Use Metadata in a Threshold

Metadata in expression-based thresholds can streamline threshold creation. The Metadata DSL (domain specific language) lets you use patterns in an expression, whereby the metadata is replaced with a corresponding value during the collection process. A single expression can behave differently based on the node being tested against.

During evaluation of an expression, the following scopes are available:

  • Node metadata

  • Interface metadata

  • Service metadata

Metadata is also supported in Value, Re-arm, and Trigger fields for Single-DS and expression-based thresholds.

For more information on metadata and how to define it, see Metadata.

Example

This procedure uses metadata to trigger an event when the number of logged-in users exceeds 1.

The expression is in the form ${context:key|context_fallback:key_fallback|…​|default}.

Before using metadata in a threshold, you need to add the metadata context pair, in this case, a requisition key called userLimit (see Adding metadata through the web UI).

  1. Click the gear icon in the top-right menu.

  2. Under Performance Measurement, choose Configure Thresholds.

  3. Click Edit beside the netsnmp group.

  4. Click Create New Expression-based Threshold.

  5. Fill in the following information:

    • Type: High

    • Expression: hrSystemNumUsers / ${requisition:userLimit|1}

    • Datasource type: Node

    • Value: 1

    • Rearm: 1

    • Description: Too many logged-in users

    meta expression1

  6. Click Save.

This expression will trigger an event when the number of logged-in users exceeds 1.

meta expression2