Zero-downtime deployment betyr å gi ut nye versjoner uten noen avbrudd for brukere — applikasjonen forblir tilgjengelig hele tiden. For å oppnå det kreves nøye deploymentstrategier, bakoverkompatible endringer, helsesjekker og grasiøs håndtering av forespørsler som allerede er i gang.
Hva zero-downtime krever
GOAL: deploy a new version with NO user-facing downtime (always-available service):
→ never take the whole service offline to deploy
→ always have healthy instances serving while updating others
→ handle in-flight requests gracefully (don't drop them mid-request)
→ Combine several techniques (below).
Viktige teknikker
✓ DEPLOYMENT STRATEGY — rolling, blue-green, or canary (never all-at-once downtime):
→ rolling: update instances gradually (others keep serving)
→ blue-green: switch traffic to the new environment instantly
✓ LOAD BALANCER + HEALTH CHECKS → route traffic only to healthy/ready instances;
new instances join only when READY (readiness checks)
✓ GRACEFUL SHUTDOWN → draining: stop sending new requests to an instance, let it FINISH
in-flight requests, THEN stop it (handle SIGTERM) → no dropped requests
✓ BACKWARD-COMPATIBLE changes → old and new versions coexist during the rollout
(API and DATABASE compatibility — expand-contract migrations)
