Cardinality Explosions

• Every unique label combination creates a new time series — user_id as label kills Prometheus
• Avoid high-cardinality labels: user IDs, email addresses, request IDs, timestamps, UUIDs
• Check cardinality: prometheus_tsdb_head_series metric — above 1M series needs attention
• Use histograms for latency, not per-request labels — buckets are fixed cardinality
• Relabeling can drop dangerous labels before ingestion: labeldrop in scrape config

Histogram vs Summary

• Histograms: use for SLOs, aggregatable across instances, buckets defined upfront
• Summaries: use when you need exact percentiles, cannot aggregate across instances
• Histogram bucket boundaries must be defined before data arrives — wrong buckets = wrong percentiles
• Default buckets (.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5, 10) assume HTTP latency — adjust for your use case

Rate and Increase

• rate() requires range selector at least 4x scrape interval — rate(metric[1m]) with 30s scrape misses data
• rate() is per-second, increase() is total over range — don't confuse them
• Counter resets on restart — rate() handles this, raw delta doesn't
• irate() uses only last two samples — too spiky for alerting, use rate() for alerts

Alerting Mistakes

• Alert on symptoms, not causes — "high latency" not "high CPU"
• for clause prevents flapping: for: 5m means condition must hold 5 minutes before firing
• Missing for clause = fires immediately on first match = noisy
• Alerts need runbook_url label — on-call needs to know what to do, not just that something's wrong
• Test alerts with promtool check rules — syntax errors discovered at 3am are bad

PromQL Traps

• and is intersection by labels, not boolean AND — results must have matching label sets
• or fills in missing series, doesn't do boolean OR on values
• {} without metric name is expensive — scans all metrics
• offset goes back in time: metric offset 1h is value from 1 hour ago
• Comparison operators filter series: http_requests > 100 drops series below 100, doesn't return boolean

Scrape Configuration

• honor_labels: true trusts source labels — use only when source is authoritative (e.g., Pushgateway)
• scrape_timeout must be less than scrape_interval — otherwise overlapping scrapes
• Static configs don't reload without restart — use file_sd or service discovery for dynamic targets
• TLS verification disabled (insecure_skip_verify) should be temporary, never permanent

Pushgateway Pitfalls

• Pushgateway is for batch jobs, not services — services should expose /metrics
• Metrics persist until deleted — stale metrics from dead jobs confuse dashboards
• Add job and instance labels to distinguish sources — default grouping hides failures
• Delete metrics when job completes: curl -X DELETE http://pushgateway/metrics/job/myjob

Recording Rules

• Pre-compute expensive queries: record: job:request_duration_seconds:rate5m
• Naming convention: level:metric:operations — helps identify what rules produce
• Recording rules update every evaluation interval — not instant, plan for slight delay
• Reduce cardinality with recording rules: aggregate away labels you don't need for alerting

Federation and Remote Write

• Federation for pulling from other Prometheus — use sparingly, adds latency
• Remote write for long-term storage — Prometheus local storage is not durable
• Remote write can buffer during outages — but buffer is finite, data loss on extended outages
• Prometheus is not highly available by default — run two instances scraping same targets

Common Operational Issues

• TSDB corruption on unclean shutdown — use --storage.tsdb.wal-compression and monitor disk space
• Memory grows with series count — each series costs ~3KB RAM
• Compaction pauses during high load — leave 40% disk headroom
• Scrape targets stuck "Unknown" — check network, firewall, target actually exposing /metrics

Label Best Practices

• Use labels for dimensions you'll filter/aggregate by — environment, service, instance
• Keep label values low-cardinality — tens or hundreds, not thousands
• Consistent naming: snake_case, prefix with domain: http_requests_total, node_cpu_seconds_total
• le label is reserved for histogram buckets — don't use for other purposes