What is idempotency? And why it matters for durable systems

What this post added

This post elaborates on idempotency as a critical property for durable systems like Temporal. It explains the concept of idempotency with real-world examples and details how Temporal's 'at-least-once' execution guarantee for Activities necessitates idempotent design. The post provides practical implementation patterns for building idempotent Activities, including using idempotency keys derived from workflow and activity IDs, and checking for pre-existing results. It also includes a SQL example demonstrating how to prevent duplicate side effects using an operations table with a uniqueness constraint and discusses potential race conditions and mitigation strategies.