Hammer

A rate-limiter for Elixir, with pluggable storage backends.

Installation

Hammer is available in Hex, the package can be installed by adding hammer to your list of dependencies in mix.exs:

def deps do
  [{:hammer, "~> 0.1.0"}]
end

Documentation

On hexdocs: https://hexdocs.pm/hammer/frontpage.html

The Tutorial is an especially good place to start.

Usage

To use Hammer, you need to do two things:

• Start a backend process
• use the Hammer module

The example below combines both in a MyApp.RateLimiter module:

defmodule MyApp.RateLimiter do
  use Supervisor
  use Hammer, backend: Hammer.Backend.ETS

  def start_link() do
    Supervisor.start_link(__MODULE__, :ok)
  end

  def init(:ok) do
    children = [
      worker(Hammer.Backend.ETS, [[expiry_ms: 1000 * 60 * 60
                                   cleanup_interval_ms: 1000 * 60 * 10]]),
    ]
    supervise(children, strategy: :one_for_one, name: MyApp.RateLimiter)
  end
end

The Hammer module provides the following functions (via use):

• check_rate(id, scale_ms, limit)
• inspect_bucket(id, scale_ms, limit)
• delete_buckets(id)

The rate-limiter is then used in the app by calling the check_rate function:

defmodule MyApp.VideoUpload do

  alias MyApp.RateLimiter

  def upload(video_data, user_id) do
    case RateLimiter.check_rate("upload_video:#{user_id}", 60_000, 5) do
      {:allow, _count} ->
        # upload the video, somehow
      {:deny, _limit} ->
        # deny the request
    end
  end

end

See the Tutorial for more.

See the Hammer Testbed app for an example of using Hammer in a Phoenix application.

Available Backends

• Hammer.Backend.ETS (provided with Hammer)
• Hammer.Backend.Redis