Behaviour implemented by every concrete sketch module.

This is the unified contract referenced by ExDataSketch.sketches/0 and by the top-level facade in ExDataSketch (new/2, update/2, merge/2, and so on). Elixir's @behaviour has no runtime dispatch machinery of its own -- implemented?/1 below is the only place this module actually calls function_exported?/3, to verify a candidate module's conformance (used by the sketches/0 registry test). Everywhere else -- the facade, and ExDataSketch.GenStage.SketchConsumer's direct sketch_module.merge/2/ update_many/2 calls -- calls the concrete module's function directly, relying on the behaviour purely as a compile-time-checked contract that the call will succeed, not as something with its own dispatch behavior. ExDataSketch.FilterChain additionally implements this behaviour itself (@behaviour ExDataSketch.Sketch) so the facade and ExDataSketch.Server can dispatch on it the same way they do for any other sketch module.

Which functions are required

Every concrete sketch module (ExDataSketch.HLL, ExDataSketch.Bloom, and so on) implements serialize/1, deserialize/1, size_bytes/1, and capabilities/0 unconditionally. new/1, update/2, update_many/2, and merge/2 are declared @optional_callbacks because not every family supports every operation with that exact shape:

• ExDataSketch.XorFilter is immutable once built (build/2) and has no update/2, update_many/2, or merge/2.
• ExDataSketch.FilterChain constructs with new/0 (no options), not new/1.
• ExDataSketch.Cuckoo has no merge/2 (per-bucket state is not associatively mergeable).

A module's capabilities/0 is the source of truth for which of these optional operations it actually supports at runtime; @optional_callbacks only relaxes the compile-time contract so dialyzer does not require every module to implement every function.

capabilities/0 returns a MapSet, not a boolean map

Seven filter modules (ExDataSketch.Bloom, Cuckoo, Quotient, CQF, XorFilter, IBLT, FilterChain) already ship capabilities/0 returning a MapSet.t(atom()) of supported operation names (:put, :merge, :member?, :delete, and so on), consumed today by ExDataSketch.FilterChain's stage-composition checks. This behaviour standardizes on that existing, tested shape for all 16 families rather than introducing a second, incompatible capabilities/0 return type.

Adding @behaviour ExDataSketch.Sketch to a module

defmodule MyApp.Sketch do
  @behaviour ExDataSketch.Sketch

  defstruct [:state]

  @impl true
  def new(_opts \\ []), do: %__MODULE__{state: <<>>}

  @impl true
  def update(sketch, _item), do: sketch

  @impl true
  def serialize(%__MODULE__{state: state}), do: state

  @impl true
  def deserialize(binary), do: {:ok, %__MODULE__{state: binary}}

  @impl true
  def size_bytes(%__MODULE__{state: state}), do: byte_size(state)

  @impl true
  def capabilities, do: MapSet.new([:new, :update, :serialize, :deserialize])
end

Examples

iex> ExDataSketch.Sketch.implemented?(ExDataSketch.Bloom)
true

iex> ExDataSketch.Sketch.implemented?(String)
false