From 15-Minute Lambda Timeouts to Sub-Second Runs — A DynamoDB Optimization Story

Short summary

Two AWS Lambda functions timing out at the 15-minute ceiling failed to process daily stats due to fetching full DynamoDB items with unused fields and recomputing history daily. The author fixed three compounding issues: ProjectionExpression to fetch only needed fields (80-90% payload reduction), S3-based state tracking to query only new data since last run, and incremental processing to avoid full table scans. Result: calculatePartnerNotificationStats reduced from timeout to sub-second execution.

• •Lambda functions hit 15-minute timeout due to inefficient DynamoDB queries and full daily history recomputation
• •Three fixes: ProjectionExpression for lean payloads, S3 delta tracking, incremental queries instead of full scans
• •Performance: 900+ second runs reduced to sub-second completion

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