# `ExDataSketch.GenStage`
[🔗](https://github.com/thanos/ex_data_sketch/blob/main/lib/ex_data_sketch/gen_stage.ex#L1)

GenStage integration for sketch aggregation.

This module provides consumer and producer stages that accumulate sketch
data from event streams. GenStage is an optional dependency; the
sub-modules are only compiled when `:gen_stage` is available.

## Dependency

This module requires the `:gen_stage` package. If GenStage is not
available, the sub-modules will not be compiled and calls to
`require_gen_stage!/0` will raise an error.

## Module Overview

- `ExDataSketch.GenStage.SketchConsumer` -- A GenStage consumer that
  accumulates events into a sketch and supports periodic flushing.
- `ExDataSketch.GenStage.SketchProducer` -- A GenStage producer that
  emits merged sketches on demand.
- `ExDataSketch.GenStage.SketchStage` -- A combined producer-consumer
  that accumulates events and periodically emits merged sketches.

## Quick Start

    # Consumer that builds an HLL from events
    {:ok, consumer} = GenStage.SketchConsumer.start_link(
      sketch_module: ExDataSketch.HLL,
      sketch_opts: [p: 14],
      subscribe_to: [{some_producer, max_demand: 100}]
    )

    # Read current estimate
    GenStage.SketchConsumer.estimate(consumer)

    # Flush and reset
    flushed = GenStage.SketchConsumer.flush(consumer)

See individual module documentation for details.

# `available?`
*since 0.9.0* 

```elixir
@spec available?() :: boolean()
```

Returns whether GenStage is available at runtime.

Checks compile-time availability and runtime configuration. Setting
`gen_stage: true` in config does not override compile-time unavailability.

## Examples

    iex> is_boolean(ExDataSketch.GenStage.available?())
    true

---

*Consult [api-reference.md](api-reference.md) for complete listing*