chore(elasticloadbalancingv2): add warning for missing ApplicationTargetGroup protocol

packages/aws-cdk-lib/aws-elasticloadbalancingv2/lib/alb/application-target-group.ts

@@ -404,6 +404,14 @@ export class ApplicationTargetGroup extends TargetGroupBase implements IApplicat
         this.setAttribute('load_balancing.algorithm.anomaly_mitigation', props.enableAnomalyMitigation ? 'on' : 'off');
       }
     }
+
+    // Add a warning if protocol wasn't explicitly specified for non-Lambda targets
+    if (props.protocol === undefined && this.targetType !== TargetType.LAMBDA) {
+      Annotations.of(this).addWarningV2(
+        '@aws-cdk/aws-elbv2:target-group-protocol-default',
+        'ApplicationTargetGroup protocol not specified. Please specify protocol explicitly (e.g., ApplicationProtocol.HTTP or ApplicationProtocol.HTTPS). Note: Missing protocol will cause deployment failures.',
+      );
+    }
   }
 
   public get metrics(): IApplicationTargetGroupMetrics {

packages/aws-cdk-lib/aws-elasticloadbalancingv2/test/alb/target-group.test.ts

@@ -77,6 +77,81 @@ describe('tests', () => {
     expect(() => app.synth()).toThrow(/port\/protocol should not be specified for Lambda targets/);
   });
 
+  test('ApplicationTargetGroup shows warning when protocol not specified', () => {
+    // GIVEN
+    const app = new cdk.App();
+    const stack = new cdk.Stack(app, 'Stack');
+    const vpc = new ec2.Vpc(stack, 'Vpc');
+
+    // WHEN - Create ApplicationTargetGroup without protocol but with port
+    new elbv2.ApplicationTargetGroup(stack, 'TG', {
+      vpc,
+      port: 80, // Providing port satisfies the "at least one of port or protocol" requirement
+      // protocol is missing - this should generate a warning
+    });
+
+    // THEN - Should synthesize successfully but show warning
+    const assembly = app.synth();
+    const stackArtifact = assembly.getStackByName(stack.stackName);
+    const warnings = stackArtifact.messages.filter(m => m.level === 'warning');
+    expect(warnings.some(w => w.entry.data.includes('ApplicationTargetGroup protocol not specified'))).toBe(true);
+  });
+
+  test('ApplicationTargetGroup with protocol should not throw validation error', () => {
+    // GIVEN
+    const app = new cdk.App();
+    const stack = new cdk.Stack(app, 'Stack');
+    const vpc = new ec2.Vpc(stack, 'Vpc');
+
+    // WHEN - Create ApplicationTargetGroup with protocol
+    new elbv2.ApplicationTargetGroup(stack, 'TG', {
+      vpc,
+      port: 80,
+      protocol: elbv2.ApplicationProtocol.HTTP,
+    });
+
+    // THEN - Should not throw
+    expect(() => app.synth()).not.toThrow();
+  });
+
+  test('ApplicationTargetGroup with Lambda target does not require protocol', () => {
+    // GIVEN
+    const app = new cdk.App();
+    const stack = new cdk.Stack(app, 'Stack');
+
+    // WHEN - Create ApplicationTargetGroup for Lambda without protocol
+    new elbv2.ApplicationTargetGroup(stack, 'TG', {
+      targetType: elbv2.TargetType.LAMBDA,
+      // protocol is missing but should be allowed for Lambda
+    });
+
+    // THEN - Should not throw
+    expect(() => app.synth()).not.toThrow();
+  });
+
+  test('ApplicationTargetGroup requires protocol even when targets added later', () => {
+    // GIVEN
+    const app = new cdk.App();
+    const stack = new cdk.Stack(app, 'Stack');
+    const vpc = new ec2.Vpc(stack, 'Vpc');
+
+    // WHEN - Create ApplicationTargetGroup without protocol but with port, then add non-Lambda target
+    const tg = new elbv2.ApplicationTargetGroup(stack, 'TG', {
+      vpc,
+      port: 80, // Providing port satisfies the "at least one of port or protocol" requirement
+      // protocol is missing - this should generate a warning
+    });
+
+    // Add a non-Lambda target
+    tg.addTarget(new elbv2.InstanceTarget('i-1234'));
+
+    // THEN - Should synthesize successfully but show warning
+    const assembly = app.synth();
+    const stackArtifact = assembly.getStackByName(stack.stackName);
+    const warnings = stackArtifact.messages.filter(m => m.level === 'warning');
+    expect(warnings.some(w => w.entry.data.includes('ApplicationTargetGroup protocol not specified'))).toBe(true);
+  });
+
   test('Can add self-registering target to imported TargetGroup', () => {
     // GIVEN
     const app = new cdk.App();