libring - A fast consistent hash ring for Elixir

This library implements a stateful consistent hash ring. It’s extremely fast (in benchmarks it’s faster than all other implementations I’ve tested against), it has no external dependencies, and is written in Elixir.

The algorithm is based on libketama. Nodes on the ring are broken into shards and each one is assigned an integer value in the keyspace, which is the set of integers from 1 to 2^32-1. The distribution of these shards is random.

Keys are then mapped to a shard by converting the key to a binary, hashing it with SHA-256, converting the hash to an integer in the keyspace, then finding the shard which is assigned the next highest value, if there is no next highest value, the lowest integer is used, which is how the “ring” is formed.

This implementation uses a general balanced tree, via Erlang’s :gb_tree module. Each shard is inserted into the tree, and we use this data structure to efficiently lookup next-highest key and smallest key. I suspect this is why libring is faster than other implementations I’ve benchmarked against.

Usage

Add :libring to your deps, and run mix deps.get.

def deps do
  [{:libring, "~> 0.1"}]
end

You have two choices for managing hash rings in your application:

HashRing

This API works with the raw ring datastructure. It is the fastest implementation, and is best suited for when you have a single process which will need to access the ring, and which can hold the ring in it’s internal state.

ring = HashRing.new()
       |> HashRing.add_node("a")
       |> HashRing.add_node("b")

"a" = HashRing.key_to_node({:myworker, 123})

You can also specify the weight of each node, and add nodes in bulk:

ring = HashRing.new()
       |> HashRing.add_nodes(["a", {"b", 64}])
       |> HashRing.add_node("c", 200)
"c" = HashRing.key_to_node({:myworker, 123})

HashRing.Managed

This API works with rings which are held in the internal state of a GenServer process. It supports the same API as HashRing. Because of this, there is a performance overhead due to the messaging, and the GenServer can be a potential bottleneck. If this is the case you are better exploring ways to use the raw HashRing API. However this API is best suited for situations where you have multiple processes accessing the ring, or need to maintain multiple rings.

NOTE: You must have the :libring application started to use this API.

{:ok, pid} = HashRing.Managed.new(:myring)
:ok = HashRing.Managed.add_node(:myring, "a")
:ok = HashRing.Managed.add_node(:myring, "b", 64)
:ok = HashRing.Managed.add_node(:myring, "c", 200)
"c" = HashRing.Managed.key_to_node(:myring, {:myworker, 123})

If your rings map 1:1 with Erlang cluster membership, you can configure rings to automatically monitor node up/down events and update the hash ring accordingly, filtered by either a whitelist or blacklist. You configure this at the ring level in your config.exs, like so:

config :libring,
  rings: [
    myring: [monitor_nodes: true,
             node_blacklist: [~r/^remsh.*$/]]
  ]

If you provide a whitelist, the blacklist will have no effect, and only nodes matching the whitelist will be added. If you do not provide a whitelist, the blacklist will be used to filter nodes. If you do not provide either, a default blacklist containing the ~r/^remsh.*$/ pattern from the example above, which is a good default to prevent remote shell sessions from causing the ring to change.

The whitelist and blacklist only have an effect when monitor_nodes: true.

Configuration

Below is an example configuration:

config :libring,
  rings: [
    # A ring which automatically changes based on Erlang cluster membership,
    # but does not allow nodes named "a" or "remsh*" to be added to the ring
    ring_a: [monitor_nodes: true,
             node_blacklist: ["a", ~r/^remsh.*$/]],
    # A ring which is composed of three nodes, of which "c" has a non-default weight of 200
    # The default weight is 128
    ring_b: [nodes: ["a", "b", {"c", 200}]]
  ]

License

MIT

Please see the LICENSE file for more info.