The principles for working with a policy model can be illustrated through the following example model.
In this example model, the node named
Temperature is an OPC Tag
node which represents an OPC Tag record. When this policy is executed,
if the DA Reading Value that is associated with the OPC Tag record is greater
than or equal to 200, the GE Digital APM system will send an email message to the email
address that is specified in the Human Resource record that is associated
with the Analyst User node.
The following table explains each of the policy model basic principles in the context of this example.
Policy Model Principle |
Example |
Policy models do not reference specific records.
Rather, they can contain nodes that reference families.
You must use 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 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.
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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. |
With the exception of the Or node and the Case 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.
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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.
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