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Section 5 · Correlate signals

Keep metrics low-cardinality and put high-cardinality detail on correlated logs

Estimated time: 1 min

Put detail on the right signal

Sometimes you genuinely need a high-cardinality value, like a request ID, when you’re debugging. The answer isn’t to delete it, and it isn’t to put it on a metric label. It’s to put the value on the signal designed to hold it, and correlate across signals when you investigate.

The pattern

When a high-cardinality value is attached to a metric label, every unique value multiplies your active series. The same value attached to a log costs a fraction of that, because Loki doesn’t index log content. Here’s how to use structured metadata to keep your metrics low-cardinality and by leveraging logs:

  • Keep the metric low-cardinality. The metric answers “how many” and “how fast” per bounded dimension, such as service and status.
  • Put the high-cardinality detail on the correlated log. Attach it as structured metadata, or leave it in the log line. The log answers “which exact request”.
  • Correlate at query time. Shared bounded labels, such as service and env, connect the metric that alerted you to the logs that explain it.