feat(store-indexer): add cache headers (#2669)

Co-authored-by: Kevin Ingersoll <[email protected]>

.changeset/fifty-dryers-enjoy.md

@@ -0,0 +1,5 @@
+---
+"@latticexyz/store-indexer": patch
+---
+
+Added `Cache-Control` and `Content-Type` headers to the postgres indexer API.

packages/store-indexer/src/postgres/apiRoutes.ts

@@ -21,6 +21,7 @@ export function apiRoutes(database: Sql): Middleware {
       options = input.parse(typeof ctx.query.input === "string" ? JSON.parse(ctx.query.input) : {});
     } catch (e) {
       ctx.status = 400;
+      ctx.set("Content-Type", "application/json");
       ctx.body = JSON.stringify(e);
       debug(e);
       return;
@@ -33,6 +34,11 @@ export function apiRoutes(database: Sql): Middleware {
       const logs = records.map(recordToLog);
       benchmark("map records to logs");
 
+      // Ideally we would immediately return an error if the request is for a Store that the indexer
+      // is not configured to index. Since we don't have easy access to this information here,
+      // we return an error if there are no logs found for a given Store, since that would never
+      // be the case for a Store that is being indexed (since there would at least be records for the
+      // Tables table with tables created during Store initialization).
       if (records.length === 0) {
         ctx.status = 404;
         ctx.body = "no logs found";
@@ -45,10 +51,27 @@ export function apiRoutes(database: Sql): Middleware {
       }
 
       const blockNumber = records[0].chainBlockNumber;
-      ctx.body = JSON.stringify({ blockNumber, logs });
       ctx.status = 200;
+
+      // max age is set to several multiples of the uncached response time (currently ~10s, but using 60s for wiggle room) to ensure only ~one origin request at a time
+      // and stale-while-revalidate below means that the cache is refreshed under the hood while still responding fast (cached)
+      const maxAgeSeconds = 60 * 5;
+      // we set stale-while-revalidate to the time elapsed by the number of blocks we can fetch from the RPC in the same amount of time as an uncached response
+      // meaning it would take ~the same about of time to get an uncached response from the origin as it would to catch up from the currently cached response
+      // if an uncached response takes ~10 seconds, we have ~10s to catch up, so let's say we can do enough RPC calls to fetch 4000 blocks
+      // with a block per 2 seconds, that means we can serve a stale/cached response for 8000 seconds before we should require the response be returned by the origin
+      const staleWhileRevalidateSeconds = 4000 * 2;
+
+      ctx.set(
+        "Cache-Control",
+        `public, max-age=${maxAgeSeconds}, stale-while-revalidate=${staleWhileRevalidateSeconds}`,
+      );
+
+      ctx.set("Content-Type", "application/json");
+      ctx.body = JSON.stringify({ blockNumber, logs });
     } catch (e) {
       ctx.status = 500;
+      ctx.set("Content-Type", "application/json");
       ctx.body = JSON.stringify(e);
       error(e);
     }