fix(core): Emit leader-takeover on leadership mismatch in checkLeader (#27126)

packages/cli/src/scaling/__tests__/multi-main-setup.ee.test.ts

@@ -0,0 +1,110 @@
+import { mockLogger } from '@n8n/backend-test-utils';
+import type { GlobalConfig } from '@n8n/config';
+import { MultiMainMetadata } from '@n8n/decorators';
+import { mock } from 'jest-mock-extended';
+import type { ErrorReporter, InstanceSettings } from 'n8n-core';
+
+import type { Publisher } from '@/scaling/pubsub/publisher.service';
+import type { RedisClientService } from '@/services/redis-client.service';
+
+import { MultiMainSetup } from '../multi-main-setup.ee';
+
+describe('MultiMainSetup', () => {
+	const hostId = 'main-n8n-main-0';
+
+	const logger = mockLogger();
+	const publisher = mock<Publisher>();
+	const redisClientService = mock<RedisClientService>();
+	const errorReporter = mock<ErrorReporter>();
+	const metadata = new MultiMainMetadata();
+
+	const globalConfig = mock<GlobalConfig>({
+		redis: { prefix: 'n8n' },
+		multiMainSetup: { ttl: 10, interval: 3, enabled: true },
+	});
+
+	redisClientService.toValidPrefix.mockReturnValue('n8n');
+
+	let instanceSettings: InstanceSettings;
+	let multiMainSetup: MultiMainSetup;
+
+	beforeEach(() => {
+		instanceSettings = mock<InstanceSettings>({ hostId, isLeader: false });
+		multiMainSetup = new MultiMainSetup(
+			logger,
+			instanceSettings,
+			publisher,
+			redisClientService,
+			globalConfig,
+			metadata,
+			errorReporter,
+		);
+		jest.clearAllMocks();
+	});
+
+	describe('checkLeader', () => {
+		beforeEach(async () => {
+			await multiMainSetup.init();
+		});
+
+		it('should emit `leader-takeover` when Redis has own `hostId` but instance thinks it is follower', async () => {
+			publisher.get.mockResolvedValue(hostId);
+			const emit = jest.spyOn(multiMainSetup, 'emit');
+
+			// @ts-expect-error - private method
+			await multiMainSetup.checkLeader();
+
+			expect(instanceSettings.markAsLeader).toHaveBeenCalled();
+			expect(emit).toHaveBeenCalledWith('leader-takeover');
+		});
+
+		it('should not emit `leader-takeover` when already leader', async () => {
+			publisher.get.mockResolvedValue(hostId);
+			Object.defineProperty(instanceSettings, 'isLeader', { get: () => true });
+			const emit = jest.spyOn(multiMainSetup, 'emit');
+
+			// @ts-expect-error - private method
+			await multiMainSetup.checkLeader();
+
+			expect(instanceSettings.markAsLeader).not.toHaveBeenCalled();
+			expect(emit).not.toHaveBeenCalledWith('leader-takeover');
+			expect(publisher.setExpiration).toHaveBeenCalled();
+		});
+
+		it('should emit `leader-stepdown` when another instance is leader', async () => {
+			publisher.get.mockResolvedValue('main-n8n-main-1');
+			Object.defineProperty(instanceSettings, 'isLeader', { get: () => true });
+			const emit = jest.spyOn(multiMainSetup, 'emit');
+
+			// @ts-expect-error - private method
+			await multiMainSetup.checkLeader();
+
+			expect(instanceSettings.markAsFollower).toHaveBeenCalled();
+			expect(emit).toHaveBeenCalledWith('leader-stepdown');
+		});
+
+		it('should not emit `leader-stepdown` when already a follower', async () => {
+			publisher.get.mockResolvedValue('main-n8n-main-1');
+			const emit = jest.spyOn(multiMainSetup, 'emit');
+
+			// @ts-expect-error - private method
+			await multiMainSetup.checkLeader();
+
+			expect(emit).not.toHaveBeenCalledWith('leader-stepdown');
+		});
+
+		it('should attempt to become leader when leadership is vacant', async () => {
+			publisher.get.mockResolvedValue(null);
+			publisher.setIfNotExists.mockResolvedValue(true);
+			const emit = jest.spyOn(multiMainSetup, 'emit');
+
+			// @ts-expect-error - private method
+			await multiMainSetup.checkLeader();
+
+			expect(instanceSettings.markAsFollower).toHaveBeenCalled();
+			expect(emit).toHaveBeenCalledWith('leader-stepdown');
+			expect(emit).toHaveBeenCalledWith('leader-takeover');
+			expect(instanceSettings.markAsLeader).toHaveBeenCalled();
+		});
+	});
+});

packages/cli/src/scaling/multi-main-setup.ee.ts

@@ -84,8 +84,11 @@ export class MultiMainSetup extends TypedEmitter<MultiMainEvents> {
 						shouldReport: true,
 					},
 				);
+
+				this.instanceSettings.markAsLeader();
+
+				this.emit('leader-takeover');
 			}
-			this.instanceSettings.markAsLeader();
 
 			this.logger.debug(`[Instance ID ${hostId}] Leader is this instance`);