reasoning-about-kubernetes-cost-reliability

# Kubernetes Cost & Reliability Reasoning **Core principle:** Every resource request has a dollar cost. Every reliability gap has an outage risk. Always quantify both. ## Workflow ### Step 1: Calculate Cost Extract `requests.cpu` and `requests.memory` (not limits) for each container. Multiply by replicas. **Quick estimate:** `(CPU x $22 + Memory_GiB x $3) x replicas = monthly cost` See `references/cost-calculation-formulas.md` for per-provider pricing (AWS/GCP/Azure). ### Step 2: Detect Waste | Signal | Threshold | Action | |--------|-----------|--------| | limits >> requests | limits > 3x requests | Flag overprovisioning risk | | No resource requests | Missing entirely | Flag -- cannot calculate cost | | High replica count | > 5 replicas without HPA | Question if static scaling needed | | Unused storage | PVC without matching Pod mount | Flag orphaned storage cost | See `references/right-sizing-guide.md` for optimization heuristics. ### Step 3: Evaluate Reliability | Check | Production Minimum | Risk if Missing | |-------|-------------------|-----------------| | replicas >= 3 | Required | No HA, single-point-of-failure | | PodDisruptionBudget | Required | Node drain kills all Pods | | Pod anti-affinity | Required | All replicas on same node = no HA | | Liveness + readiness probes | Required | No auto-recovery from crashes | | Topology spread constraints | Recommended | Zone failure takes entire app down | See `references/reliability-patterns.md` for HA patterns and SLA calculations. ### Step 4: Present Tradeoff Table Always present a table with columns: Option | Monthly Cost | Availability | Risk. Show current state, recommended state, and high-HA option with calculated dollar amounts and SLA percentages. ## Red Flags -- STOP and Investigate - Production namespace + replicas: 1 + no PDB - CPU/memory requests missing entirely (cannot reason about cost) - limits set but requests missing (limits become requests, likely unintentional) - Storage class "gp3" or "premium" without justification for cost tier ## Non-Negotiable Rules 1. **Every analysis MUST include a dollar amount.** "Significant resources" is not analysis. Calculate it. 2. **Every reliability finding MUST include an SLA percentage.** "Not highly available" is not analysis. Quantify it. 3. **Always present cost AND reliability together** as a tradeoff table. Never analyze one without the other. 4. **Use the quick estimate formula** inline -- do not defer to "see the reference file." ## Common Mistakes | Mistake | Reality | |---------|---------| | "Resources look reasonable" without dollar amounts | Always calculate: `(CPU x $22 + Mem_GiB x $3) x replicas` | | Noting "single replica" without SLA impact | Quantify: 1 replica = ~99.5%, 3 replicas = ~99.95% | | Ignoring memory when only CPU seems high | Memory is often the larger cost driver | | Not mentioning PDB for any multi-replica workload | PDB is required for safe node maintenance | | Analyzing cost without reliability (or vice versa) | Always present both together in tradeoff table |