3.3. Available Node Types

This is the start of the rule flow. (A rule flow must have only one start node. It cannot have incoming connections and must have one outgoing connection.) Whenever a rule flow process is started, execution will commence at this node and automatically continue to the first node linked from it, and so on.

The Start Event node possesses the following properties:

This is the end of the rule flow. A rule flow must have at least one end node. The End node must have one incoming connection and cannot have any outgoing connections.

This node possesses the following properties:

Use this node to represent a set of rules you wish to have evaluated. A RuleFlowGroup node should have one incoming connection and one outgoing connection.

To make rules part of a specific rule flow group, use the ruleflow-group header attribute. When a RuleFlowGroup node is reached, the engine will start executing any rules that are part of the corresponding ruleflow-group. Execution will automatically continue to the next node once there are no more active rules in that group.

This means that you can add new activations (belonging to the currently active rule flow group) to the Agenda even if the facts have been modified by other rules.

This contains the following properties:

Use this to create branches in your rule flow. A Split node must have one incoming connection and two or more outgoing connections.

There are three types of Split node: This node contains the following properties:

Use this to synchronize multiple branches. A join node must have two or more incoming connections and one outgoing connection. Four types of split are available to you:

This node contains the following properties:

this node represents a wait state. A state must have one incoming connection and one or more outgoing connections.

For each of the outgoing connections, you can specify a rule constraint to define how long the process should wait before continuing. For example, a constraint in an order entry application might specify that the process should wait until no more errors are found in the given order.

To specify a constraint, use the same syntax as you would for the left-hand side of a rule.

When it reaches this node, the engine will check the associated constraints. If one of the constraint evaluates to true directly, the flow will continue immediately. Otherwise, the flow will continue if one of the constraints is satisfied later on, for example when a fact is inserted, updated or removed from the working memory.

A state contains the following properties:

This represents the invocation of another process from within the parent process. A sub-process node must have one incoming connection and one outgoing connection.

When a SubFlow node is reached, the engine will start the process with the given ID.

This node contains the following properties:

This node represents an action that should be executed in this rule flow. An action node should have one incoming connection and one outgoing connection. The associated action specifies what should be executed, the dialect used for coding the action (such as Java or MVEL), and the actual action code.

This code can access any global, the predefined variable called drools referring to a KnowledgeHelper object (which can, for example, be used to retrieve the Working Memory by calling drools.getWorkingMemory()), and the variable kcontext that references the ProcessContext object. (This latter object can, for example, be used to access the current ProcessInstance or NodeInstance, and to obtain and set variables).

When the rule flow reaches an Action node, it will execute the action and then continue to the next node.

The Action node possesses the following properties:

This node represents a timer that can trigger one or multiple times after a given period. A Timer node must have one incoming connection and one outgoing connection.

The timer delay specifies how long (in milliseconds) the timer should wait before triggering the first time. The timer period specifies the time between two subsequent triggers. A period of 0 means that the timer should only be triggered once. When the rule flow reaches a Timer node, it starts the associated timer.

The timer is cancelled if the timer node is cancelled (by, for instance, completing or aborting the process).

The Timer node contains the following properties:

Use a Fault node to signal an exceptional condition in the process. It must have one incoming connection and no outgoing connections.

When the rule flow reaches a fault node, it will throw a fault with the given name. The process will search for an appropriate exception handler that is capable of handling this kind of fault. If no fault handler is found, the process instance is aborted.

A Fault node contains the following properties:

Use this Event node to respond to internal or external events during the execution of the process. An Event node must have no incoming connections and one outgoing connection. It specifies the type of event that is expected. Whenever that type of event is detected, the node connected to this Event node is triggered.

It contains the following properties:

A Composite node is a node that can contain other nodes so that it acts as a node container. This allows not only the embedding of a part of the flow within such a Composite node, but also the definition of additional variables and exception handlers that are accessible for all nodes inside this container. A Composite node should have one incoming connection and one outgoing connection. It contains the following properties:

A ForEach node is a special composite that allows you to execute the flow contained therein multiple times, once for each element in a collection. A ForEach node must have one incoming connection and one outgoing connection.

A ForEach node awaits the completion of the embedded flow for each of the collection''s elements before continuing.

This node contains the following properties: