On March 12, 2019 Google experienced a 4 hour and 10 minute service disruption in their internal blob storage service. The outage was caused by a configuration change made by SREs on March 12th to reduce storage usage, which had the unintended side effect of overloading a key system. The document discusses principles for building highly available systems drawn from Google's experiences, including embracing constant change, hybrid architectures, observability, and value-based decision making.