Policy Model Basic Principles

A policy model is made up of nodes and connections that define the policy logic. In order to build a functioning policy model, you must understand several basic principles.

Details

The following principles apply to working with a policy model:

Policy models do not reference specific records. Rather, they can contain nodes that reference families. You must use policy instancesIdentifies the records to which logic in the policy model will be applied. to identify the individual records whose values are evaluated when the policy is executed.
The first node in a policy model must be an Input node other than a Query node.
Most nodes have outputs that successor nodes can use as inputs. You must specify inputs for each successor node using the Properties window that appears when you select the node in the policy model.

Note: Outputs and inputs may represent either a single value or a collectionA set of results from a Query node or a set of readings associated with a Measurement Location, Health Indicator, or OPC Tag node. of values. The types of outputs that each node generates and the types of inputs that each node accepts is different for each node. When building a policy model, you must use corresponding input and output types.
A node can use an input from any predecessor node in the same logic path, even if the nodes are not directly connected.
Any number of nodes can use an input from the same predecessor node.
A node will be executed only when all necessary preceding nodes have been successfully executed.
A policy model can often be arranged in various configurations without impacting the execution results. You may want to arrange the policy model in the configuration that provides the best visual representation of the policy.
Connections that start at a Condition or Logic node can be configured to create separate logic paths in a policy model. Specifically:
- If the connection property is Yes, the corresponding path will be followed when the logical result of the Condition or Logic node is yes. If you do not configure a logic path for a connection, a Yes path is assumed but does not appear on the model.
- If the connection property is No, the corresponding path will be followed when the logical result of the Condition or Logic node is no.

Example: Principles Illustrated

Policy Model Principle	Example
Policy models do not reference specific records. Rather, they can contain nodes that reference families. You must use policy instancesIdentifies the records to which logic in the policy model will be applied. to identify the individual records whose values are evaluated when the policy is executed.	The Temperature and Analyst nodes represent families. The specific records whose values are evaluated are determined by policy instances.
The first node in a policy model must be an Input node other than a Query node.	The Temperature node is an OPC Tag node, which is a type of Input node.
Most nodes have outputs that successor nodes can use as inputs. You must specify inputs for each successor node using the Properties window that appears when you select the node in the policy model. Note: Outputs and inputs may represent either a single value or a collectionA set of results from a Query node or a set of readings associated with a Measurement Location, Health Indicator, or OPC Tag node. of values. The types of outputs that each node generates and the types of inputs that each node accepts is different for each node. When building a policy model, you must use corresponding input and output types.	The output DA Reading Value from the Temperature node is used as an input to the Condition node. This output represents a single value, which corresponds to the type of input that the Condition node accepts. The value 200 is used as the second input to the Condition node, but it is not an output from another node. Instead, it is a constant value that is specified directly in the Properties window for the Condition node.
A node can use an input from any predecessor node in the same logic path, even if the nodes are not directly connected.	The Email Contact node can use an input from the Temperature node even though the two nodes are not directly connected.
Any number of nodes can use an input from the same predecessor node.	The Email Contact node and the Condition node can both use inputs from the Temperature node.
A node will be executed only when all necessary preceding nodes have been successfully executed.	The Email Contact node will be executed only when all of the nodes preceding it have been successfully executed. If, for example, the condition defined in the Condition node was not met or if an error occurred when executing the Analystnode, the Email Contact node would not be executed.
A policy model can often be arranged in various configurations without impacting the execution results. You may want to arrange the policy model in the configuration that provides the best visual representation of the policy.	The execution results of the policy would be identical even if the Analyst node were connected to the Temperature node or the Condition node. The current configuration, however, provides a clear visual representation of the policy because the Email Contact node is the only node in the model that uses an input value from the Analyst node.
Connections that start at a Condition or Logic node can be configured to create separate logic paths in a policy model. Specifically: If the connection property is Yes, the corresponding path will be followed when the logical result of the Condition or Logic node is yes. If you do not configure a logic path for a connection, a Yes path is assumed but does not appear on the model. If the connection property is No, the corresponding path will be followed when the logical result of the Condition or Logic node is no.	A property is not defined for the connection between the Condition node and the Email Contact node, therefore, a value of Yes is assumed. This means that an email message is sent only if the preceding condition is true. If the condition is false, policy execution will not continue.