Cloud cost
Metric cardinality
The number of unique combinations of metric name and tag value that a monitoring platform stores and bills for. It is not the number of metric names your code emits.
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Definition
One metric.submit call in code can become thousands of billable series. Cardinality is the count of every distinct combination of a metric's name and its tag values. On most platforms it includes the host tag. Tag one metric by customer_id or request_id across a growing user base. It then multiplies into a five- or six-figure billable count. You never create a new metric name.
Histogram and distribution metric types multiply cardinality further. By default, each one bills as several underlying metrics for every unique tag combination. These are commonly max, median, avg, a percentile, and count.
Where it shows up
Metric cardinality shows up in a monitoring platform's usage dashboard as a billable series count. It is distinct from the number of metric names in your source code. Datadog and similar platforms report this count directly. It includes the host tag that most platforms add to every series automatically.
What makes it expensive
Tagging a metric by customer_id or request_id is a costly habit. As the user base grows, one metric name becomes a five- or six-figure billable series count. The cost does not creep. It steps on the day the tag ships. Read a cardinality report straight after any tagging change.
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