# ExHashRing
[](https://github.com/discord/ex_hash_ring/actions)
[](https://hex.pm/packages/ex_hash_ring)
[](https://hex.pm/packages/ex_hash_ring)
[](https://hexdocs.pm/ex_hash_ring)
A pure Elixir consistent hash ring implemention based on the excellent [C hash-ring lib](https://github.com/chrismoos/hash-ring) by [Chris Moos](https://github.com/chrismoos).
ExHashRing is a production ready library actively maintained and in use at [Discord](https://discord.com).
ExHashRing provides the following features.
- Lookup optimized ring storage. A ring is stored in an [ETS table](https://erlang.org/doc/man/ets.html) 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`.
```elixir
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](/pages/upgrade.md) for instructions.
## Quickstart
Each Ring is managed by a GenServer, here's an example of starting an empty Ring.
```elixir
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`
```elixir
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`
```elixir
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.
```elixir
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](https://hexdocs.pm/ex_hash_ring)
## 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](LICENSE). Check [LICENSE](LICENSE) file for more information.
