Week 13 addressed "how do you design systems that survive failures, and how do you validate that survival actually works?" There is one single insight threading through this entire week — availability isn't "preventing failures from occurring" but "assuming failure will occur, designing tolerance through replication, quorum, and failover, and validating that tolerance through experiments." From single AZ to multi-AZ, from single region to multi-region, from backup to Active-Active, and from "it should work" belief to FIS chaos experiments — the week's flow progressively assumes stronger failures and validates more rigorously.