Add compatibility with Docker dns servers

crates/ott-balancer/src/discovery/dns.rs

@@ -0,0 +1,51 @@
+use async_trait::async_trait;
+use tracing::info;
+use trust_dns_resolver::TokioAsyncResolver;
+
+use super::*;
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct DockerDiscoveryConfig {
+    /// The port that monoliths should be listening on for load balancer connections.
+    pub monolith_port: u16,
+    pub dns_server: Option<String>,
+}
+
+pub struct DockerMonolithDiscoverer {
+    config: DockerDiscoveryConfig,
+    query: Option<String>,
+}
+
+impl DockerMonolithDiscoverer {
+    pub fn new(config: DockerDiscoveryConfig) -> Self {
+        info!(
+            "Creating DockerMonolithDiscoverer, Docker DNS server: {}",
+            &config.docker_dns_server
+        );
+        let query = format!("{}", &config.dns_server);
+        Self { config, query }
+    }
+}
+
+#[async_trait]
+impl MonolithDiscoverer for DockerMonolithDiscoverer {
+    async fn discover(&mut self) -> anyhow::Result<Vec<MonolithConnectionConfig>> {
+        let resolver =
+            TokioAsyncResolver::tokio_from_system_conf().expect("failed to create resolver");
+
+        let lookup = resolver.ipv4_lookup(&self.query).await?;
+        let monoliths = lookup
+            .iter()
+            .map(|ip| MonolithConnectionConfig {
+                host: HostOrIp::Ip(IpAddr::V4(*ip)),
+                port: self.config.monolith_port,
+            })
+            .collect::<Vec<_>>();
+
+        Ok(monoliths)
+    }
+
+    fn mode(&self) -> DiscoveryMode {
+        DiscoveryMode::Polling(Duration::from_secs(10))
+    }
+}