ExHashRing

A pure Elixir consistent hash ring implemention based on the excellent C hash-ring lib by Chris Moos.

ExHashRing is a production ready library actively maintained and in use at Discord.

ExHashRing provides the following features.

Lookup optimized ring storage. A ring is stored in an ETS table which provides excellent lookup performance.
Key overrides that allow the client to pin a key to a member.
Configurable replica count for virtual nodes.
Configurable history that allows for stable lookups over time.

Installation

Add it to mix.exs.

defp deps do
  [{:ex_hash_ring, "~> 6.0"}]
end

Upgrading to 6.0.0

Version 6.0.0 introduces a number of breaking changes. Refer to the Upgrade Guide for instructions.

Quickstart

Each Ring is managed by a GenServer, here's an example of starting an empty Ring.

iex(1)> alias ExHashRing.Ring
ExHashRing.Ring
iex(2)> {:ok, ring} = Ring.start_link()
{:ok, #PID<0.166.0>}

We can add a single node with add_node/2

iex(3)> Ring.add_node(ring, "a")
{:ok, [{"a", 512}]}

The 512 above is the number of replicas for this node. Since we did not specify a custom number of replicas, it was added with the default for this Ring, which itself defaults to 512. We can control the number of default replicas when we start_link the Ring and we can control the number of replicas on a per-node basis.

We can add another node with a custom replica count with add_node/3

iex(4)> Ring.add_node(ring, "b", 100)
{:ok, [{"b", 100}, {"a", 512}]}

Now that we have some nodes we can use our Ring to map keys to nodes with the find_node/2 function.

iex(5)> Ring.find_node(ring, "key1")
{:ok, "a"}
iex(6)> Ring.find_node(ring, "key37")
{:ok, "b"}

Documentation

The Quickstart above just scratches the surface of the functionality that ExHashRing provides. For more details see the HexDocs

Configuration

ExHashRing exposes some configuration options under the :ex_hash_ring key.

Key	Description	Default
`:depth`	Default history depth for new rings	1
`:gc_delay`	The amount of time, in milliseconds, to wait before garbage collecting stale generations	10_000
`:replicas`	Default replicas setting for new rings	512

License

Hash Ring is released under the MIT License. Check LICENSE file for more information.