Stage 2 · Lesson 4 of 7
Cardinality and what belongs in metrics
is the number of unique a metric creates. Every distinct combination of values is a separate series that the backend keeps in memory. A metric with labels for 10 routes, 5 status codes and 50 pods is already 2,500 series.
Add a user ID label and it becomes millions, and the server runs out of memory or your hosted bill jumps. The rule: labels should have a small, bounded set of values. Unbounded details, such as user IDs, full URLs, order numbers and request IDs, belong in logs and . Check series counts per metric regularly with the TSDB status page.
Common trapadding a "helpful" label in one service that quietly multiplies series across the whole cluster.
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Resources for this stage
Start with· follow one of these
- GuideFreePrometheus Documentation
Prometheus · Intermediate
Free: the first steps guide, then the querying basics, functions and histograms pages
- CoursePaidPrometheus Certified Associate (PCA) Prep Course
KodeKloud · Intermediate · Subscription
Paid, hands-on labs for PromQL, recording rules, Alertmanager and Kubernetes monitoring
Practise
Keep handy
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