How we made feature flags faster and more reliable

The text discusses the challenges and solutions related to making feature flags fast and reliable, specifically in the context of PostHog's feature flag service. Feature flags are essential for determining application behavior, and latency in their evaluation can significantly impact user experience. To address this, PostHog has implemented strategies such as local evaluation, where the SDKs download and evaluate flag definitions locally, reducing dependency on network I/O and cutting latency from 500ms to 10-20ms. They also introduced bootstrapped feature flags so that flags are instantly available on the client side. Reliability is enhanced by caching flag definitions and project mappings to maintain functionality even when databases are down, although caching person properties is impractical due to scale. To mitigate issues with database and connection pooler failures, PostHog has implemented statement timeouts and separated deployments for feature flags. These efforts have significantly reduced latency and improved reliability, ensuring that the core functionality remains solid despite ongoing challenges and potential system failures.

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