fix: add stale-while-revalidate to limits (#906)

* fix: add stale-while-revalidate to limits

* improve: body send

* improve: tune parameters

* chore: update docs

* improve: removed defaults

* chore: make simple function

api/_utils.ts

@@ -1246,3 +1246,31 @@ export function getDefaultRelayerAddress(
     return sdk.constants.DEFAULT_SIMULATED_RELAYER_ADDRESS;
   }
 }
+
+/**
+ * Performs the needed function calls to return a Vercel Response
+ * @param response The response client provided by Vercel
+ * @param body A payload in JSON format to send to the client
+ * @param statusCode The status code - defaults to 200
+ * @param cacheSeconds The cache time in non-negative whole seconds
+ * @param staleWhileRevalidateSeconds The stale while revalidate time in non-negative whole seconds
+ * @returns The response object
+ * @see https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
+ * @see https://datatracker.ietf.org/doc/html/rfc7234
+ * @note Be careful to not set anything negative please. The comment in the fn explains why
+ */
+export function sendResponse(
+  response: VercelResponse,
+  body: Record<string, unknown>,
+  statusCode: number,
+  cacheSeconds: number,
+  staleWhileRevalidateSeconds: number
+) {
+  // Invalid (non-positive/non-integer) values will be considered undefined per RFC-7234.
+  // Most browsers will consider these invalid and will request fresh data.
+  response.setHeader(
+    "Cache-Control",
+    `s-maxage=${cacheSeconds}, stale-while-revalidate=${staleWhileRevalidateSeconds}`
+  );
+  return response.status(statusCode).json(body);
+}

api/limits.ts

@@ -31,6 +31,7 @@ import {
   HUB_POOL_CHAIN_ID,
   ENABLED_ROUTES,
   getDefaultRelayerAddress,
+  sendResponse,
 } from "./_utils";
 
 const LimitsQueryParamsSchema = object({
@@ -225,18 +226,14 @@ const handler = async (
         liquidReserves
       ).toString(),
     };
-
-    // Instruct Vercel to cache limit data for this token for 5 minutes. Caching can be used to limit number of
-    // Vercel invocations and run time for this serverless function and trades off potential inaccuracy in times of
-    // high volume. "max-age=0" instructs browsers not to cache, while s-maxage instructs Vercel edge caching
-    // to cache the responses and invalidate when deployments update.
     logger.debug({
       at: "Limits",
       message: "Response data",
       responseJson,
     });
-    response.setHeader("Cache-Control", "s-maxage=300");
-    response.status(200).json(responseJson);
+    // Respond with a 200 status code and 4 minutes of cache cache with
+    // a minute of stale-while-revalidate.
+    sendResponse(response, responseJson, 200, 240, 60);
   } catch (error: unknown) {
     return handleErrorCondition("limits", response, logger, error);
   }