How I Fixed The Bottleneck That Killed 700+ Lambdas

A migration on a single DynamoDB table serving 700+ Lambda functions caused a platform-wide outage when the change stream processor fell behind, overwhelming all downstream systems. The fix was an 'ActionRunner' pattern: data sources drop serialized messages into an SQS FIFO queue instead of doing work directly, and a dedicated Lambda processor handles execution with controlled concurrency. This decouples request from execution, eliminates API Gateway timeout pressure, enables throughput throttling, absorbs traffic spikes, and provides ordered parallelism via FIFO message groups. Key implementation advice includes idempotent actions, strategic message group IDs, dead-letter queues with CloudWatch alarms, and structured logging.