Runtime context - AWS Cloud Development Kit (CDK) v2

This is the AWS CDK v2 Developer Guide. CDK v1 entered maintenance on June 1, 2022 and will now receive only critical bug fixes and security patches. New features will be developed for CDK v2 exclusively.

Runtime context

Context values are key-value pairs that can be associated with an app, stack, or construct. They may be supplied to your app from a file (usually either cdk.json or cdk.context.json in your project directory) or on the command line.

The CDK Toolkit uses context to cache values retrieved from your AWS account during synthesis, such as the Availability Zones in your account or the Amazon Machine Image (AMI) IDs currently available for Amazon EC2 instances. Because these values are provided by your AWS account, they can change between runs of your CDK application, which makes them a potential source of unintended change. The CDK Toolkit's caching behavior "freezes" these values for your CDK app until you decide to accept the new values.

Without context caching, if you had specified "latest Amazon Linux" as the AMI for your Amazon EC2 instances, and a new version of this AMI were released, then the next time you deployed your CDK stack, your already-deployed instances would be using the outdated ("wrong") AMI and would therefore need to be upgraded. Upgrading would result in replacing all your existing instances with new ones, which would probably be unexpected and undesired.

Instead, the CDK records your account's available AMIs in your project's cdk.context.json file, and uses the stored value for future synthesis operations. This way, the list of AMIs is no longer a potential source of change, and you can be sure that your stacks will always synthesize to the same AWS CloudFormation templates.

Not all context values are cached values from your AWS environment. Feature flags are also context values. You can also create your own context values for use by your apps or constructs.

Context keys are strings. Values may be any type supported by JSON: numbers, strings, arrays, or objects.

Tip

If your constructs create their own context values, incorporate your library's package name in its keys so they won't conflict with other packages' context values.

Many context values are associated with a particular AWS environment, and a given CDK app can be deployed in more than one environment. The key for such values includes the AWS account and region so that values from different environments do not conflict.

The context key below illustrates the format used by the AWS CDK, including the account and region.

availability-zones:account=123456789012:region=eu-central-1
Important

Cached context values are managed by the AWS CDK and its constructs, including constructs you may write. Do not add or change cached context values by manually editing files. It can be useful, however, to review cdk.context.json occasionally to see what values are being cached. Context values that do not represent cached values should be stored under the context key of cdk.json so they won't be cleared when cached values are cleared.

Sources of context values

Context values can be provided to your AWS CDK app in six different ways:

  • Automatically from the current AWS account.

  • Through the --context option to the cdk command. (These values are always strings.)

  • In the project's cdk.context.json file.

  • In the context key of the project's cdk.json file.

  • In the context key of your ~/.cdk.json file.

  • In your AWS CDK app using the construct.node.setContext() method.

The project file cdk.context.json is where the AWS CDK caches context values retrieved from your AWS account. This practice avoids unexpected changes to your deployments when, for example, a new Availability Zone is introduced. The AWS CDK does not write context data to any of the other files listed.

Important

Because they are part of your application's state, cdk.json and cdk.context.json must be committed to source control along with the rest of your app's source code. Otherwise, deployments in other environments (for example, a CI pipeline) may produce inconsistent results.

Context values are scoped to the construct that created them; they are visible to child constructs, but not to parents or siblings. Context values set by the AWS CDK Toolkit (the cdk command), whether automatically, from a file, or from the --context option, are implicitly set on the App construct, and so are visible to every construct in every stack in the app.

Your app can read a context value using the construct.node.tryGetContext method. If the requested entry is not found on the current construct or any of its parents, the result is undefined (or your language's equivalent, such as None in Python).

Context methods

The AWS CDK supports several context methods that enable AWS CDK apps to obtain contextual information from the AWS environment. For example, you can get a list of Availability Zones that are available in a given AWS account and region, using the stack.availabilityZones method.

The following are the context methods:

HostedZone.fromLookup

Gets the hosted zones in your account.

stack.availabilityZones

Gets the supported Availability Zones.

StringParameter.valueFromLookup

Gets a value from the current Region's Amazon EC2 Systems Manager Parameter Store.

Vpc.fromLookup

Gets the existing Amazon Virtual Private Clouds in your accounts.

LookupMachineImage

Looks up a machine image for use with a NAT instance in an Amazon Virtual Private Cloud.

If a required context value isn't available, the AWS CDK app notifies the CDK Toolkit that the context information is missing. The CLI then queries the current AWS account for the information, stores the resulting context information in the cdk.context.json file, and executes the AWS CDK app again with the context values.

Viewing and managing context

Use the cdk context command to view and manage the information in your cdk.context.json file. To see this information, use the cdk context command without any options. The output should be something like the following.

Context found in cdk.json:

┌───┬─────────────────────────────────────────────────────────────┬─────────────────────────────────────────────────────────┐
│ # │ Key                                                         │ Value                                                   │
├───┼─────────────────────────────────────────────────────────────┼─────────────────────────────────────────────────────────┤
│ 1 │ availability-zones:account=123456789012:region=eu-central-1 │ [ "eu-central-1a", "eu-central-1b", "eu-central-1c" ]   │
├───┼─────────────────────────────────────────────────────────────┼─────────────────────────────────────────────────────────┤
│ 2 │ availability-zones:account=123456789012:region=eu-west-1    │ [ "eu-west-1a", "eu-west-1b", "eu-west-1c" ]            │
└───┴─────────────────────────────────────────────────────────────┴─────────────────────────────────────────────────────────┘

Run cdk context --reset KEY_OR_NUMBER to remove a context key. If it is a cached value, it will be refreshed on the next cdk synth.

To remove a context value, run cdk context --reset, specifying the value's corresponding key or number. The following example removes the value that corresponds to the second key in the preceding example, which is the list of availability zones in the Ireland region.

cdk context --reset 2
Context value
availability-zones:account=123456789012:region=eu-west-1
reset. It will be refreshed on the next SDK synthesis run.

Therefore, if you want to update to the latest version of the Amazon Linux AMI, you can use the preceding example to do a controlled update of the context value and reset it, and then synthesize and deploy your app again.

cdk synth

To clear all of the stored context values for your app, run cdk context --clear, as follows.

cdk context --clear

Only context values stored in cdk.context.json can be reset or cleared. The AWS CDK does not touch other context values. To protect a context value from being reset using these commands, then, you might copy the value to cdk.json.

AWS CDK Toolkit --context flag

Use the --context (-c for short) option to pass runtime context values to your CDK app during synthesis or deployment.

cdk synth --context key=value MyStack

To specify multiple context values, repeat the --context option any number of times, providing one key-value pair each time.

cdk synth --context key1=value1 --context key2=value2 MyStack

When synthesizing multiple stacks, the specified context values are passed to all stacks. To provide different context values to individual stacks, either use different keys for the values, or use multiple cdk synth or cdk deploy commands.

Context values passed from the command line are always strings. If a value is usually of some other type, your code must be prepared to convert or parse the value. To allow non-string context values provided in other ways (for example, in cdk.context.json) to work as expected, make sure the value is a string before converting it.

Example

Below is an example of using an existing Amazon VPC using AWS CDK context.

TypeScript
import * as cdk from 'aws-cdk-lib'; import * as ec2 from 'aws-cdk-lib/aws-ec2'; import { Construct } from 'constructs'; export class ExistsVpcStack extends cdk.Stack { constructor(scope: Construct, id: string, props?: cdk.StackProps) { super(scope, id, props); const vpcid = this.node.tryGetContext('vpcid'); const vpc = ec2.Vpc.fromLookup(this, 'VPC', { vpcId: vpcid, }); const pubsubnets = vpc.selectSubnets({subnetType: ec2.SubnetType.PUBLIC}); new cdk.CfnOutput(this, 'publicsubnets', { value: pubsubnets.subnetIds.toString(), }); } }
JavaScript
const cdk = require('aws-cdk-lib'); const ec2 = require('aws-cdk-lib/aws-ec2'); class ExistsVpcStack extends cdk.Stack { constructor(scope, id, props) { super(scope, id, props); const vpcid = this.node.tryGetContext('vpcid'); const vpc = ec2.Vpc.fromLookup(this, 'VPC', { vpcId: vpcid }); const pubsubnets = vpc.selectSubnets({subnetType: ec2.SubnetType.PUBLIC}); new cdk.CfnOutput(this, 'publicsubnets', { value: pubsubnets.subnetIds.toString() }); } } module.exports = { ExistsVpcStack }
Python
import aws_cdk as cdk import aws_cdk.aws_ec2 as ec2 from constructs import Construct class ExistsVpcStack(cdk.Stack): def __init__(scope: Construct, id: str, **kwargs): super().__init__(scope, id, **kwargs) vpcid = self.node.try_get_context("vpcid") vpc = ec2.Vpc.from_lookup(self, "VPC", vpc_id=vpcid) pubsubnets = vpc.select_subnets(subnetType=ec2.SubnetType.PUBLIC) cdk.CfnOutput(self, "publicsubnets", value=pubsubnets.subnet_ids.to_string())
Java
import software.amazon.awscdk.CfnOutput; import software.amazon.awscdk.services.ec2.Vpc; import software.amazon.awscdk.services.ec2.VpcLookupOptions; import software.amazon.awscdk.services.ec2.SelectedSubnets; import software.amazon.awscdk.services.ec2.SubnetSelection; import software.amazon.awscdk.services.ec2.SubnetType; import software.constructs.Construct; public class ExistsVpcStack extends Stack { public ExistsVpcStack(Construct context, String id) { this(context, id, null); } public ExistsVpcStack(Construct context, String id, StackProps props) { super(context, id, props); String vpcId = (String)this.getNode().tryGetContext("vpcid"); Vpc vpc = (Vpc)Vpc.fromLookup(this, "VPC", VpcLookupOptions.builder() .vpcId(vpcId).build()); SelectedSubnets pubSubNets = vpc.selectSubnets(SubnetSelection.builder() .subnetType(SubnetType.PUBLIC).build()); CfnOutput.Builder.create(this, "publicsubnets") .value(pubSubNets.getSubnetIds().toString()).build(); } }
C#
using Amazon.CDK; using Amazon.CDK.AWS.EC2; using Constructs; class ExistsVpcStack : Stack { public ExistsVpcStack(Construct scope, string id, StackProps props) : base(scope, id, props) { var vpcId = (string)this.Node.TryGetContext("vpcid"); var vpc = Vpc.FromLookup(this, "VPC", new VpcLookupOptions { VpcId = vpcId }); SelectedSubnets pubSubNets = vpc.SelectSubnets([new SubnetSelection { SubnetType = SubnetType.PUBLIC }]); new CfnOutput(this, "publicsubnets", new CfnOutputProps { Value = pubSubNets.SubnetIds.ToString() }); } }

You can use cdk diff to see the effects of passing in a context value on the command line:

cdk diff -c vpcid=vpc-0cb9c31031d0d3e22
Stack ExistsvpcStack
Outputs
[+] Output publicsubnets publicsubnets: {"Value":"subnet-06e0ea7dd302d3e8f,subnet-01fc0acfb58f3128f"}

The resulting context values can be viewed as shown here.

cdk context -j
{
  "vpc-provider:account=123456789012:filter.vpc-id=vpc-0cb9c31031d0d3e22:region=us-east-1": {
    "vpcId": "vpc-0cb9c31031d0d3e22",
    "availabilityZones": [
      "us-east-1a",
      "us-east-1b"
    ],
    "privateSubnetIds": [
      "subnet-03ecfc033225be285",
      "subnet-0cded5da53180ebfa"
    ],
    "privateSubnetNames": [
      "Private"
    ],
    "privateSubnetRouteTableIds": [
      "rtb-0e955393ced0ada04",
      "rtb-05602e7b9f310e5b0"
    ],
    "publicSubnetIds": [
      "subnet-06e0ea7dd302d3e8f",
      "subnet-01fc0acfb58f3128f"
    ],
    "publicSubnetNames": [
      "Public"
    ],
    "publicSubnetRouteTableIds": [
      "rtb-00d1fdfd823c82289",
      "rtb-04bb1969b42969bcb"
    ]
  }
}