fix(ec2): descriptive error message when selecting 0 subnets

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

@@ -53,7 +53,7 @@ export interface CommonAutoScalingGroupProps {
   /**
    * Where to place instances within the VPC
    */
-  vpcPlacement?: ec2.VpcPlacementStrategy;
+  vpcSubnets?: ec2.SubnetSelection;
 
   /**
    * SNS topic to send notifications about fleet changes
@@ -291,8 +291,7 @@ export class AutoScalingGroup extends cdk.Construct implements IAutoScalingGroup
       });
     }
 
-    const subnets = props.vpc.subnets(props.vpcPlacement);
-    asgProps.vpcZoneIdentifier = subnets.map(n => n.subnetId);
+    asgProps.vpcZoneIdentifier = props.vpc.subnetIds(props.vpcSubnets);
 
     this.autoScalingGroup = new CfnAutoScalingGroup(this, 'ASG', asgProps);
     this.osType = machineImage.os.type;

packages/@aws-cdk/aws-ec2/README.md

@@ -21,7 +21,7 @@ instances for your project.
 
 Our default `VpcNetwork` class creates a private and public subnet for every
 availability zone. Classes that use the VPC will generally launch instances
-into all private subnets, and provide a parameter called `vpcPlacement` to
+into all private subnets, and provide a parameter called `vpcSubnets` to
 allow you to override the placement. [Read more about
 subnets](https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html).
 

packages/@aws-cdk/aws-ec2/lib/vpc-ref.ts

@@ -1,5 +1,5 @@
 import { Construct, IConstruct, IDependable } from "@aws-cdk/cdk";
-import { subnetName } from './util';
+import { DEFAULT_SUBNET_NAME, subnetName } from './util';
 import { VpnConnection, VpnConnectionOptions } from './vpn';
 
 export interface IVpcSubnet extends IConstruct {
@@ -61,9 +61,20 @@ export interface IVpcNetwork extends IConstruct {
   readonly vpnGatewayId?: string;
 
   /**
-   * Return the subnets appropriate for the placement strategy
+   * Return IDs of the subnets appropriate for the given selection strategy
+   *
+   * Requires that at least once subnet is matched, throws a descriptive
+   * error message otherwise.
+   *
+   * Prefer to use this method over {@link subnets} if you need to pass subnet
+   * IDs to a CloudFormation Resource.
    */
-  subnets(placement?: VpcPlacementStrategy): IVpcSubnet[];
+  subnetIds(selection?: SubnetSelection): string[];
+
+  /**
+   * Return a dependable object representing internet connectivity for the given subnets
+   */
+  subnetInternetDependencies(selection?: SubnetSelection): IDependable;
 
   /**
    * Return whether the given subnet is one of this VPC's public subnets.
@@ -125,29 +136,29 @@ export enum SubnetType {
 }
 
 /**
- * Customize how instances are placed inside a VPC
+ * Customize subnets that are selected for placement of ENIs
  *
  * Constructs that allow customization of VPC placement use parameters of this
  * type to provide placement settings.
  *
  * By default, the instances are placed in the private subnets.
  */
-export interface VpcPlacementStrategy {
+export interface SubnetSelection {
   /**
    * Place the instances in the subnets of the given type
    *
-   * At most one of `subnetsToUse` and `subnetName` can be supplied.
+   * At most one of `subnetType` and `subnetName` can be supplied.
    *
    * @default SubnetType.Private
    */
-  subnetsToUse?: SubnetType;
+  subnetType?: SubnetType;
 
   /**
    * Place the instances in the subnets with the given name
    *
    * (This is the name supplied in subnetConfiguration).
    *
-   * At most one of `subnetsToUse` and `subnetName` can be supplied.
+   * At most one of `subnetType` and `subnetName` can be supplied.
    *
    * @default name
    */
@@ -200,31 +211,30 @@ export abstract class VpcNetworkBase extends Construct implements IVpcNetwork {
   public readonly natDependencies = new Array<IConstruct>();
 
   /**
-   * Return the subnets appropriate for the placement strategy
+   * Returns IDs of selected subnets
    */
-  public subnets(placement: VpcPlacementStrategy = {}): IVpcSubnet[] {
-    if (placement.subnetsToUse !== undefined && placement.subnetName !== undefined) {
-      throw new Error('At most one of subnetsToUse and subnetName can be supplied');
-    }
+  public subnetIds(selection: SubnetSelection = {}): string[] {
+    selection = reifySelectionDefaults(selection);
 
-    // Select by name
-    if (placement.subnetName !== undefined) {
-      const allSubnets = this.privateSubnets.concat(this.publicSubnets).concat(this.isolatedSubnets);
-      const selectedSubnets = allSubnets.filter(s => subnetName(s) === placement.subnetName);
-      if (selectedSubnets.length === 0) {
-        throw new Error(`No subnets with name: ${placement.subnetName}`);
-      }
-      return selectedSubnets;
+    const nets = this.subnets(selection);
+    if (nets.length === 0) {
+      throw new Error(`There are no ${describeSelection(selection)} in this VPC. Use a different VPC subnet selection.`);
     }
 
-    // Select by type
-    if (placement.subnetsToUse === undefined) { return this.privateSubnets; }
+    return nets.map(n => n.subnetId);
+  }
 
-    return {
-      [SubnetType.Isolated]: this.isolatedSubnets,
-      [SubnetType.Private]: this.privateSubnets,
-      [SubnetType.Public]: this.publicSubnets,
-    }[placement.subnetsToUse];
+  /**
+   * Return a dependable object representing internet connectivity for the given subnets
+   */
+  public subnetInternetDependencies(selection: SubnetSelection = {}): IDependable {
+    selection = reifySelectionDefaults(selection);
+
+    const ret = new CompositeDependable();
+    for (const subnet of this.subnets(selection)) {
+      ret.add(subnet.internetConnectivityEstablished);
+    }
+    return ret;
   }
 
   /**
@@ -260,6 +270,31 @@ export abstract class VpcNetworkBase extends Construct implements IVpcNetwork {
     return this.node.stack.region;
   }
 
+  /**
+   * Return the subnets appropriate for the placement strategy
+   */
+  protected subnets(selection: SubnetSelection = {}): IVpcSubnet[] {
+    selection = reifySelectionDefaults(selection);
+
+    // Select by name
+    if (selection.subnetName !== undefined) {
+      const allSubnets = this.privateSubnets.concat(this.publicSubnets).concat(this.isolatedSubnets);
+      const selectedSubnets = allSubnets.filter(s => subnetName(s) === selection.subnetName);
+      if (selectedSubnets.length === 0) {
+        throw new Error(`No subnets with name: ${selection.subnetName}`);
+      }
+      return selectedSubnets;
+    }
+
+    // Select by type
+    if (selection.subnetType === undefined) { return this.privateSubnets; }
+
+    return {
+      [SubnetType.Isolated]: this.isolatedSubnets,
+      [SubnetType.Private]: this.privateSubnets,
+      [SubnetType.Public]: this.publicSubnets,
+    }[selection.subnetType];
+  }
 }
 
 /**
@@ -335,3 +370,56 @@ export interface VpcSubnetImportProps {
    */
   subnetId: string;
 }
+
+/**
+ * If the placement strategy is completely "default", reify the defaults so
+ * consuming code doesn't have to reimplement the same analysis every time.
+ *
+ * Returns "private subnets" by default.
+ */
+function reifySelectionDefaults(placement: SubnetSelection): SubnetSelection {
+    if (placement.subnetType !== undefined && placement.subnetName !== undefined) {
+      throw new Error('Only one of subnetType and subnetName can be supplied');
+    }
+
+    if (placement.subnetType === undefined && placement.subnetName === undefined) {
+      return { subnetType: SubnetType.Private };
+    }
+
+    return placement;
+}
+
+/**
+ * Describe the given placement strategy
+ */
+function describeSelection(placement: SubnetSelection): string {
+  if (placement.subnetType !== undefined) {
+    return `'${DEFAULT_SUBNET_NAME[placement.subnetType]}' subnets`;
+  }
+  if (placement.subnetName !== undefined) {
+    return `subnets named '${placement.subnetName}'`;
+  }
+  return JSON.stringify(placement);
+}
+
+class CompositeDependable implements IDependable {
+  private readonly dependables = new Array<IDependable>();
+
+  /**
+   * Add a construct to the dependency roots
+   */
+  public add(dep: IDependable) {
+    this.dependables.push(dep);
+  }
+
+  /**
+   * Retrieve the current set of dependency roots
+   */
+  public get dependencyRoots(): IConstruct[] {
+    const ret = [];
+    for (const dep of this.dependables) {
+      ret.push(...dep.dependencyRoots);
+    }
+    return ret;
+  }
+}

packages/@aws-cdk/aws-ec2/lib/vpc.ts

@@ -5,7 +5,7 @@ import { CfnRouteTable, CfnSubnet, CfnSubnetRouteTableAssociation, CfnVPC, CfnVP
 import { NetworkBuilder } from './network-util';
 import { DEFAULT_SUBNET_NAME, ExportSubnetGroup, ImportSubnetGroup, subnetId  } from './util';
 import { VpcNetworkProvider, VpcNetworkProviderProps } from './vpc-network-provider';
-import { IVpcNetwork, IVpcSubnet, SubnetType, VpcNetworkBase, VpcNetworkImportProps, VpcPlacementStrategy, VpcSubnetImportProps } from './vpc-ref';
+import { IVpcNetwork, IVpcSubnet, SubnetSelection, SubnetType, VpcNetworkBase, VpcNetworkImportProps, VpcSubnetImportProps } from './vpc-ref';
 import { VpnConnectionOptions, VpnConnectionType } from './vpn';
 
 /**
@@ -80,7 +80,7 @@ export interface VpcNetworkProps {
    *
    * @default All public subnets
    */
-  natGatewayPlacement?: VpcPlacementStrategy;
+  natGatewaySubnets?: SubnetSelection;
 
   /**
    * Configure the subnets to build for each AZ
@@ -365,7 +365,7 @@ export class VpcNetwork extends VpcNetworkBase {
       });
 
       // if gateways are needed create them
-      this.createNatGateways(props.natGateways, props.natGatewayPlacement);
+      this.createNatGateways(props.natGateways, props.natGatewaySubnets);
 
       (this.privateSubnets as VpcPrivateSubnet[]).forEach((privateSubnet, i) => {
         let ngwId = this.natGatewayByAZ[privateSubnet.availabilityZone];
@@ -445,7 +445,7 @@ export class VpcNetwork extends VpcNetworkBase {
     };
   }
 
-  private createNatGateways(gateways?: number, placement?: VpcPlacementStrategy): void {
+  private createNatGateways(gateways?: number, placement?: SubnetSelection): void {
     const useNatGateway = this.subnetConfiguration.filter(
       subnet => (subnet.subnetType === SubnetType.Private)).length > 0;
 

packages/@aws-cdk/aws-ec2/test/integ.import-default-vpc.lit.ts

@@ -18,7 +18,7 @@ new ec2.SecurityGroup(stack, 'SecurityGroup', {
 });
 
 // Try subnet selection
-new cdk.CfnOutput(stack, 'PublicSubnets', { value: 'ids:' + vpc.subnets({ subnetsToUse: ec2.SubnetType.Public }).map(s => s.subnetId).join(',') });
-new cdk.CfnOutput(stack, 'PrivateSubnets', { value: 'ids:' + vpc.subnets({ subnetsToUse: ec2.SubnetType.Private }).map(s => s.subnetId).join(',') });
+new cdk.CfnOutput(stack, 'PublicSubnets', { value: 'ids:' + vpc.publicSubnets.map(s => s.subnetId).join(',') });
+new cdk.CfnOutput(stack, 'PrivateSubnets', { value: 'ids:' + vpc.privateSubnets.map(s => s.subnetId).join(',') });
 
 app.run();