Common Expression Language for Metric Filtering with Telegraf

Telegraf is an open-source plugin-driven agent for collecting, processing, aggregating, and writing time series data. When collecting metrics it is common to filter out or pass through metrics with specific names, tags, fields, or timestamp values.

The Common Expression Language (CEL) is an open-source language that provides a set of semantics for expression evaluation. Kubernetes users may already be familiar with the language as it is used to declare validation rules, policy rules, and other constraints or conditions.

In v1.27, Telegraf now has the ability to parse CEL expressions to filter metrics. The following post outlines how users can take advantage of CEL to filter metrics easier than ever with Telegraf.

Telegraf configuration

The new metricpass option enables CEL filtering. This is similar to the existing namepass or tagpass options.

Telegraf evaluates the metrics based on the expression. Metrics that match the expression are passed on, while other metrics are dropped.

  metricpass = "fields.usage_idle < 90"

This option is valid for any input, output, processor, and aggregator plugin. For inputs the filtering takes place at the very end of an input, while for the other plugin types the filter is run before those plugins are run.

While the CEL language is incredibly powerful, it does not alter metrics. It only determines which metrics should continue on from an input or to other plugins.

CEL syntax by example

The following are a set of examples to introduce CEL usage for Telegraf. For the complete set of types, logic, and functions users can reference the cel-spec language definition in the CEL repo. There is even a CEL Discussion Forum for help with specific questions about the language.

Referencing metric components

In order to filter metrics, users can reference specific components of a metric. Consider the following example metric:

example,host=a value=42 1686252961000000000

The following table shows how to reference the specific components of the metric:

Component CEL Reference Example
Metric name name name == "example"
Tags tags == "a"
Fields fields fields.value > 0
Timestamp time time >= now() - duration("24h")

Logical operators

CEL supports C- and Go-style logic operators to make comparisons. The following example checks that the field “id” exists and the field “id” contains the substring “nwr”.

"id" in fields &&"nwr")

This example ensures the metric name does not start with a “t” and that the tag “state” is equal to “on”.

!name.startsWith("t") || tags.state == "on"

Numeric operations

For numeric values, users can do both arithmetic and equality operations. For example:

fields.value / 8 >= 1024
fields.bits % 2 == 0

String operations

To aid in testing strings, users have a number of helper functions available. The following table lists a few common ones:

Function Test Description Example
contains String contains substring"domain")
endsWith Test string suffix".com")
lowerAscii Lower case string
matches Compare string against a regular expression tags.source.matches("^[0-9a-zA-z-_]$")
startsWith Test string prefix"subdomain")
size String length

For the full list of functions, check out string.go source, which includes examples as well.

Time operations

A possible use case for this type of filtering would be to remove metrics older than a specific date. CEL includes a variety of helper functions to pick apart a date. For example:

time.getFullYear() == 2023

There is a large list of these helper functions available on the CEL list of standard definitions.

Additionally, a user can compare the current time to the timestamp. With this a user can see if a timestamp is less than a day old:

time >= now() - duration("24h")


To test for a specific type, the type() function returns what a particular tag or field is. This might be useful to ensure a field is a numeric type:

type(fields.bits) in [double, int, uint]
type(fields.cluster) == string

The first line ensures that the bits field is of a numeric type, while the second ensures the cluster field is a string.

Performance impact

Users should keep in mind that CEL usage comes at a cost. CEL is an interpreted language and uses overhead when implemented. If you only filter based on a specific tag or metric name, continue to use the namepass/namedrop and tagpass/tagdrop configuration options to achieve the best performance. However, in most cases, these CEL filters should be faster than using the Starlark processor to do the same task.

Get started with metric filtering and Telegraf

With the addition of the Common Expression Language for metric filtering in v1.27, it is now even easier for users to customize their metrics. Download Telegraf and give the new metricpass option a try today!