---
title: Kafka Golang Hash Strategy
description: Figuring out how HashPartitioner works
slug: kafka-golang-hash-strategy
lastmod: 2024-11-16
tags:
- "kafka"
- "events"
- "messaging"
- "kafka-go"
- "sarama"
- "golang"
---
Nowadays, the default partition is apparently [UniformStickyPartitioner](https://cwiki.apache.org/confluence/display/KAFKA/KIP-794%3A+Strictly+Uniform+Sticky+Partitioner) although at the time of writing [IBM/sarama](https://github.com/IBM/sarama) defaults to [HashPartitioner](https://github.com/IBM/sarama/blob/main/config.go#L529) and [segmentio/kafka-go](https://github.com/segmentio/kafka-go) has round-robin as [its default](https://github.com/segmentio/kafka-go/blob/main/writer.go#L96).

When it comes to the hash strategy, I recently found myself wondering how to determine the assigned partition for a program using kafka-go's hash partition.

This lead to me slicing up the [default hasher implementation](https://github.com/segmentio/kafka-go/blob/main/balancer.go#L153-L181) into a small [Go playground program](https://go.dev/play/p/k-3AgdzKjB0).

**NOTE**: While I've manually run the [testcases](https://github.com/segmentio/kafka-go/blob/main/balancer_test.go#L10-L50) through this small program and they all passed, I haven't exercised it in any depth. It's just a reference for myself in the future. You should probably use the actual library directly.

```go
package main

import (
	"fmt"
	"hash"
	"hash/fnv"
)

func main() {
	partitions := generatePartitions(3)
	key := "blah"

	hasher := fnv.New32a().(hash.Hash32)
	hasher.Reset()

	if _, err := hasher.Write([]byte(key)); err != nil {
		panic(err)
	}

	partition := int32(hasher.Sum32()) % int32(len(partitions))
	if partition < 0 {
		partition = -partition
	}
	fmt.Println(partition)
}

func generatePartitions(partitionCount int) []int {
	partitions := []int{}
	for i := 0; i <= partitionCount-1; i++ {
		partitions = append(partitions, i)
	}
	return partitions
}
```