chore(apollo_l1_endpoint_monitor): implement the core of l1 endpoint monitor

crates/apollo_l1_endpoint_monitor/Cargo.toml

@@ -6,6 +6,11 @@ repository.workspace = true
 license.workspace = true
 
 [dependencies]
+alloy.workspace = true
+serde.workspace = true
+thiserror.workspace = true
+tracing.workspace = true
+url = { workspace = true, features = ["serde"] }
 
 [dev-dependencies]
 

crates/apollo_l1_endpoint_monitor/src/lib.rs

@@ -1 +1 @@
-
+pub mod monitor;

crates/apollo_l1_endpoint_monitor/src/monitor.rs

@@ -0,0 +1,69 @@
+use alloy::providers::{Provider, ProviderBuilder};
+use serde::{Deserialize, Serialize};
+use thiserror::Error;
+use tracing::{error, warn};
+use url::Url;
+
+type L1EndpointMonitorResult<T> = Result<T, L1EndpointMonitorError>;
+
+#[derive(Debug, Clone)]
+pub struct L1EndpointMonitor {
+    pub current_l1_endpoint_index: usize,
+    pub config: L1EndpointMonitorConfig,
+}
+
+impl L1EndpointMonitor {
+    /// Returns a functional L1 endpoint, or fails if all configured endpoints are non-operational.
+    /// The method cycles through the configured endpoints, starting from the currently selected one
+    /// and returns the first one that is operational.
+    pub async fn get_active_l1_endpoint(&mut self) -> L1EndpointMonitorResult<Url> {
+        let current_l1_endpoint_index = self.current_l1_endpoint_index;
+        // This check can be done async, instead of blocking the user, but this requires an
+        // additional "active" component or async task in our infra.
+        if self.is_operational(current_l1_endpoint_index).await {
+            return Ok(self.get_node_url(current_l1_endpoint_index).clone());
+        }
+
+        let n_urls = self.config.ordered_l1_endpoint_urls.len();
+        for offset in 1..n_urls {
+            let idx = (current_l1_endpoint_index + offset) % n_urls;
+            if self.is_operational(idx).await {
+                warn!(
+                    "L1 endpoint {} down; switched to {}",
+                    self.get_node_url(current_l1_endpoint_index),
+                    self.get_node_url(idx)
+                );
+
+                self.current_l1_endpoint_index = idx;
+                return Ok(self.get_node_url(idx).clone());
+            }
+        }
+
+        error!("No operational L1 endpoints found in {:?}", self.config.ordered_l1_endpoint_urls);
+        Err(L1EndpointMonitorError::NoActiveL1Endpoint)
+    }
+
+    fn get_node_url(&self, index: usize) -> &Url {
+        &self.config.ordered_l1_endpoint_urls[index]
+    }
+
+    async fn is_operational(&self, l1_endpoint_index: usize) -> bool {
+        let l1_endpoint_url = self.get_node_url(l1_endpoint_index);
+        let l1_client = ProviderBuilder::new().on_http(l1_endpoint_url.clone());
+        // Is this fast enough? we can use something to just check connectivity, but a recent infura
+        // bug failed on this API even though connectivity was fine. Besides, this API is called for
+        // most of our operations anyway.
+        l1_client.get_block_number().await.is_ok()
+    }
+}
+
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq, Eq)]
+pub struct L1EndpointMonitorConfig {
+    pub ordered_l1_endpoint_urls: Vec<Url>,
+}
+
+#[derive(Debug, Clone, Error, Serialize, Deserialize, PartialEq, Eq)]
+pub enum L1EndpointMonitorError {
+    #[error("All L1 endpoints are non-operational")]
+    NoActiveL1Endpoint,
+}