ExRated, the OTP GenServer with the naughty name that allows you to rate-limit calls to any service that requires it. For example, rate-limit calls to your favorite API which requires no more than `limit` API calls within a `scale` milliseconds time window.

api, bucket, elixir, otp, phoenix, rate, rate-limit, rate-limiting, rate-limits



ExRated is:

  1. A port of the Erlang 'raterlimiter' project to Elixir.
  2. An OTP GenServer process that allows you to rate limit calls to something like an external API.
  3. The Hex.pm package with the naughty name.

You can learn more about the concept for this rate limiter in the Token Bucket article on Wikipedia

If you use the PhoenixFramework there is also a great blog post on Rate Limiting a Phoenix API by danielberkompas describing how to write a plug to use ExRated in your own API. Its fast and its easy.


Call the ExRated application with ExRated.check_rate/3. This function takes three arguments:

  1. A bucket name (String). You can have as many buckets as you need.
  2. A scale (Integer). The time scale in milliseconds that the bucket is valid for.
  3. A limit (Integer). How many actions you want to limit your app to in the time scale provided.

For example, if you have to enforce a rate limit of no more than 5 calls in 10 seconds to your API:

iex> ExRated.check_rate("my-rate-limited-api", 10_000, 5)
{:ok, 1}

The ExRated.check_rate function will return an {:ok, Integer} tuple if its OK to proceed with your rate limited function. The Integer returned is the current value of the incrementing counter showing how many times in the time scale window your function has already been called. If you are over limit a {:error, Integer} tuple will be returned where the Integer is always the limit you have specified in the function call.

Call the ExRated application with ExRated.inspect_bucket/3. This function takes the same three arguments as check_rate:

For example, if you want to inspect the bucket for your API:

iex> ExRated.inspect_bucket("my-rate-limited-api", 10_000, 5)
{0, 5, 2483, nil, nil}
iex> ExRated.check_rate("my-rate-limited-api", 10_000, 5)
{:ok, 1}
iex> ExRated.inspect_bucket("my-rate-limited-api", 10_000, 5)
{1, 4, 723, 1450282268397, 1450282268397}

The ExRated.inspect_bucket function will return a {count, count_remaining, ms_to_next_bucket, created_at, updated_at} tuple, count and count_remaining are integers, ms_to_next_bucket is the number of milliseconds before the bucket resets, created_at and updated_at are timestamps in millseconds.

Call the ExRated application with ExRated.delete_bucket/1. This function takes one argument:

  1. A bucket name (String). You can have as many buckets as you need.

For example, if you want to reset the counter for your API:

iex> ExRated.delete_bucket("my-rate-limited-api")

The ExRated.delete_bucket function will return an :ok on success or :error if the bucket doesn't exist


You can use ExRated in your projects in two steps:

  1. Add ExRated to your mix.exs dependencies:

    def deps do
      [{:ex_rated, "~> 1.2"}]
  2. List :ex_rated in your application dependencies:

    def application do
      [applications: [:ex_rated]]

You can also start the GenServer manually, and pass it custom config, with something like:

{:ok, pid} = GenServer.start_link(ExRated, [ {:timeout, 10_000}, {:cleanup_rate, 10_000}, {:ets_table_name, :ex_rated_buckets}, {:persistent, false} ], [name: :ex_rated])

Where the args and their defaults are:

{:timeout, 90_000_000} : buckets older than this in milliseconds will be automatically pruned.

{:cleanup_rate, 60_000} : how often, in milliseconds, the bucket pruning process will be run.

{:ets_table_name, :ex_rated_buckets} : The atom name of the ETS table.

{:persistent, false} : Whether to persist ETS table to disk with DETS on server stop/restart.

[name: :ex_rated] : The registered name of the ExRated GenServer.


It is important that the OTP doesn't get automatically started by Mix.

mix test --no-start

Is it fast?

You can use the Benchfella library to do a quick performance test.

On a 2017 Macbook Pro I can do 1,000,000 checks, averaging 2.31 µs/op.

$ mix bench
  duration:      1.0 s

## BasicBench
[15:09:49] 1/1: Basic Bench

Finished in 2.57 seconds

## BasicBench
benchmark na iterations   average time
Basic Bench     1000000   2.31 µs/op



  • Eliminate warning: function timestamp/1 is unused. [@brianberlin]


  • Automatic application inference
  • Update ex_doc dependency
  • Update minimum Elixer to 1.6+
  • Update "Rate Limiting" blogpost URL reference


  • Update ex2ms to v1.5


  • Fix compilation warnings. [@walkr]
  • Start app properly with no args [@walkr]
  • Modify start_link to be callable by Supervisor.Spec.worker fun [@walkr]


  • Update Elixir to v1.2
  • Update ex2ms to v1.4


  • Change ETS Table to private.
  • Change ETS table name to a non-test name.


  • Added {:persistent, false} option to server config to allow persisting data to disk.
  • Fixed minor compilation warning.


  • Added delete_bucket/1 function. Takes a bucket name and removes it now instead of waiting for pruning (Nick Sanders).
  • Added inspect_bucket/3 function. Returns metadata about buckets (Nick Sanders).


  • [BREAKING] Return {:error, limit} instead of {:fail, limit} to be a bit more idiomatic. Requires semver major version number change.
  • Support Elixir version 1.1 in addition to 1.0


  • ExRated internally calls :erlang.system_time(:milli_seconds) provided by the new Time API in OTP 18 and greater if available, and will fall back gracefully to the old :erlang.now() in older versions. Thanks to Mitchell Henke (mitchellhenke) for the enhancement.


ExRated source code is released under Apache 2 License. Check LICENSE file for more information.