fix: now hydration event is being returned if revision does not exist in cache (#9203)

Now when customer is coming with revision ID that does not exist in
cache, we return hydration.

src/lib/features/client-feature-toggles/delta/client-feature-delta-api.e2e.test.ts

@@ -270,3 +270,54 @@ test('should get segment updated and removed events', async () => {
         ],
     });
 });
+
+test('should return hydration if revision not in cache', async () => {
+    await app.createFeature('base_feature');
+    await syncRevisions();
+    const { body, headers } = await app.request
+        .get('/api/client/delta')
+        .expect(200);
+    const etag = headers.etag;
+
+    expect(body).toMatchObject({
+        events: [
+            {
+                type: DELTA_EVENT_TYPES.HYDRATION,
+                features: [
+                    {
+                        name: 'base_feature',
+                    },
+                ],
+            },
+        ],
+    });
+
+    await app.createFeature('not_important1');
+    await syncRevisions();
+
+    const { body: deltaBody } = await app.request
+        .get('/api/client/delta')
+        .set('If-None-Match', etag)
+        .expect(200);
+
+    expect(deltaBody).toMatchObject({
+        events: [
+            {
+                type: DELTA_EVENT_TYPES.FEATURE_UPDATED,
+            },
+        ],
+    });
+
+    const { body: rehydrationBody } = await app.request
+        .get('/api/client/delta')
+        .set('If-None-Match', '1')
+        .expect(200);
+
+    expect(rehydrationBody).toMatchObject({
+        events: [
+            {
+                type: DELTA_EVENT_TYPES.HYDRATION,
+            },
+        ],
+    });
+});

src/lib/features/client-feature-toggles/delta/client-feature-toggle-delta.ts

@@ -154,8 +154,12 @@ export class ClientFeatureToggleDelta {
         if (requiredRevisionId >= this.currentRevisionId) {
             return undefined;
         }
-        if (requiredRevisionId === 0) {
-            const hydrationEvent = this.delta[environment].getHydrationEvent();
+        const delta = this.delta[environment];
+        if (
+            requiredRevisionId === 0 ||
+            delta.isMissingRevision(requiredRevisionId)
+        ) {
+            const hydrationEvent = delta.getHydrationEvent();
             const filteredEvent = filterHydrationEventByQuery(
                 hydrationEvent,
                 projects,
@@ -168,7 +172,7 @@ export class ClientFeatureToggleDelta {
 
             return Promise.resolve(response);
         } else {
-            const environmentEvents = this.delta[environment].getEvents();
+            const environmentEvents = delta.getEvents();
             const events = filterEventsByQuery(
                 environmentEvents,
                 requiredRevisionId,
@@ -299,8 +303,7 @@ export class ClientFeatureToggleDelta {
         });
     }
 
-    public async initEnvironmentDelta(environment: string) {
-        // Todo: replace with method that gets all features for an environment
+    private async initEnvironmentDelta(environment: string) {
         const baseFeatures = await this.getClientFeatures({
             environment,
         });

src/lib/features/client-feature-toggles/delta/delta-cache.ts

@@ -12,6 +12,22 @@ export class DeltaCache {
     constructor(hydrationEvent: DeltaHydrationEvent, maxLength: number = 20) {
         this.hydrationEvent = hydrationEvent;
         this.maxLength = maxLength;
+
+        this.addBaseEventFromHydration(hydrationEvent);
+    }
+
+    private addBaseEventFromHydration(
+        hydrationEvent: DeltaHydrationEvent,
+    ): void {
+        const lastFeature =
+            hydrationEvent.features[hydrationEvent.features.length - 1];
+        this.addEvents([
+            {
+                eventId: hydrationEvent.eventId,
+                type: 'feature-updated',
+                feature: lastFeature,
+            },
+        ]);
     }
 
     public addEvents(events: DeltaEvent[]): void {
@@ -27,6 +43,10 @@ export class DeltaCache {
         return this.events;
     }
 
+    public isMissingRevision(revisionId: number): boolean {
+        return !this.events.some((event) => event.eventId === revisionId);
+    }
+
     public getHydrationEvent(): DeltaHydrationEvent {
         return this.hydrationEvent;
     }