feat: delta rework (#9133)

We are changing how the Delta API works, as discussed:

1. We have removed the `updated` and `removed` arrays and now keep
everything in the `events` array.
2. We decided to keep the hydration cache separate from the events array
internally. Since the hydration cache has a special structure and may
contain not just one feature but potentially 1,000 features, it behaved
differently, requiring a lot of special logic to handle it.
3. Implemented `nameprefix` filtering, which we were missing before.


Things still to implement:

1.  Segment hydration and updates to it.

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

@@ -41,6 +41,7 @@ import {
 } from '../../internals';
 import isEqual from 'lodash.isequal';
 import { diff } from 'json-diff';
+import type { DeltaHydrationEvent } from './delta/client-feature-toggle-delta-types';
 
 const version = 2;
 
@@ -191,22 +192,34 @@ export default class FeatureController extends Controller {
                 const sortedToggles = features.sort((a, b) =>
                     a.name.localeCompare(b.name),
                 );
-                const sortedNewToggles = delta?.updated.sort((a, b) =>
+                if (delta?.events[0].type === 'hydration') {
-                    a.name.localeCompare(b.name),
+                    const hydrationEvent: DeltaHydrationEvent =
+                        delta?.events[0];
+                    const sortedNewToggles = hydrationEvent.features.sort(
+                        (a, b) => a.name.localeCompare(b.name),
                     );
 
                     if (
-                    !this.deepEqualIgnoreOrder(sortedToggles, sortedNewToggles)
+                        !this.deepEqualIgnoreOrder(
+                            sortedToggles,
+                            sortedNewToggles,
+                        )
                     ) {
                         this.logger.warn(
                             `old features and new features are different. Old count ${
                                 features.length
-                        }, new count ${delta?.updated.length}, query ${JSON.stringify(query)},
+                            }, new count ${hydrationEvent.features.length}, query ${JSON.stringify(query)},
                         diff ${JSON.stringify(
                             diff(sortedToggles, sortedNewToggles),
                         )}`,
                         );
                     }
+                } else {
+                    this.logger.warn(
+                        `Delta diff should have only hydration event, query ${JSON.stringify(query)}`,
+                    );
+                }
+
                 this.storeFootprint();
             } catch (e) {
                 this.logger.error('Delta diff failed', e);

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

@@ -37,7 +37,11 @@ const setupFeatures = async (
         {
             name: 'default',
             constraints: [
-                { contextName: 'userId', operator: 'IN', values: ['123'] },
+                {
+                    contextName: 'userId',
+                    operator: 'IN',
+                    values: ['123'],
+                },
             ],
             parameters: {},
         },
@@ -88,17 +92,19 @@ test('should match with /api/client/delta', async () => {
         .expect('Content-Type', /json/)
         .expect(200);
 
-    expect(body.features).toMatchObject(deltaBody.updated);
+    expect(body.features).toMatchObject(deltaBody.events[0].features);
 });
 
 test('should get 304 if asked for latest revision', async () => {
     await setupFeatures(db, app);
 
-    const { body } = await app.request.get('/api/client/delta').expect(200);
+    const { body, headers } = await app.request
-    const currentRevisionId = body.revisionId;
+        .get('/api/client/delta')
+        .expect(200);
+    const etag = headers.etag;
 
     await app.request
-        .set('If-None-Match', `"${currentRevisionId}"`)
+        .set('If-None-Match', etag)
         .get('/api/client/delta')
         .expect(304);
 });
@@ -106,71 +112,102 @@ test('should get 304 if asked for latest revision', async () => {
 test('should return correct delta after feature created', async () => {
     await app.createFeature('base_feature');
     await syncRevisions();
-    const { body } = await app.request.get('/api/client/delta').expect(200);
+    const { body, headers } = await app.request
-    const currentRevisionId = body.revisionId;
+        .set('If-None-Match', null)
+        .get('/api/client/delta')
+        .expect(200);
+    const etag = headers.etag;
 
     expect(body).toMatchObject({
-        updated: [
+        events: [
+            {
+                type: 'hydration',
+                features: [
                     {
                         name: 'base_feature',
                     },
                 ],
+            },
+        ],
     });
 
     await app.createFeature('new_feature');
 
     await syncRevisions();
+    //@ts-ignore
+    await app.services.clientFeatureToggleService.clientFeatureToggleDelta.onUpdateRevisionEvent();
 
     const { body: deltaBody } = await app.request
         .get('/api/client/delta')
-        .set('If-None-Match', `"${currentRevisionId}"`)
+        .set('If-None-Match', etag)
         .expect(200);
 
     expect(deltaBody).toMatchObject({
-        updated: [
+        events: [
             {
+                type: 'feature-updated',
+                feature: {
                     name: 'new_feature',
                 },
+            },
+            {
+                type: 'feature-updated',
+                feature: {
+                    name: 'new_feature',
+                },
+            },
         ],
     });
 });
 
 const syncRevisions = async () => {
     await app.services.configurationRevisionService.updateMaxRevisionId();
-    // @ts-ignore
-    await app.services.clientFeatureToggleService.clientFeatureToggleDelta.onUpdateRevisionEvent();
 };
 
 test('archived features should not be returned as updated', async () => {
     await app.createFeature('base_feature');
     await syncRevisions();
-    const { body } = await app.request.get('/api/client/delta').expect(200);
+    const { body, headers } = await app.request
-    const currentRevisionId = body.revisionId;
+        .get('/api/client/delta')
+        .expect(200);
+    const etag = headers.etag;
 
     expect(body).toMatchObject({
-        updated: [
+        events: [
+            {
+                features: [
                     {
                         name: 'base_feature',
                     },
                 ],
+            },
+        ],
     });
 
     await app.archiveFeature('base_feature');
+    await syncRevisions();
     await app.createFeature('new_feature');
 
     await syncRevisions();
+    await app.getProjectFeatures('new_feature'); // TODO: this is silly, but events syncing and tests do not work nicely. this is basically a setTimeout
 
     const { body: deltaBody } = await app.request
         .get('/api/client/delta')
-        .set('If-None-Match', `"${currentRevisionId}"`)
+        .set('If-None-Match', etag)
         .expect(200);
 
     expect(deltaBody).toMatchObject({
-        updated: [
+        events: [
             {
+                type: 'feature-removed',
+                featureName: 'base_feature',
+            },
+            {
+                type: 'feature-updated',
+                feature: {
                     name: 'new_feature',
                 },
+            },
         ],
-        removed: ['base_feature'],
     });
 });

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

@@ -103,15 +103,16 @@ export default class ClientFeatureToggleDeltaController extends Controller {
             res.end();
             return;
         }
-
+        const lastEventId =
-        if (changedFeatures.revisionId === currentSdkRevisionId) {
+            changedFeatures.events[changedFeatures.events.length - 1].eventId;
+        if (lastEventId === currentSdkRevisionId) {
             res.status(304);
             res.getHeaderNames().forEach((header) => res.removeHeader(header));
             res.end();
             return;
         }
 
-        res.setHeader('ETag', `"${changedFeatures.revisionId}"`);
+        res.setHeader('ETag', `"${lastEventId}"`);
         this.openApiService.respondWithValidation(
             200,
             res,

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

@@ -0,0 +1,63 @@
+import type { ClientFeatureSchema } from '../../../openapi';
+import type { IClientSegment } from '../../../types';
+
+export type DeltaHydrationEvent = {
+    eventId: number;
+    type: 'hydration';
+    features: ClientFeatureSchema[];
+};
+
+export type DeltaEvent =
+    | {
+          eventId: number;
+          type: 'feature-updated';
+          feature: ClientFeatureSchema;
+      }
+    | {
+          eventId: number;
+          type: 'feature-removed';
+          featureName: string;
+          project: string;
+      }
+    | {
+          eventId: number;
+          type: 'segment-updated';
+          segment: IClientSegment;
+      }
+    | {
+          eventId: number;
+          type: 'segment-removed';
+          segmentId: number;
+      };
+
+export const DELTA_EVENT_TYPES = {
+    FEATURE_UPDATED: 'feature-updated',
+    FEATURE_REMOVED: 'feature-removed',
+    SEGMENT_UPDATED: 'segment-updated',
+    SEGMENT_REMOVED: 'segment-removed',
+    HYDRATION: 'hydration',
+} as const;
+
+export const isDeltaFeatureUpdatedEvent = (
+    event: DeltaEvent,
+): event is Extract<DeltaEvent, { type: 'feature-updated' }> => {
+    return event.type === DELTA_EVENT_TYPES.FEATURE_UPDATED;
+};
+
+export const isDeltaFeatureRemovedEvent = (
+    event: DeltaEvent,
+): event is Extract<DeltaEvent, { type: 'feature-removed' }> => {
+    return event.type === DELTA_EVENT_TYPES.FEATURE_REMOVED;
+};
+
+export const isDeltaSegmentUpdatedEvent = (
+    event: DeltaEvent,
+): event is Extract<DeltaEvent, { type: 'segment-updated' }> => {
+    return event.type === DELTA_EVENT_TYPES.SEGMENT_UPDATED;
+};
+
+export const isDeltaSegmentRemovedEvent = (
+    event: DeltaEvent,
+): event is Extract<DeltaEvent, { type: 'segment-removed' }> => {
+    return event.type === DELTA_EVENT_TYPES.SEGMENT_REMOVED;
+};

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

@@ -1,4 +1,9 @@
-import { calculateRequiredClientRevision } from './client-feature-toggle-delta';
+import {
+    type DeltaEvent,
+    filterEventsByQuery,
+    filterHydrationEventByQuery,
+} from './client-feature-toggle-delta';
+import type { DeltaHydrationEvent } from './client-feature-toggle-delta-types';
 
 const mockAdd = (params): any => {
     const base = {
@@ -16,166 +21,86 @@ const mockAdd = (params): any => {
     return { ...base, ...params };
 };
 
-test('compresses multiple revisions to a single update', () => {
-    const revisionList = [
-        {
-            revisionId: 1,
-            updated: [mockAdd({ type: 'release' })],
-            removed: [],
-        },
-        {
-            revisionId: 2,
-            updated: [mockAdd({ type: 'test' })],
-            removed: [],
-        },
-    ];
-
-    const revisions = calculateRequiredClientRevision(revisionList, 0, [
-        'default',
-    ]);
-
-    expect(revisions).toEqual({
-        revisionId: 2,
-        updated: [mockAdd({ type: 'test' })],
-        removed: [],
-    });
-});
-
-test('revision that adds, removes then adds again does not end up with the remove', () => {
-    const revisionList = [
-        {
-            revisionId: 1,
-            updated: [mockAdd({ name: 'some-toggle' })],
-            removed: [],
-        },
-        {
-            revisionId: 2,
-            updated: [],
-            removed: [
-                {
-                    name: 'some-toggle',
-                    project: 'default',
-                },
-            ],
-        },
-        {
-            revisionId: 3,
-            updated: [mockAdd({ name: 'some-toggle' })],
-            removed: [],
-        },
-    ];
-
-    const revisions = calculateRequiredClientRevision(revisionList, 0, [
-        'default',
-    ]);
-
-    expect(revisions).toEqual({
-        revisionId: 3,
-        updated: [mockAdd({ name: 'some-toggle' })],
-        removed: [],
-    });
-});
-
-test('revision that removes, adds then removes again does not end up with the remove', () => {
-    const revisionList = [
-        {
-            revisionId: 1,
-            updated: [],
-            removed: [
-                {
-                    name: 'some-toggle',
-                    project: 'default',
-                },
-            ],
-        },
-        {
-            revisionId: 2,
-            updated: [mockAdd({ name: 'some-toggle' })],
-            removed: [],
-        },
-        {
-            revisionId: 3,
-            updated: [],
-            removed: [
-                {
-                    name: 'some-toggle',
-                    project: 'default',
-                },
-            ],
-        },
-    ];
-
-    const revisions = calculateRequiredClientRevision(revisionList, 0, [
-        'default',
-    ]);
-
-    expect(revisions).toEqual({
-        revisionId: 3,
-        updated: [],
-        removed: [
-            {
-                name: 'some-toggle',
-                project: 'default',
-            },
-        ],
-    });
-});
-
 test('revision equal to the base case returns only later revisions ', () => {
-    const revisionList = [
+    const revisionList: DeltaEvent[] = [
         {
-            revisionId: 1,
+            eventId: 2,
-            updated: [
+            type: 'feature-updated',
-                mockAdd({ name: 'feature1' }),
+            feature: mockAdd({ name: 'feature4' }),
-                mockAdd({ name: 'feature2' }),
+        },
-                mockAdd({ name: 'feature3' }),
+        {
+            eventId: 3,
+            type: 'feature-updated',
+            feature: mockAdd({ name: 'feature5' }),
+        },
+    ];
+
+    const revisions = filterEventsByQuery(revisionList, 1, ['default'], '');
+
+    expect(revisions).toEqual([
+        {
+            eventId: 2,
+            type: 'feature-updated',
+            feature: mockAdd({ name: 'feature4' }),
+        },
+        {
+            eventId: 3,
+            type: 'feature-updated',
+            feature: mockAdd({ name: 'feature5' }),
+        },
+    ]);
+});
+
+test('project filter removes features not in project and nameprefix', () => {
+    const revisionList: DeltaEvent[] = [
+        {
+            eventId: 1,
+            type: 'feature-updated',
+            feature: mockAdd({ name: 'feature1', project: 'project1' }),
+        },
+        {
+            eventId: 2,
+            type: 'feature-updated',
+            feature: mockAdd({ name: 'feature2', project: 'project2' }),
+        },
+        {
+            eventId: 3,
+            type: 'feature-updated',
+            feature: mockAdd({ name: 'ffeature1', project: 'project1' }),
+        },
+    ];
+
+    const revisions = filterEventsByQuery(revisionList, 0, ['project1'], 'ff');
+
+    expect(revisions).toEqual([
+        {
+            eventId: 3,
+            type: 'feature-updated',
+            feature: mockAdd({ name: 'ffeature1', project: 'project1' }),
+        },
+    ]);
+});
+
+test('project filter removes features not in project in hydration', () => {
+    const revisionList: DeltaHydrationEvent = {
+        eventId: 1,
+        type: 'hydration',
+        features: [
+            mockAdd({ name: 'feature1', project: 'project1' }),
+            mockAdd({ name: 'feature2', project: 'project2' }),
+            mockAdd({ name: 'myfeature2', project: 'project2' }),
         ],
-            removed: [],
+    };
-        },
-        {
-            revisionId: 2,
-            updated: [mockAdd({ name: 'feature4' })],
-            removed: [],
-        },
-        {
-            revisionId: 3,
-            updated: [mockAdd({ name: 'feature5' })],
-            removed: [],
-        },
-    ];
 
-    const revisions = calculateRequiredClientRevision(revisionList, 1, [
+    const revisions = filterHydrationEventByQuery(
-        'default',
+        revisionList,
-    ]);
+        ['project2'],
+        'my',
+    );
 
     expect(revisions).toEqual({
-        revisionId: 3,
+        eventId: 1,
-        updated: [mockAdd({ name: 'feature4' }), mockAdd({ name: 'feature5' })],
+        type: 'hydration',
-        removed: [],
+        features: [mockAdd({ name: 'myfeature2', project: 'project2' })],
-    });
-});
-
-test('project filter removes features not in project', () => {
-    const revisionList = [
-        {
-            revisionId: 1,
-            updated: [mockAdd({ name: 'feature1', project: 'project1' })],
-            removed: [],
-        },
-        {
-            revisionId: 2,
-            updated: [mockAdd({ name: 'feature2', project: 'project2' })],
-            removed: [],
-        },
-    ];
-
-    const revisions = calculateRequiredClientRevision(revisionList, 0, [
-        'project1',
-    ]);
-
-    expect(revisions).toEqual({
-        revisionId: 2,
-        updated: [mockAdd({ name: 'feature1', project: 'project1' })],
-        removed: [],
     });
 });

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

@@ -9,7 +9,7 @@ import type {
 } from '../../../types';
 import type ConfigurationRevisionService from '../../feature-toggle/configuration-revision-service';
 import { UPDATE_REVISION } from '../../feature-toggle/configuration-revision-service';
-import { RevisionDelta } from './revision-delta';
+import { DeltaCache } from './delta-cache';
 import type {
     FeatureConfigurationDeltaClient,
     IClientFeatureToggleDeltaReadModel,
@@ -18,92 +18,75 @@ import { CLIENT_DELTA_MEMORY } from '../../../metric-events';
 import type EventEmitter from 'events';
 import type { Logger } from '../../../logger';
 import type { ClientFeaturesDeltaSchema } from '../../../openapi';
+import {
+    DELTA_EVENT_TYPES,
+    type DeltaEvent,
+    type DeltaHydrationEvent,
+    isDeltaFeatureRemovedEvent,
+    isDeltaFeatureUpdatedEvent,
+} from './client-feature-toggle-delta-types';
 
-type DeletedFeature = {
+type EnvironmentRevisions = Record<string, DeltaCache>;
-    name: string;
-    project: string;
-};
 
-export type RevisionDeltaEntry = {
+export const filterEventsByQuery = (
-    updated: FeatureConfigurationDeltaClient[];
+    events: DeltaEvent[],
-    revisionId: number;
-    removed: DeletedFeature[];
-    segments: IClientSegment[];
-};
-
-export type Revision = {
-    revisionId: number;
-    updated: any[];
-    removed: DeletedFeature[];
-};
-
-type Revisions = Record<string, RevisionDelta>;
-
-const applyRevision = (first: Revision, last: Revision): Revision => {
-    const updatedMap = new Map(
-        [...first.updated, ...last.updated].map((feature) => [
-            feature.name,
-            feature,
-        ]),
-    );
-    const removedMap = new Map(
-        [...first.removed, ...last.removed].map((feature) => [
-            feature.name,
-            feature,
-        ]),
-    );
-
-    for (const feature of last.updated) {
-        removedMap.delete(feature.name);
-    }
-
-    for (const feature of last.removed) {
-        updatedMap.delete(feature.name);
-        removedMap.set(feature.name, feature);
-    }
-
-    return {
-        revisionId: last.revisionId,
-        updated: Array.from(updatedMap.values()),
-        removed: Array.from(removedMap.values()),
-    };
-};
-
-const filterRevisionByProject = (
-    revision: Revision,
-    projects: string[],
-): Revision => {
-    const updated = revision.updated.filter(
-        (feature) =>
-            projects.includes('*') || projects.includes(feature.project),
-    );
-    const removed = revision.removed.filter(
-        (feature) =>
-            projects.includes('*') || projects.includes(feature.project),
-    );
-
-    return { ...revision, updated, removed };
-};
-
-export const calculateRequiredClientRevision = (
-    revisions: Revision[],
     requiredRevisionId: number,
     projects: string[],
+    namePrefix: string,
 ) => {
-    const targetedRevisions = revisions.filter(
+    const targetedEvents = events.filter(
-        (revision) => revision.revisionId > requiredRevisionId,
+        (revision) => revision.eventId > requiredRevisionId,
     );
-    const projectFeatureRevisions = targetedRevisions.map((revision) =>
+    const allProjects = projects.includes('*');
-        filterRevisionByProject(revision, projects),
+    const startsWithPrefix = (revision: DeltaEvent) => {
+        return (
+            (isDeltaFeatureUpdatedEvent(revision) &&
+                revision.feature.name.startsWith(namePrefix)) ||
+            (isDeltaFeatureRemovedEvent(revision) &&
+                revision.featureName.startsWith(namePrefix))
         );
+    };
 
-    return projectFeatureRevisions.reduce(applyRevision);
+    const isInProject = (revision: DeltaEvent) => {
+        return (
+            (isDeltaFeatureUpdatedEvent(revision) &&
+                projects.includes(revision.feature.project!)) ||
+            (isDeltaFeatureRemovedEvent(revision) &&
+                projects.includes(revision.project))
+        );
+    };
+
+    return targetedEvents.filter((revision) => {
+        return (
+            startsWithPrefix(revision) && (allProjects || isInProject(revision))
+        );
+    });
+};
+
+export const filterHydrationEventByQuery = (
+    event: DeltaHydrationEvent,
+    projects: string[],
+    namePrefix: string,
+): DeltaHydrationEvent => {
+    const allProjects = projects.includes('*');
+    const { type, features, eventId } = event;
+
+    return {
+        eventId,
+        type,
+        features: features.filter((feature) => {
+            return (
+                feature.name.startsWith(namePrefix) &&
+                (allProjects || projects.includes(feature.project!))
+            );
+        }),
+    };
 };
 
 export class ClientFeatureToggleDelta {
     private clientFeatureToggleDeltaReadModel: IClientFeatureToggleDeltaReadModel;
 
-    private delta: Revisions = {};
+    private delta: EnvironmentRevisions = {};
 
     private segments: IClientSegment[];
 
@@ -159,7 +142,8 @@ export class ClientFeatureToggleDelta {
     ): Promise<ClientFeaturesDeltaSchema | undefined> {
         const projects = query.project ? query.project : ['*'];
         const environment = query.environment ? query.environment : 'default';
-        // TODO: filter by tags, what is namePrefix? anything else?
+        const namePrefix = query.namePrefix ? query.namePrefix : '';
+
         const requiredRevisionId = sdkRevisionId || 0;
 
         const hasDelta = this.delta[environment] !== undefined;
@@ -171,26 +155,48 @@ export class ClientFeatureToggleDelta {
         if (!hasSegments) {
             await this.updateSegments();
         }
-
         if (requiredRevisionId >= this.currentRevisionId) {
             return undefined;
         }
-
+        if (requiredRevisionId === 0) {
-        const environmentRevisions = this.delta[environment].getRevisions();
+            const hydrationEvent = this.delta[environment].getHydrationEvent();
-
+            const filteredEvent = filterHydrationEventByQuery(
-        const compressedRevision = calculateRequiredClientRevision(
+                hydrationEvent,
-            environmentRevisions,
-            requiredRevisionId,
                 projects,
+                namePrefix,
             );
 
-        const revisionResponse: ClientFeaturesDeltaSchema = {
+            const response: ClientFeaturesDeltaSchema = {
-            ...compressedRevision,
+                events: [
+                    {
+                        ...filteredEvent,
                         segments: this.segments,
-            removed: compressedRevision.removed.map((feature) => feature.name),
+                    },
+                ],
             };
 
-        return Promise.resolve(revisionResponse);
+            return Promise.resolve(response);
+        } else {
+            const environmentEvents = this.delta[environment].getEvents();
+            const events = filterEventsByQuery(
+                environmentEvents,
+                requiredRevisionId,
+                projects,
+                namePrefix,
+            );
+
+            const response: ClientFeaturesDeltaSchema = {
+                events: events.map((event) => {
+                    if (event.type === 'feature-removed') {
+                        const { project, ...rest } = event;
+                        return rest;
+                    }
+                    return event;
+                }),
+            };
+
+            return Promise.resolve(response);
+        }
     }
 
     public async onUpdateRevisionEvent() {
@@ -220,7 +226,7 @@ export class ClientFeatureToggleDelta {
             latestRevision,
         );
 
-        const changedToggles = [
+        const featuresUpdated = [
             ...new Set(
                 changeEvents
                     .filter((event) => event.featureName)
@@ -230,25 +236,49 @@ export class ClientFeatureToggleDelta {
             ),
         ];
 
-        const removed = changeEvents
+        const featuresRemovedEvents: DeltaEvent[] = changeEvents
             .filter((event) => event.featureName && event.project)
             .filter((event) => event.type === 'feature-archived')
             .map((event) => ({
-                name: event.featureName!,
+                eventId: latestRevision,
+                type: DELTA_EVENT_TYPES.FEATURE_REMOVED,
+                featureName: event.featureName!,
                 project: event.project!,
             }));
 
+        // TODO: implement single segment fetching
+        // const segmentsUpdated = changeEvents
+        //     .filter((event) => event.type === 'segment-updated')
+        //     .map((event) => ({
+        //         name: event.featureName!,
+        //         project: event.project!,
+        //     }));
+        //
+        // const segmentsRemoved = changeEvents
+        //     .filter((event) => event.type === 'segment-deleted')
+        //     .map((event) => ({
+        //         name: event.featureName!,
+        //         project: event.project!,
+        //     }));
+        //
+
         // TODO: we might want to only update the environments that had events changed for performance
         for (const environment of keys) {
             const newToggles = await this.getChangedToggles(
                 environment,
-                changedToggles,
+                featuresUpdated,
             );
-            this.delta[environment].addRevision({
+            const featuresUpdatedEvents: DeltaEvent[] = newToggles.map(
-                updated: newToggles,
+                (toggle) => ({
-                revisionId: latestRevision,
+                    eventId: latestRevision,
-                removed,
+                    type: DELTA_EVENT_TYPES.FEATURE_UPDATED,
-            });
+                    feature: toggle,
+                }),
+            );
+            this.delta[environment].addEvents([
+                ...featuresUpdatedEvents,
+                ...featuresRemovedEvents,
+            ]);
         }
         this.currentRevisionId = latestRevision;
     }
@@ -257,11 +287,10 @@ export class ClientFeatureToggleDelta {
         environment: string,
         toggles: string[],
     ): Promise<FeatureConfigurationDeltaClient[]> {
-        const foundToggles = await this.getClientFeatures({
+        return this.getClientFeatures({
             toggleNames: toggles,
             environment,
         });
-        return foundToggles;
     }
 
     public async initEnvironmentDelta(environment: string) {
@@ -273,14 +302,11 @@ export class ClientFeatureToggleDelta {
         this.currentRevisionId =
             await this.configurationRevisionService.getMaxRevisionId();
 
-        const delta = new RevisionDelta([
+        this.delta[environment] = new DeltaCache({
-            {
+            eventId: this.currentRevisionId,
-                revisionId: this.currentRevisionId,
+            type: DELTA_EVENT_TYPES.HYDRATION,
-                updated: baseFeatures,
+            features: baseFeatures,
-                removed: [],
+        });
-            },
-        ]);
-        this.delta[environment] = delta;
 
         this.storeFootprint();
     }
@@ -313,3 +339,5 @@ export class ClientFeatureToggleDelta {
         return Buffer.byteLength(jsonString, 'utf8');
     }
 }
+
+export type { DeltaEvent };

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

@@ -0,0 +1,141 @@
+import { DeltaCache } from './delta-cache';
+import type {
+    DeltaEvent,
+    DeltaHydrationEvent,
+} from './client-feature-toggle-delta-types';
+
+describe('RevisionCache', () => {
+    it('should always update the hydration event and remove event when over limit', () => {
+        const baseEvent: DeltaHydrationEvent = {
+            eventId: 1,
+            features: [
+                {
+                    name: 'test-flag',
+                    type: 'release',
+                    enabled: false,
+                    project: 'default',
+                    stale: false,
+                    strategies: [
+                        {
+                            name: 'flexibleRollout',
+                            constraints: [],
+                            parameters: {
+                                groupId: 'test-flag',
+                                rollout: '100',
+                                stickiness: 'default',
+                            },
+                            variants: [],
+                        },
+                    ],
+                    variants: [],
+                    description: null,
+                    impressionData: false,
+                },
+                {
+                    name: 'my-feature-flag',
+                    type: 'release',
+                    enabled: true,
+                    project: 'default',
+                    stale: false,
+                    strategies: [
+                        {
+                            name: 'flexibleRollout',
+                            constraints: [],
+                            parameters: {
+                                groupId: 'my-feature-flag',
+                                rollout: '100',
+                                stickiness: 'default',
+                            },
+                            variants: [],
+                        },
+                    ],
+                    variants: [],
+                    description: null,
+                    impressionData: false,
+                },
+            ],
+            type: 'hydration',
+        };
+        const initialEvents: DeltaEvent[] = [
+            {
+                eventId: 2,
+                feature: {
+                    name: 'my-feature-flag',
+                    type: 'release',
+                    enabled: true,
+                    project: 'default',
+                    stale: false,
+                    strategies: [
+                        {
+                            name: 'flexibleRollout',
+                            constraints: [],
+                            parameters: {
+                                groupId: 'my-feature-flag',
+                                rollout: '100',
+                                stickiness: 'default',
+                            },
+                            variants: [],
+                        },
+                    ],
+                    variants: [],
+                    description: null,
+                    impressionData: false,
+                },
+                type: 'feature-updated',
+            },
+        ];
+
+        const maxLength = 2;
+        const deltaCache = new DeltaCache(baseEvent, maxLength);
+        deltaCache.addEvents(initialEvents);
+
+        // Add a new revision to trigger changeBase
+        deltaCache.addEvents([
+            {
+                eventId: 3,
+                type: 'feature-updated',
+                feature: {
+                    name: 'another-feature-flag',
+                    type: 'release',
+                    enabled: true,
+                    project: 'default',
+                    stale: false,
+                    strategies: [
+                        {
+                            name: 'flexibleRollout',
+                            constraints: [],
+                            parameters: {
+                                groupId: 'another-feature-flag',
+                                rollout: '100',
+                                stickiness: 'default',
+                            },
+                        },
+                    ],
+                    variants: [],
+                    description: null,
+                    impressionData: false,
+                },
+            },
+            {
+                eventId: 4,
+                type: 'feature-removed',
+                featureName: 'test-flag',
+                project: 'default',
+            },
+        ]);
+
+        const events = deltaCache.getEvents();
+
+        // Check that the base has been changed and merged correctly
+        expect(events.length).toBe(2);
+
+        const hydrationEvent = deltaCache.getHydrationEvent();
+        expect(hydrationEvent.features).toHaveLength(2);
+        expect(hydrationEvent.features).toEqual(
+            expect.arrayContaining([
+                expect.objectContaining({ name: 'my-feature-flag' }),
+                expect.objectContaining({ name: 'another-feature-flag' }),
+            ]),
+        );
+    });
+});

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

@@ -0,0 +1,77 @@
+import type { DeltaEvent } from './client-feature-toggle-delta';
+import {
+    DELTA_EVENT_TYPES,
+    type DeltaHydrationEvent,
+} from './client-feature-toggle-delta-types';
+
+const mergeWithoutDuplicates = (arr1: any[], arr2: any[]) => {
+    const map = new Map();
+    arr1.concat(arr2).forEach((item) => {
+        map.set(item.name, item);
+    });
+    return Array.from(map.values());
+};
+
+export class DeltaCache {
+    private events: DeltaEvent[] = [];
+    private maxLength: number;
+    private hydrationEvent: DeltaHydrationEvent;
+
+    constructor(hydrationEvent: DeltaHydrationEvent, maxLength: number = 20) {
+        this.hydrationEvent = hydrationEvent;
+        this.maxLength = maxLength;
+    }
+
+    public addEvents(events: DeltaEvent[]): void {
+        this.events = [...this.events, ...events];
+
+        this.updateHydrationEvent(events);
+        while (this.events.length > this.maxLength) {
+            this.events.splice(1, 1);
+        }
+    }
+
+    public getEvents(): DeltaEvent[] {
+        return this.events;
+    }
+
+    public getHydrationEvent(): DeltaHydrationEvent {
+        return this.hydrationEvent;
+    }
+
+    private updateHydrationEvent(events: DeltaEvent[]): void {
+        for (const appliedEvent of events) {
+            switch (appliedEvent.type) {
+                case DELTA_EVENT_TYPES.FEATURE_UPDATED: {
+                    const featureToUpdate = this.hydrationEvent.features.find(
+                        (feature) => feature.name === appliedEvent.feature.name,
+                    );
+                    if (featureToUpdate) {
+                        Object.assign(featureToUpdate, appliedEvent.feature);
+                    } else {
+                        this.hydrationEvent.features.push(appliedEvent.feature);
+                    }
+                    break;
+                }
+                case DELTA_EVENT_TYPES.FEATURE_REMOVED: {
+                    this.hydrationEvent.features =
+                        this.hydrationEvent.features.filter(
+                            (feature) =>
+                                feature.name !== appliedEvent.featureName,
+                        );
+                    break;
+                }
+                case DELTA_EVENT_TYPES.SEGMENT_UPDATED: {
+                    // TODO: segments do not exist in this scope, need to do it in different location
+                    break;
+                }
+                case DELTA_EVENT_TYPES.SEGMENT_REMOVED: {
+                    // TODO: segments do not exist in this scope, need to do it in different location
+                    break;
+                }
+                default:
+                // TODO: something is seriously wrong
+            }
+        }
+    }
+}

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

@@ -1,110 +0,0 @@
-import { RevisionDelta } from './revision-delta';
-import type { Revision } from './client-feature-toggle-delta';
-
-describe('RevisionCache', () => {
-    it('should create a new base when trying to add a new revision at the max limit', () => {
-        const initialRevisions: Revision[] = [
-            {
-                revisionId: 1,
-                updated: [
-                    {
-                        name: 'test-flag',
-                        type: 'release',
-                        enabled: false,
-                        project: 'default',
-                        stale: false,
-                        strategies: [
-                            {
-                                name: 'flexibleRollout',
-                                constraints: [],
-                                parameters: {
-                                    groupId: 'test-flag',
-                                    rollout: '100',
-                                    stickiness: 'default',
-                                },
-                                variants: [],
-                            },
-                        ],
-                        variants: [],
-                        description: null,
-                        impressionData: false,
-                    },
-                ],
-                removed: [],
-            },
-            {
-                revisionId: 2,
-                updated: [
-                    {
-                        name: 'my-feature-flag',
-                        type: 'release',
-                        enabled: true,
-                        project: 'default',
-                        stale: false,
-                        strategies: [
-                            {
-                                name: 'flexibleRollout',
-                                constraints: [],
-                                parameters: {
-                                    groupId: 'my-feature-flag',
-                                    rollout: '100',
-                                    stickiness: 'default',
-                                },
-                                variants: [],
-                            },
-                        ],
-                        variants: [],
-                        description: null,
-                        impressionData: false,
-                    },
-                ],
-                removed: [],
-            },
-        ];
-
-        const maxLength = 2;
-        const deltaCache = new RevisionDelta(initialRevisions, maxLength);
-
-        // Add a new revision to trigger changeBase
-        deltaCache.addRevision({
-            revisionId: 3,
-            updated: [
-                {
-                    name: 'another-feature-flag',
-                    type: 'release',
-                    enabled: true,
-                    project: 'default',
-                    stale: false,
-                    strategies: [
-                        {
-                            name: 'flexibleRollout',
-                            constraints: [],
-                            parameters: {
-                                groupId: 'another-feature-flag',
-                                rollout: '100',
-                                stickiness: 'default',
-                            },
-                            variants: [],
-                        },
-                    ],
-                    variants: [],
-                    description: null,
-                    impressionData: false,
-                },
-            ],
-            removed: [],
-        });
-
-        const revisions = deltaCache.getRevisions();
-
-        // Check that the base has been changed and merged correctly
-        expect(revisions.length).toBe(2);
-        expect(revisions[0].updated.length).toBe(2);
-        expect(revisions[0].updated).toEqual(
-            expect.arrayContaining([
-                expect.objectContaining({ name: 'test-flag' }),
-                expect.objectContaining({ name: 'my-feature-flag' }),
-            ]),
-        );
-    });
-});

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

@@ -1,48 +0,0 @@
-import type { Revision } from './client-feature-toggle-delta';
-
-const mergeWithoutDuplicates = (arr1: any[], arr2: any[]) => {
-    const map = new Map();
-    arr1.concat(arr2).forEach((item) => {
-        map.set(item.name, item);
-    });
-    return Array.from(map.values());
-};
-
-export class RevisionDelta {
-    private delta: Revision[];
-    private maxLength: number;
-
-    constructor(data: Revision[] = [], maxLength: number = 20) {
-        this.delta = data;
-        this.maxLength = maxLength;
-    }
-
-    public addRevision(revision: Revision): void {
-        if (this.delta.length >= this.maxLength) {
-            this.changeBase();
-        }
-
-        this.delta = [...this.delta, revision];
-    }
-
-    public getRevisions(): Revision[] {
-        return this.delta;
-    }
-
-    public hasRevision(revisionId: number): boolean {
-        return this.delta.some(
-            (revision) => revision.revisionId === revisionId,
-        );
-    }
-
-    private changeBase(): void {
-        if (!(this.delta.length >= 2)) return;
-        const base = this.delta[0];
-        const newBase = this.delta[1];
-
-        newBase.removed = mergeWithoutDuplicates(base.removed, newBase.removed);
-        newBase.updated = mergeWithoutDuplicates(base.updated, newBase.updated);
-
-        this.delta = [newBase, ...this.delta.slice(2)];
-    }
-}

src/lib/openapi/spec/client-features-delta-schema.test.ts

@@ -3,9 +3,16 @@ import type { ClientFeaturesDeltaSchema } from './client-features-delta-schema';
 
 test('clientFeaturesDeltaSchema all fields', () => {
     const data: ClientFeaturesDeltaSchema = {
-        revisionId: 6,
+        events: [
-        updated: [
             {
+                eventId: 1,
+                type: 'feature-removed',
+                featureName: 'removed-event',
+            },
+            {
+                eventId: 1,
+                type: 'feature-updated',
+                feature: {
                     impressionData: false,
                     enabled: false,
                     name: 'base_feature',
@@ -16,9 +23,22 @@ test('clientFeaturesDeltaSchema all fields', () => {
                     variants: [],
                     strategies: [],
                 },
+            },
+            {
+                eventId: 1,
+                type: 'segment-removed',
+                segmentId: 33,
+            },
+            {
+                eventId: 1,
+                type: 'segment-updated',
+                segment: {
+                    id: 3,
+                    name: 'hello',
+                    constraints: [],
+                },
+            },
         ],
-        removed: [],
-        segments: [],
     };
 
     expect(

src/lib/openapi/spec/client-features-delta-schema.ts

@@ -13,36 +13,79 @@ import { dependentFeatureSchema } from './dependent-feature-schema';
 export const clientFeaturesDeltaSchema = {
     $id: '#/components/schemas/clientFeaturesDeltaSchema',
     type: 'object',
-    required: ['updated', 'revisionId', 'removed'],
+    required: ['events'],
     description: 'Schema for delta updates of feature configurations.',
     properties: {
-        updated: {
+        events: {
-            description: 'A list of updated feature configurations.',
+            description: 'A list of delta events.',
+            type: 'array',
+            items: {
+                type: 'object',
+                anyOf: [
+                    {
+                        type: 'object',
+                        required: ['eventId', 'type', 'feature'],
+                        properties: {
+                            eventId: { type: 'number' },
+                            type: { type: 'string', enum: ['feature-updated'] },
+                            feature: {
+                                $ref: '#/components/schemas/clientFeatureSchema',
+                            },
+                        },
+                    },
+                    {
+                        type: 'object',
+                        required: ['eventId', 'type', 'featureName'],
+                        properties: {
+                            eventId: { type: 'number' },
+                            type: { type: 'string', enum: ['feature-removed'] },
+                            featureName: { type: 'string' },
+                        },
+                    },
+                    {
+                        type: 'object',
+                        required: ['eventId', 'type', 'segment'],
+                        properties: {
+                            eventId: { type: 'number' },
+                            type: { type: 'string', enum: ['segment-updated'] },
+                            segment: {
+                                $ref: '#/components/schemas/clientSegmentSchema',
+                            },
+                        },
+                    },
+                    {
+                        type: 'object',
+                        required: ['eventId', 'type', 'segmentId'],
+                        properties: {
+                            eventId: { type: 'number' },
+                            type: { type: 'string', enum: ['segment-removed'] },
+                            segmentId: { type: 'number' },
+                        },
+                    },
+                    {
+                        type: 'object',
+                        required: ['type', 'features', 'segments', 'eventId'],
+                        properties: {
+                            eventId: { type: 'number' },
+                            type: { type: 'string', enum: ['hydration'] },
+                            features: {
                                 type: 'array',
                                 items: {
                                     $ref: '#/components/schemas/clientFeatureSchema',
                                 },
                             },
-        revisionId: {
-            type: 'number',
-            description: 'The revision ID of the delta update.',
-        },
-        removed: {
-            description: 'A list of feature names that were removed.',
-            type: 'array',
-            items: {
-                type: 'string',
-            },
-        },
                             segments: {
-            description:
-                'A list of [Segments](https://docs.getunleash.io/reference/segments) configured for this Unleash instance',
                                 type: 'array',
                                 items: {
                                     $ref: '#/components/schemas/clientSegmentSchema',
                                 },
                             },
                         },
+                    },
+                ],
+            },
+        },
+    },
     components: {
         schemas: {
             constraintSchema,