ExDataSketch

Production-grade streaming data sketching algorithms for Elixir.

ExDataSketch provides probabilistic data structures for approximate counting, frequency estimation, quantile computation, heavy-hitter detection, membership testing with deletion, and set reconciliation on streaming data. All sketch state is stored as Elixir-owned binaries, enabling straightforward serialization, distribution, and persistence.

Supported Algorithms

Capability Matrix

*Bloom count is a popcount-based cardinality estimate.

When to Choose

• Bloom -- Default choice for membership testing. No deletion needed, mergeable.
• Cuckoo -- Need to delete items. Better space efficiency than Bloom at low FPR.
• Quotient -- Need deletion and merge. Good when filter resizing may be needed.
• CQF -- Need to count how many times each item was inserted (multiset).
• XorFilter -- Static set known at build time. Smallest footprint, fastest lookups.
• IBLT -- Need to find differences between two sets (reconciliation).

Installation

Add ex_data_sketch to your list of dependencies in mix.exs:

def deps do
  [
    {:ex_data_sketch, "~> 0.7.1"}
  ]
end

Quick Start

# HLL: count distinct elements
hll = ExDataSketch.HLL.new() |> ExDataSketch.HLL.update_many(1..100_000)
ExDataSketch.HLL.estimate(hll)  # ~100_000

# KLL: quantile estimation
kll = ExDataSketch.KLL.new() |> ExDataSketch.KLL.update_many(1..100_000)
ExDataSketch.KLL.quantile(kll, 0.5)   # approximate median (~50_000)
ExDataSketch.KLL.quantile(kll, 0.99)  # 99th percentile (~99_000)

# Bloom: membership testing
bloom = ExDataSketch.Bloom.new(capacity: 100_000)
bloom = ExDataSketch.Bloom.put_many(bloom, 1..50_000)
ExDataSketch.Bloom.member?(bloom, 42)      # true
ExDataSketch.Bloom.member?(bloom, 99_999)  # false (probably)

# Cuckoo: membership testing with deletion
cuckoo = ExDataSketch.Cuckoo.new(capacity: 100_000)
{:ok, cuckoo} = ExDataSketch.Cuckoo.put(cuckoo, "user_42")
ExDataSketch.Cuckoo.member?(cuckoo, "user_42")  # true
{:ok, cuckoo} = ExDataSketch.Cuckoo.delete(cuckoo, "user_42")
ExDataSketch.Cuckoo.member?(cuckoo, "user_42")  # false

See the Quick Start Guide for more examples.

Documentation

Full documentation is available at HexDocs.

Architecture

• Binary state: All sketch state is canonical Elixir binaries. No opaque NIF resources.
• Backend system: Pure Elixir reference implementation with optional Rust NIF acceleration. The Rust backend falls back to Pure automatically when unavailable.
• Serialization: ExDataSketch-native format (EXSK) for all sketches, plus Apache DataSketches CompactSketch interop for Theta.
• Deterministic hashing: Stable 64-bit hash (ExDataSketch.Hash) for reproducible results.
• Backend parity: Both backends produce byte-identical serialized output for the same inputs.

Compatibility and Stability

The following guarantees apply within the v0.x release series:

• EXSK serialization: The ExDataSketch-native binary format is stable. Binaries produced by any v0.x release can be deserialized by any other v0.x release.
• Pure vs Rust parity: Given identical inputs, both backends produce byte-identical serialized state and identical estimates.
• Deterministic output: The same input sequence always produces the same sketch state and estimate, regardless of backend.

Not guaranteed:

• Cross-language interop: Only Theta supports Apache DataSketches CompactSketch format. HLL and CMS DataSketches interop is not implemented.
• Performance stability: Benchmark results may vary across hardware and OTP versions.
• EXSK format across major versions: The binary format may change in future major releases.

Development

# Get dependencies
mix deps.get

# Run tests with coverage
mix test --cover

# Run lints
mix lint

# Run benchmarks
mix bench

# Generate docs
mix docs

Roadmap

License

MIT License. See LICENSE for details.