From 5d4d24c0cf7927f5883393ed768110aa951b352f Mon Sep 17 00:00:00 2001
From: Krishan Thisera <krishan.thisera@yahoo.com>
Date: Wed, 1 Nov 2023 14:49:21 +1030
Subject: [PATCH] DO-1560: make the scaling options configurable

---
 packages/prerender-fargate/index.ts           |  6 +-
 .../lib/prerender-fargate-options.ts          | 58 +++++++++++++++++++
 .../lib/prerender-fargate.ts                  | 27 +++++++--
 3 files changed, 85 insertions(+), 6 deletions(-)

diff --git a/packages/prerender-fargate/index.ts b/packages/prerender-fargate/index.ts
index e00f084e..ed7b803c 100644
--- a/packages/prerender-fargate/index.ts
+++ b/packages/prerender-fargate/index.ts
@@ -1,9 +1,13 @@
 import { PrerenderFargate } from "./lib/prerender-fargate";
-import { PrerenderFargateOptions } from "./lib/prerender-fargate-options";
+import {
+  PrerenderFargateOptions,
+  PrerenderFargateScalingOptions,
+} from "./lib/prerender-fargate-options";
 import { PrerenderTokenUrlAssociationOptions } from "./lib/recaching/prerender-tokens";
 
 export {
   PrerenderFargate,
   PrerenderFargateOptions,
+  PrerenderFargateScalingOptions,
   PrerenderTokenUrlAssociationOptions,
 };
diff --git a/packages/prerender-fargate/lib/prerender-fargate-options.ts b/packages/prerender-fargate/lib/prerender-fargate-options.ts
index a90a320c..95d1d22b 100644
--- a/packages/prerender-fargate/lib/prerender-fargate-options.ts
+++ b/packages/prerender-fargate/lib/prerender-fargate-options.ts
@@ -1,4 +1,5 @@
 import { PrerenderTokenUrlAssociationOptions } from "./recaching/prerender-tokens";
+import * as ec2 from "aws-cdk-lib/aws-ec2";
 
 /**
  * Options for configuring the Prerender Fargate construct.
@@ -35,6 +36,10 @@ export interface PrerenderFargateOptions {
    * The ARN of the SSL certificate to use for HTTPS connections.
    */
   certificateArn: string;
+  /**
+   * The minimum number of Fargate instances to run.
+   */
+  minInstanceCount?: number;
   /**
    * The desired number of Fargate instances to run.
    */
@@ -78,4 +83,57 @@ export interface PrerenderFargateOptions {
    * ```
    */
   tokenUrlAssociation?: PrerenderTokenUrlAssociationOptions;
+  /**
+   * Prerender Fargate Scaling option
+   * This allows to alter the scaling behavior. The default configuration should be sufficient
+   * for most of the cases.
+   */
+  prerenderFargateScalingOptions?: PrerenderFargateScalingOptions;
+}
+
+/**
+ * Prerender Fargate Scaling option
+ */
+export interface PrerenderFargateScalingOptions {
+  /**
+   * Fargate service health check grace period.
+   * The minimum number of tasks, specified as a percentage of
+   * the Amazon ECS service's DesiredCount value, that must
+   * continue to run and remain healthy during a deployment.
+   * @default - 20 seconds
+   */
+  healthCheckGracePeriod?: number;
+  /**
+   * Fargate service minimum healthy percent.
+   * @default - 0
+   */
+  minHealthyPercent?: number;
+  /**
+   * Fargate service maximum healthy percent.
+   * This limits the scheduler from starting a replacement task first,
+   * the scheduler will stop an unhealthy task one at a time at random to
+   * free up capacity, and then start a replacement task
+   * @default - 200
+   */
+  maxHealthyPercent?: number;
+  /**
+   * Health check interval in seconds.
+   * @default - 50
+   */
+  healthCheckInterval?: number;
+  /**
+   * Scale in cooldown in seconds.
+   * @default - 60
+   */
+  scaleInCooldown?: number;
+  /**
+   * Scale out cooldown in seconds.
+   * @default - 60
+   */
+  scaleOutCooldown?: number;
+  /**
+   * The number of consecutive health check failures required before considering a task unhealthy.
+   * @default - 5
+   */
+  unhealthyThresholdCount?: number;
 }
diff --git a/packages/prerender-fargate/lib/prerender-fargate.ts b/packages/prerender-fargate/lib/prerender-fargate.ts
index 2868112e..2988fc51 100644
--- a/packages/prerender-fargate/lib/prerender-fargate.ts
+++ b/packages/prerender-fargate/lib/prerender-fargate.ts
@@ -98,6 +98,8 @@ export class PrerenderFargate extends Construct {
       bucketName,
       domainName,
       prerenderName,
+      minInstanceCount,
+      prerenderFargateScalingOptions,
     } = props;
 
     // Create bucket for prerender storage
@@ -160,7 +162,6 @@ export class PrerenderFargate extends Construct {
               TOKEN_LIST: tokenList.toString(),
             },
           },
-          healthCheckGracePeriod: Duration.seconds(20),
           publicLoadBalancer: true,
           assignPublicIp: true,
           listenerPort: 443,
@@ -174,6 +175,14 @@ export class PrerenderFargate extends Construct {
             "cert",
             certificateArn
           ),
+          // Scaling configuration
+          healthCheckGracePeriod: Duration.seconds(
+            prerenderFargateScalingOptions?.healthCheckGracePeriod || 20
+          ),
+          minHealthyPercent:
+            prerenderFargateScalingOptions?.minHealthyPercent || 50,
+          maxHealthyPercent:
+            prerenderFargateScalingOptions?.maxHealthyPercent || 200,
         }
       );
 
@@ -184,19 +193,27 @@ export class PrerenderFargate extends Construct {
     // It should be considered healthy when receiving a 401 response
     fargateService.targetGroup.configureHealthCheck({
       path: "/health",
-      interval: Duration.seconds(120),
-      unhealthyThresholdCount: 5,
+      interval: Duration.seconds(
+        prerenderFargateScalingOptions?.healthCheckInterval || 120
+      ),
+      unhealthyThresholdCount:
+        prerenderFargateScalingOptions?.unhealthyThresholdCount || 5,
       healthyHttpCodes: "401",
     });
 
     // Setup AutoScaling policy
     const scaling = fargateService.service.autoScaleTaskCount({
       maxCapacity: maxInstanceCount || 2,
+      minCapacity: minInstanceCount || 1,
     });
     scaling.scaleOnCpuUtilization(`${prerenderName}-scaling`, {
       targetUtilizationPercent: 50,
-      scaleInCooldown: Duration.seconds(60),
-      scaleOutCooldown: Duration.seconds(60),
+      scaleInCooldown: Duration.seconds(
+        prerenderFargateScalingOptions?.scaleInCooldown || 60
+      ),
+      scaleOutCooldown: Duration.seconds(
+        prerenderFargateScalingOptions?.scaleOutCooldown || 60
+      ),
     });
 
     /**