The Negative Shard — Java Bug Hunt

Briefing

Inspired by a bug that has paged on-call engineers at every large company: routing by Math.abs(key.hashCode()) % shards works for years — until a key whose hash is exactly Integer.MIN_VALUE arrives. Math.abs of that number is still negative, and the router indexes shard -8.

Fun fact baked into the tests: the string "polygenelubricants" hashes to exactly Integer.MIN_VALUE.

Bug report

BUG-SHARD-8 · Priority: High · Reported by: storage on-call

shardFor(key, shardCount) must ALWAYS return a value in [0, shardCount) and be stable per key.

Observed: ArrayIndexOutOfBoundsException: -8, once in a blue moon, always the same mystery user.

Logs

[router] key="polygenelubricants" -> shard -8 of 16

The code as shipped

ShardRouter.java (editable)

class ShardRouter {
    static int shardFor(String key, int shardCount) {
        return Math.abs(key.hashCode()) % shardCount;
    }
}