fix(elasticloadbalancingv2): allow alb slow start duration of 0 seconds (#29445)

Closes #29437.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: msambol

packages/aws-cdk-lib/aws-elasticloadbalancingv2/README.md

@@ -335,6 +335,26 @@ const tg2 = new elbv2.ApplicationTargetGroup(this, 'TG2', {
 });
 ```
 
+### Slow start mode for your Application Load Balancer
+
+By default, a target starts to receive its full share of requests as soon as it is registered with a target group and passes an initial health check. Using slow start mode gives targets time to warm up before the load balancer sends them a full share of requests.
+
+After you enable slow start for a target group, its targets enter slow start mode when they are considered healthy by the target group. A target in slow start mode exits slow start mode when the configured slow start duration period elapses or the target becomes unhealthy. The load balancer linearly increases the number of requests that it can send to a target in slow start mode. After a healthy target exits slow start mode, the load balancer can send it a full share of requests.
+
+The allowed range is 30-900 seconds (15 minutes). The default is 0 seconds (disabled).
+
+```ts
+declare const vpc: ec2.Vpc;
+
+// Target group with slow start mode enabled
+const tg = new elbv2.ApplicationTargetGroup(this, 'TG', {
+  targetType: elbv2.TargetType.INSTANCE,
+  slowStart: Duration.seconds(60),
+  port: 80,
+  vpc,
+});
+```
+
 For more information see: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/sticky-sessions.html#application-based-stickiness
 
 ### Setting the target group protocol version

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

@@ -330,8 +330,9 @@ export class ApplicationTargetGroup extends TargetGroupBase implements IApplicat
 
     if (props) {
       if (props.slowStart !== undefined) {
-        if (props.slowStart.toSeconds() < 30 || props.slowStart.toSeconds() > 900) {
-          throw new Error('Slow start duration value must be between 30 and 900 seconds.');
+        // 0 is allowed and disables slow start
+        if ((props.slowStart.toSeconds() < 30 && props.slowStart.toSeconds() !== 0) || props.slowStart.toSeconds() > 900) {
+          throw new Error('Slow start duration value must be between 30 and 900 seconds, or 0 to disable slow start.');
         }
         this.setAttribute('slow_start.duration_seconds', props.slowStart.toSeconds().toString());
       }

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

@@ -340,7 +340,33 @@ describe('tests', () => {
           slowStart: badDuration,
           vpc,
         });
-      }).toThrow(/Slow start duration value must be between 30 and 900 seconds./);
+      }).toThrow(/Slow start duration value must be between 30 and 900 seconds, or 0 to disable slow start./);
+    });
+  });
+
+  test('Disable slow start by setting to 0 seconds', () => {
+    const app = new cdk.App();
+    const stack = new cdk.Stack(app, 'Stack');
+    const vpc = new ec2.Vpc(stack, 'VPC', {});
+
+    // WHEN
+    new elbv2.ApplicationTargetGroup(stack, 'TargetGroup', {
+      slowStart: cdk.Duration.seconds(0),
+      vpc,
+    });
+
+    // THEN
+    Template.fromStack(stack).hasResourceProperties('AWS::ElasticLoadBalancingV2::TargetGroup', {
+      TargetGroupAttributes: [
+        {
+          Key: 'slow_start.duration_seconds',
+          Value: '0',
+        },
+        {
+          Key: 'stickiness.enabled',
+          Value: 'false',
+        },
+      ],
     });
   });