Slack developed the Service Delivery Index for Reliability (SDI-R) as a straightforward yet comprehensive metric to track and improve service reliability amid the complexity of managing a vast engineering team and numerous daily production changes. This metric evaluates successful API calls and content delivery, ensuring services are reliable by measuring their ability to handle and respond to user requests. The SDI-R was created partly to mitigate the stress and burnout associated with relying on "Hero Engineers" for incident response, aiming instead for a scalable, systemic approach to incident management and service ownership. Slack's initiative highlights the importance of a shared reliability culture and the necessity of clear, measurable reliability metrics in driving decision-making and setting customer expectations. The SDI-R, similar to Gremlin's reliability score, helps companies understand their service reliability at a glance, although each approaches this from different angles, with Slack focusing on operational data and Gremlin on predictive testing against known risks.