, any identifier bound in the query can be used to access the corresponding fact or fact field by calling the get method with the binding variable's name as its argument. If the binding refers to a fact object, its FactHandle can be retrieved by calling getFactHandle, again with the variable's name as the parameter. Illustrated below is a Query example: 
QueryResults results =
    ksession.getQueryResults( "my query", new Object[] { "string" } );
for ( QueryResultsRow row : results ) {
    System.out.println( row.get( "varName" ) );
}

8.13.2. Live Queries

Invoking queries and processing the results by iterating over the returned set is not a good way to monitor changes over time.
To alleviate this, JBoss BRMS provides Live Queries, which have a listener attached instead of returning an iterable result set. These live queries stay open by creating a view and publishing change events for the contents of this view. To activate, start your query with parameters and listen to changes in the resulting view. The dispose method terminates the query and discontinues this reactive scenario.

8.13.3. ViewChangedEventListener Implementation Example

final List updated = new ArrayList();
final List removed = new ArrayList();
final List added = new ArrayList();
 
ViewChangedEventListener listener = new ViewChangedEventListener() {           
 public void rowUpdated(Row row) {
  updated.add( row.get( "$price" ) );
 }
  
 public void rowRemoved(Row row) {
  removed.add( row.get( "$price" ) );
 }
  
 public void rowAdded(Row row) {
  added.add( row.get( "$price" ) );
 }
};       
 
// Open the LiveQuery
LiveQuery query = ksession.openLiveQuery( "cars",
                                          new Object[] { "sedan", "hatchback" },
                                          listener );
...
...
query.dispose() // calling dispose to terminate the live query

Note

For an example of Glazed Lists integration for live queries, visit http://blog.athico.com/2010/07/glazed-lists-examples-for-drools-live.html