Implementing Bloom Filters in Go for Efficient Caching

In cache design, we often encounter a troublesome problem: a large number of “invalid queries” put tremendous pressure on the database. For example, when a user requests data that doesn’t exist, we typically query the database, which returns “not found.” If there are many such requests, the database gets busy handling these meaningless queries, affecting system performance. So, is there a way to know in advance whether data “might exist” before querying? This is where Bloom filters come into play.

Problems in Traditional Cache Scenarios

Imagine the following scenario:

Our system has a cache layer (Redis) and a database layer (MySQL).
When a user requests some data, we first check the cache. If it’s a cache hit, we return the result directly.
If the data isn’t in the cache, we query the database. If the database doesn’t have it either, we return “not found” to the user.

At first glance, this design seems reasonable. But in practice, you may encounter a large number of invalid queries:

User requests data with ID=99999999
    -> Not found in cache
    -> Query database for ID=99999999
    -> Database returns: not found

If users repeatedly request data that simply doesn’t exist, you end up with:

Cache not working: Every request queries the database, making the cache layer pointless.
Excessive database load: Numerous invalid requests slow down database response.

A common optimization is to filter out invalid queries in advance, so data that we already know doesn’t exist is returned immediately, without querying the database. This is exactly where Bloom filters shine.

What is a Bloom Filter?

A Bloom filter is an efficient set-membership query algorithm. It can quickly determine whether a value might exist or definitely does not exist. Simply put:

If the Bloom filter tells you “does not exist,” then it truly doesn’t exist, and you can return an error right away—no need to query the database.
If the Bloom filter tells you “might exist,” then you query the database for confirmation.

The underlying logic of a Bloom filter is simple:

Use multiple hash functions to map data into a fixed-size bit array.
When querying, compute the same hashes for the target data and check if all the corresponding bits are set to 1.
If any of those bits are 0, the data definitely does not exist.
If all bits are 1, the data might exist (with a certain false positive rate).

Advantages:

Saves memory—occupies less space than a traditional hash table.
Fast queries—time complexity close to O(1).
Efficient filtering—reduces database pressure.

Disadvantages:

Possible false positives (but the false positive rate can be reduced by tuning the number of hash functions).
Cannot delete data (generally used for big data streams or cache scenarios).

The Data Structure of a Bloom Filter

The core data structures of a Bloom filter are two components:

Bit array: Used to record whether a certain value exists; all bits are initialized to 0.
Multiple hash functions: Used to calculate the bit positions corresponding to the data.

Example:

Insert "Leap"; after hash calculations, it maps to positions 2, 5, 8, and 9 in the bit array:
Index:   0  1  2  3  4  5  6  7  8  9
Value:   0  0  1  0  0  1  0  0  1  1

When querying "Leap":

Calculate the hashes for "Leap" and find that positions 2, 5, 8, and 9 are all 1, indicating it might exist.
Calculate the hashes for "cell" and find some bits are 0, so it’s immediately returned as “does not exist”.

Implementing a Bloom Filter in Go

Below is a Bloom filter implementation using Go:

The BloomFilter struct.
The constructor NewBloomFilter, which automatically calculates the optimal bit array size (m) and number of hash functions (k) based on the expected number of elements and the acceptable false positive rate.
The constructor NewBloomFilterWithMK, which allows you to specify m and k directly.
The Add method, for adding elements.
The MightContain method, for checking whether an element might exist (there may be false positives, but never false negatives).
The internal getHashes method, which uses double hashing to generate k hash values. Here, we use two variants of the FNV-1a hash algorithm as base hashes.

Bloom Filter Structure

package main

import (
  "fmt"
  "hash/fnv"
)

// Bloom filter struct
type BloomFilter struct {
  m    uint64 // Size of bit array (number of bits)
  k    uint64 // Number of hash functions
  bits []byte // Bit array
}

// Create a Bloom filter
func NewBloomFilter(expectedN uint64, falsePositiveRate float64) *BloomFilter {
  m, k := estimateParameters(expectedN, falsePositiveRate)
  if m == 0 || k == 0 {
    panic("Invalid parameters for Bloom filter: m or k is zero")
  }
  return NewBloomFilterWithMK(m, k)
}

// estimateParameters calculates the optimal m and k based on the expected number of elements n and the false positive rate p
// m = - (n * ln(p)) / (ln(2))^2
// k = (m / n) * ln(2)
func estimateParameters(n uint64, p float64) (m uint64, k uint64) {
  if n == 0 || p <= 0 || p >= 1 {
    return 1000, 10
  }
  mFloat := -(float64(n) * math.Log(p)) / (math.Ln2 * math.Ln2)
  kFloat := (mFloat / float64(n)) * math.Ln2

  m = uint64(math.Ceil(mFloat))
  k = uint64(math.Ceil(kFloat))

  if k < 1 {
    k = 1
  }
  return
}

// NewBloomFilterWithMK creates a Bloom filter using the specified m and k
func NewBloomFilterWithMK(m, k uint64) *BloomFilter {
  if m == 0 || k == 0 {
    panic("Invalid parameters for Bloom filter: m or k is zero")
  }
  numBytes := (m + 7) / 8
  return &BloomFilter{
    m:    m,
    k:    k,
    bits: make([]byte, numBytes),
  }
}

// getHashes uses double hashing to generate k hash values for the data
func (bf *BloomFilter) getHashes(data []byte) []uint64 {
  hashes := make([]uint64, bf.k)

  // Use two different versions (or seeds) of FNV-1a as base hash functions
  h1 := fnv.New64()
  h1.Write(data)
  hash1Val := h1.Sum64()

  h2 := fnv.New64a()
  h2.Write(data)
  hash2Val := h2.Sum64()

  for i := uint64(0); i < bf.k; i++ {
    if hash2Val == 0 && i > 0 {
      hash2Val = 1
    }
    hashes[i] = (hash1Val + i*hash2Val) % bf.m
  }
  return hashes
}

Inserting Data

// Insert data into the Bloom filter
func (bf *BloomFilter) Add(data []byte) {
  hashes := bf.getHashes(data)
  for _, h := range hashes {
    byteIndex := h / 8                     // Find the byte index
    bitOffset := h % 8                     // Find the bit offset within the byte
    bf.bits[byteIndex] |= (1 << bitOffset) // Set the corresponding position to 1
  }
}

Querying Data

// Check whether data might exist
func (bf *BloomFilter) MightContain(data []byte) bool {
  hashes := bf.getHashes(data)
  for _, h := range hashes {
    byteIndex := h / 8
    bitOffset := h % 8
    if (bf.bits[byteIndex] & (1 << bitOffset)) == 0 {
      // If any bit corresponding to a hash is 0, the element definitely does not exist
      return false
    }
  }
  // If all bits corresponding to hashes are 1, the element might exist
  return true
}

Resetting the Bloom Filter

// Reset clears the Bloom filter (sets all bits to 0)
func (bf *BloomFilter) Reset() {
  for i := range bf.bits {
    bf.bits[i] = 0
  }
}

Test Code

func main() {
  // Example: expect 1000 elements, false positive rate 1%
  expectedN := uint64(1000)
  falsePositiveRate := 0.01

  m, k := estimateParameters(expectedN, falsePositiveRate)
  fmt.Printf("Estimated parameters: m = %d, k = %d\n", m, k)

  bf := NewBloomFilter(expectedN, falsePositiveRate)
  // Or bf := NewBloomFilterWithMK(m, k)

  // Add some elements
  item1 := []byte("apple")
  item2 := []byte("banana")
  item3 := []byte("cherry")

  bf.Add(item1)
  bf.Add(item2)

  fmt.Printf("MightContain 'apple': %t\n", bf.MightContain(item1))           // true
  fmt.Printf("MightContain 'banana': %t\n", bf.MightContain(item2))          // true
  fmt.Printf("MightContain 'cherry': %t\n", bf.MightContain(item3))          // false (should be false, as it wasn't added)
  fmt.Printf("MightContain 'grape': %t\n", bf.MightContain([]byte("grape"))) // false (should also be false)

  bf.Reset()
  fmt.Println("After Reset:")
  fmt.Printf("MightContain 'apple': %t\n", bf.MightContain(item1)) // false
}

Summary

A Bloom filter can help us in cache systems by:

Reducing invalid database queries: first judge whether the data might exist, to avoid directly accessing the database.
Saving storage space: uses less memory than hash tables, suitable for large-scale data.
Improving query efficiency: O(1) time complexity for queries, no need to traverse the entire dataset.

We used Go to implement a simple Bloom filter, applying it to cache scenarios to optimize database load.

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