Designing and Managing Behavior Models - Using Alarms - Correlation Expressions -
Enabling an Alarm      Alarm Actions

Correlation Expressions

NerveCenter 3.8 provides an additional method for Alarm Definition creation, the Correlation Expression window. Correlation expressions allow the definition of alarm diagrams based on Boolean expressions. The correlation expressions do not apply in every situation, but in cases where multiple combinations of events need to be detected and acted upon, the correlation expressions save tremendous amounts of time, both in alarm diagram designing and building.

To build a correlation expression, first create the necessary trap masks and poll conditions to fire the desired triggers. Once the triggers have been created, the Correlation Expression Editor can be used to create the expression.

There are three main components of the correlation expression. First, the Boolean expression is created using and, or, parenthesis and triggers that are already existent. Second, the correlation reset period determines the time limit in which the entire expression must become true once a portion has been detected. Third, the correlation action must be specified, directing NerveCenter to act when the expression becomes true.

For example take the sample alarm in Sample Alarm: Error Rate Alarm Created with the Alarm Definition Window.

Sample Alarm: Error Rate Alarm Created with the Alarm Definition Window

In this alarm, you want an inform to be sent if you receive HighErrorRate and HighLoad triggers or if you receive a HighErrorPersists trigger. The alarm will reset to Ground if the alarm is not completed within the time period specified by the transition Reset. Creating this model takes several steps. You need to create three states and eight transitions. Three of those transitions require you to add the same action, send inform. The idea behind this model, however, can be expressed simply with a boolean expression:

If (HighLoad AND HighErrorRate) OR HighErrPersists, then Inform NC/NV/OV

Correlation expressions allow you to create simple alarms quickly.

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   After a correlation expression reaches the final state, the Alarm reverts to Ground.

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  To create a Correlation Expression:

1. From the client's Admin menu, choose Correlation Expression List.

   The Correlation Expression List Window opens.

   

2. Select the New button.

   The Edit Correlation Expression window opens.

   

3. In the Expression Name field enter a name for the expression.

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      The maximum length for correlation names is 255 characters.

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4. Enter the Reset Period (must be greater than 0) and select a time unit (seconds, minutes or hours) from the drop-down menu.

   The correlation reset period is the time in which the entire alarm must complete before the alarm resets. This counter starts when the first trigger occurs. The counter does not restart when a second trigger occurs.

   The time period must be greater than zero. You can choose between seconds, minutes or hours.

5. Enter a correlation expression.

   You enter information in the Correlation Expression field by using the buttons below the field.

   • To add a trigger:
   1. Select a trigger from the Trigger to Add drop down list.
   2. Select Add Trigger.
   • To add a boolean operator, select the AND or OR button.

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        The AND operator has precedence over the OR operator. For example, x or y and z is the same as x or (y and z).

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   • To add a parenthesis, select the ( or ) button.

     The close parenthesis ) button is not active until there is an open parenthesis ( in the correlation expression.

   • To delete the previous element of the correlation expression, select the Del button.
6. Add Correlation Actions.
   1. Select the New Action button.

      NerveCenter displays a pop-up menu that lists all the actions supported on your platform. The complete list of actions is:

   • Action Router
   • Alarm Counter
   • Beep
   • Clear Trigger
   • Command
   • Delete Node
   • EventLog
   • Fire Trigger
   • Inform
   • Inform OpC
   • Inform Platform
   • Log to Database
   • Log to File
   • Microsoft Mail
   • Notes
   • Paging
   • Perl Subroutine
   • Send Trap
   • Set Attribute
   • SMTP Mail
   • SNMP Set

     These actions are described in Chapter 12, Alarm Actions.

   1. Select an action from the pop-up menu.

      If you select the Action Router, Delete Node, or Notes action, the action is added immediately to the Actions list in the Edit Correlation Expression window. However, because most actions require you to supply parameters, NerveCenter generally displays an action dialog at this point. The dialog varies from action to action.

   2. Fill in the fields in the action dialog.

      This step is dependent on the action you've selected. For details on how to complete this step, see the appropriate section in Chapter 12, Alarm Actions.

      You can edit these selections later by selecting the Update Action button.

   3. Repeat step a through step c for any additional actions you want to add to the correlation expression.

      To delete an action, select it from the Correlation Actions list and click Delete Action.

7. Select Save.

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      The Save and Create Alarm buttons are not enabled until:
   w you give the correlation expression a name
   w you set the Reset Period to a number other than zero
   w the correlation expression is valid (for example, all open parenthesis are closed)
   w you select at least one Correlation Action

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   This saves the correlation expression.

After creating a correlation expression, you can use it as a building block to create alarms.

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   You do not have to save a correlation expression to create an Alarm. As long as the correlation expression has a name, a reset period, a valid expression and an action, you can create an Alarm from the expression.

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  To copy a correlation expression:

1. From the client's Admin menu, choose Correlation Expression List.

   The Correlation Expression List Window opens.

   

2. Select a correlation expression and right-click.
3. Select Copy from the pop-up menu.

   The Edit Correlation Expression window opens.

4. In the Expression Name field enter a new name for the expression.

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      The maximum length for correlation names is 255 characters.

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5. Select Save.

 
  To create an alarm from a correlation expression:

1. From the Edit Correlation Expression window, click Create Alarm.

   The Create Alarm using Correlation Expression window opens.

   

2. In the Alarm Name field, enter a name for the alarm.

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      The maximum length for alarm names is 255 characters.

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3. Select a property from the Property list box. Or leave the Property set to NO_PROP.

   The property you choose helps determine whether a particular trigger can cause an alarm instance to be instantiated or cause a transition in an existing alarm instance. Generally, the alarm's property must match one of the properties in the property group of the node associated with the trigger. The property NO_PROP matches any property.

   For complete information regarding the matching rules that determine whether a trigger causes an alarm transition, see Designing and Managing Behavior Models.

4. Select a scope from the Scope list box.

   The options are Enterprise, Instance, Node, and Subobject. Briefly, an alarm instance with Enterprise scope monitors all the nodes managed by the NerveCenter server. An alarm instance with Node scope monitors a single node. A subobject scope alarm monitors a subcomponent of a node, usually an interface (subobject). Instance scope lets you monitor different base objects in a single alarm instance.

   For further information on alarm scope, see Designing and Managing Behavior Models.

5. Select the Clear Triggers for Reset To Ground or Off checkbox if you want NerveCenter to clear any pending triggers fired by this alarm when the alarm is turned off or manually reset to ground. The alarm might have pending triggers if you associated a Fire Trigger alarm action with this alarm.
6. If you want to enable you alarm now, set the alarm's Enabled status to On.
7. Select Save Alarm.

When you save the alarm, you can now access it through the Alarm Definition List and edit it as any other alarm. For details on using the Alarm Definition window, see Defining Transitions.

Error Rate Correlation Expression shows the correlation expression that creates the alarm shown in Sample Alarm: Error Rate Alarm Created with the Alarm Definition Window. Error Rate Alarm Generated from the Error Rate Correlation Expression shows the alarm generated with the Error Rate correlation expression.

Error Rate Correlation Expression

Error Rate Alarm Generated from the Error Rate Correlation Expression

 
  To add Notes to a Correlation Expression:

1. From the Correlation Expression window, select Notes.

   The Correlation Expression Notes dialog box displays.

   

2. Enter your comments.
3. Select Save to close the Notes dialog box.

   A dialog box asking Are you sure? displays.

4. Select Yes.
5. Click Save in the Correlation Expression window to save the notes.

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      These notes document the correlation expression. They are not copied over to any alarm created by a correlation expression.

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