tl;dr:The article introduces the Service Delivery Index – Reliability (SDI-R), a metric designed to measure and drive service reliability at Slack. As the company grew, the need to move from a reactive to a proactive approach to reliability became evident. SDI-R, a composite metric of successful API calls, content delivery, and user workflows, provides a common understanding of reliability across the organization. It helps in spotting trends, identifying regressions, and setting customer expectations. The article details how SDI-R evolved, the tools and processes that support it, and the lessons learned.