Hot Partition
also called Hot Shard, Hot Key
One partition receiving disproportionate traffic, so the system saturates at a fraction of its aggregate capacity.
The characteristic failure of any partitioned system, and the reason "we have twenty shards" tells you nothing on its own. Aggregate capacity is irrelevant if one partition holds the celebrity, the flash-sale product, the largest tenant, or the current time bucket.
Three causes worth distinguishing because they need different fixes. Key skew — the data is unevenly distributed (a power-law tenant). Access skew — the data is even but the traffic is not (one popular product). Temporal skew — a monotonically increasing key such as a timestamp means all writes land on the newest partition.
Fixes, matched to cause: for temporal skew, prefix or salt the key so writes spread; for access skew, add a caching tier with request coalescing, or replicate the hot item; for key skew, use a directory-based mapping so the large tenant can be moved to its own shard, or sub-shard it.
The design-time habit: before choosing a partition key, ask what the 99.9th percentile partition looks like, not the average one.