Meridian supports the assignment of arbitrary metadata to nodes, interfaces and services. You can use this metadata to dynamically configure service monitoring, performance data collection, service detection, and expression-based thresholds.

The metadata is made up of a simple triad of strings containing a context, a key, and the associated value. Each node, interface, and service can have an arbitrary number of metadata elements assigned to it. The only restriction is that the tuple of context and key must be unique in the element with which it is associated.

The association of metadata with nodes, interfaces, and services happens during provisioning with the use of detectors. Users can add, query, modify, or delete metadata through the requisition editor in the web UI, or through the REST endpoints.

A simple domain-specific language (DSL) lets users access metadata that is associated with the elements they are working on, and use it as a variable in parameters and expressions. There is no limitation to defining metadata: users can decide how to define and use it in expressions.

You can view the metadata that is currently assigned to nodes, interfaces, and services on the entity’s details page:

Meridian web UI showing a node’s associated metadata
Figure 1. Node metadata


Contexts distinguish different kinds of metadata use. Meridian uses several default contexts: pattern (used with polling), requisition, node, interface, and service. Three special contexts provide details about nodes, interfaces, and service objects. Each context has keys associated with it that you can use in a metadata expression.

You can create user-defined contexts in the REST API by prefixing its name with X-. Using an X- prefix can help to avoid future Meridian contexts interfering with a user-defined context, since Meridian contexts are not prefixed in this way.

Node context

The node context provides details about the node currently processed. The following keys are available under this context:

Context:Key Description


The node’s label


The node’s foreign source name


The node’s foreign ID


The NetBIOS domain as provided by SNMP.


The NetBIOS name as provided by SNMP.


The node’s operating system


The node’s system name


The node’s system location


The system contact specified for the node.


The node’s system description


The node’s monitoring location name


The node’s monitoring location area


A Geohash of the node’s latitude or longitude.

Interface context

The interface context provides details about the interface currently processed. The following keys are available under this context:

Context:Key Description


The hostname associated with the IP address of the interface.


The interface’s IP address


The interface’s netmask


The SNMP interface index


The SNMP interface alias


The SNMP interface description


The interface’s physical address

Service context

The service context provides details about the service currently processed. The following key is available under this context:

Context:Key Description


The service’s full name

Secure credentials vault

The special context scv provides access to values stored in the secure credentials vault. The key used in this context must consist of the alias and the attribute of the credential, separated by a colon (for example, ${scv:mydevice:password} will access the password attribute of a credential named mydevice).

Add metadata through the web UI

Follow these steps to edit the requisition context in the web UI:

  1. Click the gear symbol in the top-right of the page.

  2. Under Provisioning, select Manage Provisioning Requisitions.

  3. Click Edit beside the requisition that you want to modify.

  4. Click Edit beside the node that you want to modify.

  5. Navigate to the Meta-Data tab, and click Add Meta-Data.

    Meridian UI displaying the Meta-Data Entry page with example parameters
  6. In the Scope list, select either Node or Interface.

  7. Specify the key and a value, and click Save.

Hidden passwords in metadata

Metadata values for key names that include a substring match of password, Password, secret, or Secret will be hidden by default for non-administrator users. Users with ROLE_ADMIN permissions can read these values in the web UI.

The metadata DSL

The metadata DSL lets you interpolate metadata into a parameter. Its syntax lets you use patterns like ${context:key|context_fallback:key_fallback|…​|default} in an expression.

Each expression can contain multiple references to metadata. The references will be replaced with their corresponding values during evaluation. Placeholders start with ${ and end with }, and they contain references to context-key pairs. You may choose to define multiple fallback context-key pairs and a trailing default value. Separate the context and the key using a colon. Use a vertical bar (|) to separate any fallback context-key pairs and default values.

If the first context-key value is not available (not found on a service, interface, node, or any other special context), the next value following the vertical bar is used. The final value—​the default value—​is not interpreted as a context-key, but is used as a literal. Its evaluation will always succeed.

The interpolation process supports recursive evaluation. This lets you specify a pattern inside another pattern, or provide whole expressions stored as metadata.

The DSL uses scopes to determine the resolution order of a context-key pair. First, the most recent scope is used; if it does not contain the context-key tuple, the next scope is queried. For example, a query on a service entity would be resolved in this order: service metadata > interface metadata > node metadata. On an interface, it resolves in metadata > interface metadata > node metadata order. On the node level, only the node is queried.

The available scopes depend on the environment for which an expression is evaluated; they are documented as appropriate elsewhere in this guide. Some environments also provide additional scopes that are not backed by the persisted metadata, but which provide additional metadata related to the current evaluation.


The following placeholder will resolve to the username as defined in the requisitioning UI, or an empty value if the username is not defined:


Placeholders can contain optional default values, separated from the main reference with a vertical bar (|). The following placeholder will resolve to the username as defined in the requisitioning UI, or to admin if the username is not defined:


You can use fallback context-key pairs in a placeholder to specify other references to fall back to if the primary context-key pair is not defined. Separate each reference with a vertical bar (|). The following placeholder will resolve to the username as defined in the requisitioning UI. If the username is not defined, the fallback account reference is used. If neither exist, the placeholder defaults to admin:


You can also embed placeholders within each other. For example, the embedded placeholder below resolves to the credentials as defined in the requisitioning UI, with the node label as a fallback value. Its result is used to resolve the outer pattern, and results in the device’s final password:


Test an expression

Test an expression using the following Karaf shell command:

admin@opennms> opennms:metadata-test -n 1 -i -s ICMP '${fruits:apple|fruits:banana|vegetables:tomato|blue}'

This expression resolves to the following:

Meta-Data for node (id=1)
Meta-Data for interface (ipAddress=
Meta-Data for service (name=ICMP):
Input: '${fruits:apple|fruits:banana|vegetables:tomato|blue}'
Output: 'red'
  Part: '${fruits:apple|fruits:banana|vegetables:tomato|blue}' => match='fruits:apple', value='red', scope='SERVICE'


The following Meridian components allow the use of the Metadata DSL: