AZ Failover Plus Rolling Deploy: Fence the Past, Expand the Future

AZ Failover Plus Rolling Deploy: Fence the Past, Expand the Future A sequence diagram generated by Archify. AZ-b goes dark, drain it health checks fail, stop routing drained: survivors take the peak N+1 capacity pays off promote, fence old primary epoch 7 to 8 write with stale epoch 7 split-brain attempt rejected: fenced only epoch 8 writes expand: add nullable column old and new binaries coexist migrate: backfill, dual-read compare before switching contract: drop old column rollback means routing back first Lose an AZ, fence the past Deploy without stopping Load Balancer · health checks · Sequence participant Load Balancer health checks API Fleet · surviving AZs · Sequence participant API Fleet surviving AZs New Primary · was sync standby · Sequence participant New Primary was sync standby Old Primary · fenced, epoch 7 · Sequence participant Old Primary fenced, epoch 7 Deployer · rolling release · Sequence participant Deployer rolling release Legend main request return security async trace message