Use concurrency to improve performance

Author: stevebux

lambdas/api-handler/src/services/letter-operations.ts

@@ -51,11 +51,20 @@ export const getPendingLetters = async (
   letterQueueRepo: LetterQueueRepository,
   visibilityTimeout: number,
 ): Promise<LetterBase[]> => {
+  const CONCURRENCY = 5;
+
   const pendingLetters = await letterQueueRepo.getLetters(supplierId, limit);
   const timestamp = new Date(Date.now() + visibilityTimeout * 1000);
-  for (const letter of pendingLetters) {
-    await letterQueueRepo.updateLetterTimestamp(letter, timestamp);
+
+  for (let i = 0; i < pendingLetters.length; i += CONCURRENCY) {
+    const window = pendingLetters.slice(i, i + CONCURRENCY);
+    await Promise.all(
+      window.map((letter) =>
+        letterQueueRepo.updateLetterTimestamp(letter, timestamp),
+      ),
+    );
   }
+
   return pendingLetters.map((letter) => mapPendingLetterToLetterBase(letter));
 };