Expression Rule definition

Expression rules provide a way to evaluate events, data, and conditions and then run actions based on the results of the evaluation. Expression rules are created using programmatic If -Then-Else expressions, and must evaluate to a Boolean result (that is, true or false or 1 or 0) at runtime.

For example, you can create an expression rule for an Alarm trigger (message) that evaluates the severity of alarms reported by any one of the associated, included assets. If the alarm name is "alarmXYZ", and the severity for alarmXYZ is greater than 100, and the data item "machineFan" is not 1, then the Platform needs to run an action that writes a value to a data item on the related asset. You can also specify an Else portion that directs the Platform to write a different value to the data item. In this page, you can create the expressions and conditions for this rule as follows:

If: (Alarm.name == "alarmXYZ" && Alarm.severity>100) && (DataItem.machineFan.value != 1)

Note: In addition to alarms generated by Axeda Gateway and Axeda Connector Agents, by "smart" mobile or fixed wireless assets, by a Platform rule, or by the Axeda Platform SDK, you can use faults (alarms) sent from Axeda® IDM Agents in expression rules. For example, you may want to convert the Axeda IDM Agent alarm to a different type of Platform alarm. If using an Axeda IDM Agent alarm, you need to evaluate the content of the description attribute (all four components of the IDM Agent fault are put in the description field of the Alarm when it is received at the Axeda® Enterprise Server). See also Alarm Namespace.

Expression rules are created in the Configure Expression Rule page using programmatic If-Then-Else definitions. You can use the many predefined functions and actions to create a customized set of expression rules and logic specific to your business requirements. Each rule can be saved and reused or modified and saved as a new expression rule.

Expressions are created using a combination of functions, actions, and variables, including namespaces and symbols. Triggers are assigned to Expression rules. A trigger specifies what message causes the Platform to evaluate and run the related expression rule. Refer to the syntax requirements for Expression Rules.

Note: In a Platform with Delegated Administration enabled, the names of expression rules may have prefixes; for details, refer to Names of Objects in DA Units.

Also note that Delegated Admin visibility restrictions and privileges are NOT fully supported for expression rules. See Supported Objects for Delegated Administration for more information.

Format for an Expression Rule

An Expression Rule contains three expressions: If, Then, Else. The format for an expression rule is as follows:

If <expression> Then <expression> Else <expression>

(Note that the Else expression is optional.)

The If expression defines Platform events or information that you want the Platform to evaluate to true or false.

For example:

If: DistanceToNamedGeofence("home", location) < 3

determines if the object is less than 3 miles from the geofence,"home".

Note: A geofence can be created only through Axeda® Platform Web services and through the .NET client available with the Axeda Platform Web services. Refer to the Axeda Platform Web Services Reference Guide for information about creating geofences.

An If expression can be simple or complex. You can configure a rule that runs its actions every time alarm "AlarmXYZ" occurs for model "ModelABC" (simple) or only when alarm "AlarmXYZ" occurs with a severity greater than "100" and data item "Data123" has a value of "1" for model "ModelABC" (complex).

Note: For a MobileLocation trigger, you can specify an unqualified "location". For all other triggers, you need to qualify this as "Location.location.

The Then expression specifies the action(s) the Platform runs when the If expression evaluates to true.

For example:

Then: SendDataitem("DataItemxyz", 0)

writes the value 0 to the asset data item, DataItemxyz"

The Else expression specifies the action(s) the Platform runs when the If expression evaluates to false.

For example:

Else: SendDataitem(DataItemxyz", 1)

Writes the value 1 to the asset data item, DataItemxyz"

Supported actions, functions, and variables

The functions and variables that you can define for each <expression> vary based on the type of trigger selected and whether you are defining an If, Then, or Else expression. For example, Alarm triggers support a variety of variables, including name, severity, time, and value. When you select an Alarm trigger, all functions and variables supported for that trigger are shown in the Expression Tree.

How Expression Rules Work

The Axeda Platform evaluates expression rules as events are triggered, such as receiving data, alarms or files from an asset, or as a result of other rules. In addition, many of these messages can be triggered through the Axeda Platform SDK. The If expression rule must be able to evaluate to either True or False (Boolean 1 or 0). If the expression evaluates to true, then the Platform runs the Then expression; if it evaluates to false, the Platform runs the Else expression (if one is defined). You create the If, Then, Else expressions using the Expression Tree objects and the supported operators.

For example:

An example of an Alarm trigger Expression Rule could be as follows:

If: Alarm.name == "High Tempcontactable && Alarm.severity == 10
Then: SetAlarmState("ESCALATED")

The Platform evaluates the If expression first. If both conditions are true - that is, the name of the alarm generated is "High Temp" and the severity of the alarm is equal to 10, the If expression evaluates to True and the Platform runs the Then expression. In this example, it sets the state of the Alarm to "ESCALATED".

If the If expression evaluates to False (for example, the alarm name is not "High Temp" or the severity is not equal to 10), then the Platform runs the Else expression of the Expression Rule. In this case, no Else expression has been defined, so the Platform does nothing if the expression evaluates to false.