balancer: add HashRingSelector (#1650)

Cargo.toml

@@ -14,6 +14,7 @@ figment = { version = "0.10.15", features = ["toml", "env"] }
 futures-util = "0.3.30"
 harness = { path = "crates/harness" }
 harness_macros = { path = "crates/harness_macros" }
+hashring = "0.3.3"
 hickory-resolver = { version = "0.24.0", features = ["system-config"] }
 humantime-serde = "1.1"
 hyper = { version = "1.2.0", features = ["full"] }

crates/ott-balancer/Cargo.toml

@@ -12,6 +12,7 @@ console-subscriber.workspace = true
 enum_dispatch.workspace = true
 figment.workspace = true
 futures-util.workspace = true
+hashring.workspace = true
 hyper.workspace = true
 hyper-util.workspace = true
 http-body-util.workspace = true

crates/ott-balancer/src/selection.rs

@@ -1,5 +1,6 @@
 use crate::monolith::BalancerMonolith;
 use enum_dispatch::enum_dispatch;
+use hashring::HashRing;
 use ott_balancer_protocol::RoomName;
 use rand::seq::IteratorRandom;
 use serde::Deserialize;
@@ -28,6 +29,7 @@ pub trait MonolithSelection: std::fmt::Debug {
 #[enum_dispatch]
 pub enum MonolithSelectionStrategy {
     MinRooms(MinRoomsSelector),
+    HashRing(HashRingSelector),
 }
 
 impl Default for MonolithSelectionStrategy {
@@ -57,6 +59,28 @@ impl MonolithSelection for MinRoomsSelector {
     }
 }
 
+#[derive(Debug, Default, Deserialize, Copy, Clone)]
+pub struct HashRingSelector;
+
+impl MonolithSelection for HashRingSelector {
+    fn select_monolith<'a>(
+        &'a self,
+        room: &RoomName,
+        monoliths: Vec<&'a BalancerMonolith>,
+    ) -> anyhow::Result<&BalancerMonolith> {
+        let mut ring = HashRing::new();
+        ring.batch_add(monoliths.iter().map(|m| m.id()).collect());
+
+        let id = ring.get(room).ok_or(anyhow::anyhow!("ring hash empty"))?;
+        let i = monoliths
+            .iter()
+            .position(|m| m.id() == *id)
+            .ok_or(anyhow::anyhow!("monolith not found"))?;
+
+        Ok(monoliths[i])
+    }
+}
+
 #[cfg(test)]
 mod test {
     use std::net::Ipv4Addr;