feat: keep earliest event when cache is full (#537)

* chore: drop new events when event cache is full

* add doc

* words

src/event-cache/EventCache.ts

@@ -216,8 +216,11 @@ export class EventCache {
         }
 
         if (this.events.length === this.config.eventCacheSize) {
-            // Make room in the cache by dropping the oldest event.
-            this.events.shift();
+            // Drop newest event and keep the older ones
+            // 1. Older events tend to be more relevant, such as session start
+            //    or performance entries that are attributed to web vitals
+            // 2. Dropping an old event requires linear time
+            return;
         }
 
         // The data plane service model (i.e., LogEvents) does not adhere to the

src/event-cache/__tests__/EventCache.test.ts

@@ -158,7 +158,7 @@ describe('EventCache tests', () => {
         expect(eventCache.getEventBatch()[0].type).toEqual(EVENT2_SCHEMA);
     });
 
-    test('when cache size reached, recordEvent drops oldest event', async () => {
+    test('when cache size reached, recordEvent drops the newest event', async () => {
         // Init
         const EVENT1_SCHEMA = 'com.amazon.rum.event1';
         const EVENT2_SCHEMA = 'com.amazon.rum.event2';
@@ -175,7 +175,7 @@ describe('EventCache tests', () => {
             {
                 id: expect.stringMatching(/[0-9a-f\-]+/),
                 timestamp: new Date(),
-                type: EVENT2_SCHEMA,
+                type: EVENT1_SCHEMA,
                 metadata: `{"version":"1.0.0","aws:client":"${INSTALL_MODULE}","aws:clientVersion":"${WEB_CLIENT_VERSION}"}`,
                 details: '{}'
             }