fix(autoscaling): every deployment resets capacity (#5507)

* fix(autoscaling): every deployment resets capacity

If `DesiredCapacity` is specified in the CloudFormation template, on every
deployment the capacity of the AutoScalingGroup is reset to that number,
even if the group had been scaled out at that point. The solution is to
leave DesiredCapacity empty, in which case it will remain untouched
during a deployment.

Previously, CDK would use some logic to always calculate a
DesiredCapacity for you, even if you left the `desiredCapacity` property
unset, leading to the undesirable behavior--which frankly represents
an availability risk.

Now, if you don't specify `desiredCapacity`, we won't set
`DesiredCapacity` either, avoiding the availability risk that we
introduced beforehand. In fact, if you *do* set `desiredCapacity`, we
will warn you that you probably shouldn't using a construct warning.

Fixes #5215, closes #5208.

BREAKING CHANGE: AutoScalingGroups without `desiredCapacity` are now
initially scaled to their minimum capacity (instead of their maximum
capaciety).

* Add links

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: rix0rrr

packages/@aws-cdk/aws-autoscaling/lib/auto-scaling-group.ts

@@ -44,7 +44,11 @@ export interface CommonAutoScalingGroupProps {
   /**
    * Initial amount of instances in the fleet
    *
-   * @default 1
+   * If this is set to a number, every deployment will reset the amount of
+   * instances to this number. It is recommended to leave this value blank.
+   *
+   * @default minCapacity, and leave unchanged during deployment
+   * @see https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-as-group.html#cfn-as-group-desiredcapacity
    */
   readonly desiredCapacity?: number;
 
@@ -475,30 +479,40 @@ export class AutoScalingGroup extends AutoScalingGroupBase implements
 
     launchConfig.node.addDependency(this.role);
 
-    const desiredCapacity =
-        (props.desiredCapacity !== undefined ? props.desiredCapacity :
-        (props.minCapacity !== undefined ? props.minCapacity :
-        (props.maxCapacity !== undefined ? props.maxCapacity : 1)));
+    // desiredCapacity just reflects what the user has supplied.
+    const desiredCapacity = props.desiredCapacity;
     const minCapacity = props.minCapacity !== undefined ? props.minCapacity : 1;
-    const maxCapacity = props.maxCapacity !== undefined ? props.maxCapacity : desiredCapacity;
+    const maxCapacity = props.maxCapacity !== undefined ? props.maxCapacity :
+                        desiredCapacity !== undefined ? desiredCapacity : Math.max(minCapacity, 1);
 
+    withResolved(minCapacity, maxCapacity, (min, max) => {
+      if (min > max) {
+        throw new Error(`minCapacity (${min}) should be <= maxCapacity (${max})`);
+      }
+    });
     withResolved(desiredCapacity, minCapacity, (desired, min) => {
+      if (desired === undefined) { return; }
       if (desired < min) {
         throw new Error(`Should have minCapacity (${min}) <= desiredCapacity (${desired})`);
       }
     });
     withResolved(desiredCapacity, maxCapacity, (desired, max) => {
+      if (desired === undefined) { return; }
       if (max < desired) {
         throw new Error(`Should have desiredCapacity (${desired}) <= maxCapacity (${max})`);
       }
     });
 
+    if (desiredCapacity !== undefined) {
+      this.node.addWarning(`desiredCapacity has been configured. Be aware this will reset the size of your AutoScalingGroup on every deployment. See https://github.com/aws/aws-cdk/issues/5215`);
+    }
+
     const { subnetIds, hasPublic } = props.vpc.selectSubnets(props.vpcSubnets);
     const asgProps: CfnAutoScalingGroupProps = {
       cooldown: props.cooldown !== undefined ? props.cooldown.toSeconds().toString() : undefined,
       minSize: Tokenization.stringifyNumber(minCapacity),
       maxSize: Tokenization.stringifyNumber(maxCapacity),
-      desiredCapacity: Tokenization.stringifyNumber(desiredCapacity),
+      desiredCapacity: desiredCapacity !== undefined ? Tokenization.stringifyNumber(desiredCapacity) : undefined,
       launchConfigurationName: launchConfig.ref,
       loadBalancerNames: Lazy.listValue({ produce: () => this.loadBalancerNames }, { omitEmpty: true }),
       targetGroupArns: Lazy.listValue({ produce: () => this.targetGroupArns }, { omitEmpty: true }),

packages/@aws-cdk/aws-autoscaling/test/integ.amazonlinux2.expected.json

@@ -451,7 +451,6 @@
       "Properties": {
         "MaxSize": "1",
         "MinSize": "1",
-        "DesiredCapacity": "1",
         "LaunchConfigurationName": {
           "Ref": "FleetLaunchConfig59F79D36"
         },

packages/@aws-cdk/aws-autoscaling/test/integ.asg-w-classic-loadbalancer.expected.json

@@ -631,7 +631,6 @@
       "Properties": {
         "MaxSize": "1",
         "MinSize": "1",
-        "DesiredCapacity": "1",
         "LaunchConfigurationName": {
           "Ref": "FleetLaunchConfig59F79D36"
         },

packages/@aws-cdk/aws-autoscaling/test/integ.asg-w-elbv2.expected.json

@@ -472,7 +472,6 @@
       "Properties": {
         "MaxSize": "1",
         "MinSize": "1",
-        "DesiredCapacity": "1",
         "LaunchConfigurationName": {
           "Ref": "FleetLaunchConfig59F79D36"
         },

packages/@aws-cdk/aws-autoscaling/test/integ.custom-scaling.expected.json

@@ -451,7 +451,6 @@
       "Properties": {
         "MaxSize": "1",
         "MinSize": "1",
-        "DesiredCapacity": "1",
         "LaunchConfigurationName": {
           "Ref": "FleetLaunchConfig59F79D36"
         },

packages/@aws-cdk/aws-autoscaling/test/integ.external-role.expected.json

@@ -604,7 +604,6 @@
       "Properties": {
         "MaxSize": "1",
         "MinSize": "1",
-        "DesiredCapacity": "1",
         "LaunchConfigurationName": {
           "Ref": "ASGLaunchConfigC00AF12B"
         },

packages/@aws-cdk/aws-autoscaling/test/integ.spot-instances.expected.json

@@ -452,7 +452,6 @@
       "Properties": {
         "MaxSize": "1",
         "MinSize": "1",
-        "DesiredCapacity": "1",
         "LaunchConfigurationName": {
           "Ref": "FleetLaunchConfig59F79D36"
         },

packages/@aws-cdk/aws-autoscaling/test/test.auto-scaling-group.ts

@@ -113,7 +113,6 @@ export = {
             }
           },
           "Properties": {
-            "DesiredCapacity": "1",
             "LaunchConfigurationName": {
               "Ref": "MyFleetLaunchConfig5D7F9801"
             },
@@ -252,7 +251,6 @@ export = {
     expect(stack).to(haveResource("AWS::AutoScaling::AutoScalingGroup", {
       MinSize: "10",
       MaxSize: "10",
-      DesiredCapacity: "10",
     }
     ));
 
@@ -275,8 +273,7 @@ export = {
     // THEN
     expect(stack).to(haveResource("AWS::AutoScaling::AutoScalingGroup", {
       MinSize: "1",
-      MaxSize: "10",
-      DesiredCapacity: "10",
+      MaxSize: "10"
     }
     ));
 

packages/@aws-cdk/aws-autoscaling/test/test.scheduled-action.ts

@@ -64,7 +64,6 @@ export = {
           Properties: {
             MaxSize: "1",
             MinSize: "1",
-            DesiredCapacity: "1",
             LaunchConfigurationName: { Ref: "ASGLaunchConfigC00AF12B" },
             Tags: [
               {

packages/@aws-cdk/aws-codedeploy/test/server/integ.deployment-group.expected.json

@@ -683,7 +683,6 @@
       "Properties": {
         "MaxSize": "1",
         "MinSize": "1",
-        "DesiredCapacity": "1",
         "LaunchConfigurationName": {
           "Ref": "ASGLaunchConfigC00AF12B"
         },
@@ -885,4 +884,4 @@
       "Default": "/aws/service/ami-amazon-linux-latest/amzn-ami-hvm-x86_64-gp2"
     }
   }
-}
+}