What is a hot partition, and how do you avoid one?

Summary

A hot partition receives far more traffic than others, usually because many requests share one partition key value (a celebrity user, today's date as key, a global counter). It throttles even when the table has spare capacity overall. Avoid it with high-cardinality keys and, for unavoidable hot keys, write sharding or caching.

Longer Answer

Techniques:

• Choose keys with many values and even access (user ID, not country).
• Write sharding: append a suffix (EVENT#2026-10-17#7, suffix 0-9 chosen randomly or computed from an attribute) and read all shards in parallel.
• Cache hot reads (DAX or an application cache).
• Aggregate writes (buffer counters in memory or a queue) instead of writing one item per event.

CloudWatch Contributor Insights shows the most accessed keys, which is how you find hot partitions in practice.

Key Points

• Traffic concentrated on one key.
• High-cardinality keys.
• Write sharding with suffixes.
• Cache hot reads; Contributor Insights to find them.