Drools Expert User Guide ....................................................................................................................................... vii 1. The Rule Engine .......................................................................................................... 1 1.1. What is a Rule Engine? ...................................................................................... 1 1.1.1. Introduction and Background .................................................................... 1 1.2. Why use a Rule Engine? .................................................................................... 6 1.2.1. Advantages of a Rule Engine ................................................................... 6 1.2.2. When should you use a Rule Engine? ....................................................... 7 1.2.3. When not to use a Rule Engine ................................................................ 8 1.2.4. Scripting or Process Engines .................................................................... 8 1.2.5. Strong and Loose Coupling ...................................................................... 9 2. Quick Start ................................................................................................................. 11 2.1. The Basics ....................................................................................................... 11 2.1.1. Stateless Knowledge Session ................................................................. 11 2.1.2. Stateful Knowledge Session .................................................................... 14 2.2. A Little Theory .................................................................................................. 19 2.2.1. Methods versus Rules ............................................................................ 19 2.2.2. Cross Products ...................................................................................... 20 2.2.3. Activations, Agenda and Conflict Sets. .................................................... 21 2.2.4. Inference ............................................................................................... 26 2.2.5. Inference and TruthMaintenance ............................................................. 28 2.3. More on building and deploying ......................................................................... 30 2.3.1. Knowledge Base by Configuration Using Changesets ............................... 30 2.3.2. Knowledge Agent ................................................................................... 31 3. Advanced Concepts and Theory ............................................................................... 35 3.1. Truth Maintenance with Logical Objects ............................................................. 35 3.1.1. Example Scenario .................................................................................. 37 3.1.2. Lazy Truth Maintenance ......................................................................... 38 3.1.3. Important note: Equality for Java objects ................................................. 38 3.2. Rete Algorithm .................................................................................................. 38 4. User Guide ................................................................................................................. 45 4.1. Building ............................................................................................................ 45 4.1.1. Building using Code ............................................................................... 46 4.1.2. Building using Configuration and the ChangeSet XML .............................. 49 4.2. Deploying ......................................................................................................... 52 4.2.1. KnowledgePackage and Knowledge Definitions ........................................ 52 4.2.2. KnowledgeBase ..................................................................................... 53 4.2.3. In-Process Building and Deployment ....................................................... 55 4.2.4. Building and Deployment in Separate Processes ...................................... 56 4.2.5. StatefulknowledgeSessions and KnowledgeBase Modifications ................. 57 4.2.6. KnowledgeAgent .................................................................................... 57 4.3. Running ............................................................................................................ 61 4.3.1. KnowledgeBase ..................................................................................... 61 4.3.2. StatefulKnowledgeSession ...................................................................... 62 4.3.3. KnowledgeRuntime ................................................................................. 62 iii Drools Expert User Guide 4.3.4. Agenda .................................................................................................. 71 4.3.5. Event Model .......................................................................................... 76 4.3.6. KnowledgeRuntimeLogger ...................................................................... 78 4.3.7. StatelessKnowledgeSession .................................................................... 79 4.3.8. Commands and the CommandExecutor ................................................... 83 4.3.9. Marshalling ............................................................................................ 91 4.3.10. Persistence and Transactions ............................................................... 93 4.3.11. Drools Clips ......................................................................................... 95 5. The Rule Language .................................................................................................... 97 5.1. Overview .......................................................................................................... 97 5.1.1. A rule file ............................................................................................... 97 5.1.2. What makes a rule ................................................................................. 97 5.2. Keywords ......................................................................................................... 98 5.3. Comments ...................................................................................................... 100 5.3.1. Single line comment ............................................................................. 100 5.3.2. Multi-line comment ............................................................................... 101 5.4. Error Messages .............................................................................................. 101 5.4.1. Message format ................................................................................... 101 5.4.2. Error Messages Description .................................................................. 102 5.4.3. Other Messages ................................................................................... 106 5.5. Package ......................................................................................................... 106 5.5.1. import .................................................................................................. 107 5.5.2. global .................................................................................................. 108 5.6. Function ......................................................................................................... 109 5.7. Type Declaration ............................................................................................. 110 5.7.1. Declaring New Types ........................................................................... 111 5.7.2. Declaring Metadata .............................................................................. 113 5.7.3. Declaring Metadata for Existing Types ................................................... 115 5.7.4. Parameterized constructors for declared types ....................................... 115 5.7.5. Non Typesafe Classes .......................................................................... 116 5.7.6. Accessing Declared Types from the Application Code ............................. 116 5.7.7. Type Declaration 'extends' .................................................................... 117 5.8. Rule ............................................................................................................... 118 5.8.1. Rule Attributes ..................................................................................... 119 5.8.2. Timers and Calendars .......................................................................... 123 5.8.3. Left Hand Side (when) syntax ............................................................... 124 5.8.4. The Right Hand Side (then) .................................................................. 171 5.8.5. A Note on Auto-boxing and Primitive Types ........................................... 174 5.9. Query ............................................................................................................. 174 5.10. Domain Specific Languages ........................................................................... 177 5.10.1. When to Use a DSL ........................................................................... 177 5.10.2. DSL Basics ........................................................................................ 178 5.10.3.