feat(store-indexer): add metric for distance from block tag to follow

.changeset/good-fireants-retire.md

@@ -0,0 +1,5 @@
+---
+"@latticexyz/store-indexer": patch
+---
+
+Added a `distance_from_follow_block` metric to compare the latest stored block number with the block number corresponding to the block tag the indexer follows.

packages/store-indexer/bin/postgres-indexer.ts

@@ -66,6 +66,12 @@ async function getLatestStoredBlockNumber(): Promise<bigint | undefined> {
   }
 }
 
+async function getDistanceFromFollowBlock(): Promise<bigint | undefined> {
+  const latestStoredBlockNumber = (await getLatestStoredBlockNumber()) ?? -1n;
+  const latestFollowBlockNumber = (await publicClient.getBlock({ blockTag: env.FOLLOW_BLOCK_TAG })).number;
+  return latestFollowBlockNumber - latestStoredBlockNumber;
+}
+
 const latestStoredBlockNumber = await getLatestStoredBlockNumber();
 if (latestStoredBlockNumber != null) {
   startBlock = latestStoredBlockNumber + 1n;
@@ -117,6 +123,7 @@ if (env.HEALTHCHECK_HOST != null || env.HEALTHCHECK_PORT != null) {
       isHealthy: () => true,
       isReady: () => isCaughtUp,
       getLatestStoredBlockNumber,
+      getDistanceFromFollowBlock,
       followBlockTag: env.FOLLOW_BLOCK_TAG,
     }),
   );

packages/store-indexer/bin/sqlite-indexer.ts

@@ -73,6 +73,12 @@ async function getLatestStoredBlockNumber(): Promise<bigint | undefined> {
   return currentChainState?.lastUpdatedBlockNumber ?? undefined;
 }
 
+async function getDistanceFromFollowBlock(): Promise<bigint | undefined> {
+  const latestStoredBlockNumber = (await getLatestStoredBlockNumber()) ?? -1n;
+  const latestFollowBlockNumber = (await publicClient.getBlock({ blockTag: env.FOLLOW_BLOCK_TAG })).number;
+  return latestFollowBlockNumber - latestStoredBlockNumber;
+}
+
 const currentChainState = await getCurrentChainState();
 if (currentChainState) {
   // Reset the db if the version changed
@@ -132,6 +138,7 @@ server.use(
     isHealthy: () => true,
     isReady: () => isCaughtUp,
     getLatestStoredBlockNumber,
+    getDistanceFromFollowBlock,
     followBlockTag: env.FOLLOW_BLOCK_TAG,
   }),
 );

packages/store-indexer/src/koa-middleware/metrics.ts

@@ -5,6 +5,7 @@ type MetricsOptions = {
   isHealthy?: () => boolean;
   isReady?: () => boolean;
   getLatestStoredBlockNumber?: () => Promise<bigint | undefined>;
+  getDistanceFromFollowBlock?: () => Promise<bigint | undefined>;
   followBlockTag?: "latest" | "safe" | "finalized";
 };
 
@@ -15,6 +16,7 @@ export function metrics({
   isHealthy,
   isReady,
   getLatestStoredBlockNumber,
+  getDistanceFromFollowBlock,
   followBlockTag,
 }: MetricsOptions = {}): Middleware {
   promClient.collectDefaultMetrics();
@@ -61,6 +63,16 @@ export function metrics({
     blockTagGauge.set(blockTagToValue[followBlockTag]);
   }
 
+  if (getDistanceFromFollowBlock != null) {
+    new promClient.Gauge({
+      name: "distance_from_follow_block",
+      help: "Block distance from the block tag this the indexer is following",
+      async collect(): Promise<void> {
+        this.set(Number(await getDistanceFromFollowBlock()));
+      },
+    });
+  }
+
   return async function metricsMiddleware(ctx, next): Promise<void> {
     if (ctx.path === "/metrics") {
       ctx.status = 200;