# 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:

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

## 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

6. Click Save.

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