feat(aws-ec2): Control over VPC AZs

With this change, the `Vpc` construct gains a new constructor prop,
`availabilityZones`, which gives more control over AZs than the existing
`maxAzs` prop.

closes aws#5847

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

@@ -841,10 +841,21 @@ export interface VpcProps {
  * only 2 AZs, so to use more than 2 AZs, be sure to specify the account and
  * region on your stack.
  *
+ * Specify this option only if you do not specify `availabilityZones`.
+ *
  * @default 3
  */
  readonly maxAzs?: number;
 
+ /**
+ * Availability zones this VPC spans.
+ *
+ * This option overrides `maxAzs`.
+ *
+ * @default (a subset of) AZs of the stack
+ */
+ readonly availabilityZones?: string[];
+
  /**
  * The number of NAT Gateways/Instances to create.
  *
@@ -1289,6 +1300,10 @@ export class Vpc extends VpcBase {
  throw new Error('To use DNS Hostnames, DNS Support must be enabled, however, it was explicitly disabled.');
  }
 
+ if (props.availabilityZones && props.maxAzs) {
+ throw new Error('Vpc supports either specific \'availabilityZones\' or \'maxAzs\' number, but not both.');
+ }
+
  const cidrBlock = ifUndefined(props.cidr, Vpc.DEFAULT_CIDR_RANGE);
  if (Token.isUnresolved(cidrBlock)) {
  throw new Error('\'cidr\' property must be a concrete CIDR string, got a Token (we need to parse it for automatic subdivision)');
@@ -1317,10 +1332,13 @@ export class Vpc extends VpcBase {
 
  Tags.of(this).add(NAME_TAG, props.vpcName || this.node.path);
 
- this.availabilityZones = stack.availabilityZones;
+ if (props.availabilityZones) {
+ this.availabilityZones = props.availabilityZones;
+ } else {
+ const maxAZs = props.maxAzs ?? 3;
+ this.availabilityZones = stack.availabilityZones.slice(0, maxAZs);
+ }
 
- const maxAZs = props.maxAzs ?? 3;
- this.availabilityZones = this.availabilityZones.slice(0, maxAZs);
 
  this.vpcId = this.resource.ref;
  this.vpcArn = Arn.format({

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

@@ -588,6 +588,30 @@ describe('vpc', () => {
  });
 
  });
+
+ test('with availabilityZones and maxAzs set, throws error', () => {
+ const stack = getTestStack();
+ expect(() => {
+ new Vpc(stack, 'VPC', {
+ availabilityZones: stack.availabilityZones,
+ maxAzs: 1,
+ });
+ }).toThrow(/Vpc supports either specific 'availabilityZones' or 'maxAzs'/);
+ });
+
+ test('with availabilityZones set', () => {
+ const stack = getTestStack();
+ new Vpc(stack, 'VPC', {
+ availabilityZones: [`${stack.region}b`],
+ });
+ Template.fromStack(stack).resourceCountIs('AWS::EC2::Subnet', 2);
+ for (let i = 1; i < 2; i++) {
+ Template.fromStack(stack).hasResourceProperties('AWS::EC2::Subnet', {
+ AvailabilityZone: `${stack.region}b`,
+ });
+ }
+ });
+
  test('with natGateway set to 1', () => {
  const stack = getTestStack();
  new Vpc(stack, 'VPC', {