Amazon Elastic Load Balancing V2 Construct Library

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The @aws-cdk/aws-elasticloadbalancingv2 package provides constructs for configuring application and network load balancers.

For more information, see the AWS documentation for Application Load Balancers and Network Load Balancers.

Defining an Application Load Balancer

You define an application load balancer by creating an instance of ApplicationLoadBalancer, adding a Listener to the load balancer and adding Targets to the Listener:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import aws_cdk.aws_ec2 as ec2
import aws_cdk.aws_elasticloadbalancingv2 as elbv2
import aws_cdk.aws_autoscaling as autoscaling

# ...

vpc = ec2.Vpc(...)

# Create the load balancer in a VPC. 'internetFacing' is 'false'
# by default, which creates an internal load balancer.
lb = elbv2.ApplicationLoadBalancer(self, "LB",
    vpc=vpc,
    internet_facing=True
)

# Add a listener and open up the load balancer's security group
# to the world.
listener = lb.add_listener("Listener",
    port=80,

    # 'open: true' is the default, you can leave it out if you want. Set it
    # to 'false' and use `listener.connections` if you want to be selective
    # about who can access the load balancer.
    open=True
)

# Create an AutoScaling group and add it as a load balancing
# target to the listener.
asg = autoscaling.AutoScalingGroup(...)
listener.add_targets("ApplicationFleet",
    port=8080,
    targets=[asg]
)

The security groups of the load balancer and the target are automatically updated to allow the network traffic.

Conditions

It’s possible to route traffic to targets based on conditions in the incoming HTTP request. For example, the following will route requests to the indicated AutoScalingGroup only if the requested host in the request is either for example.com/ok or example.com/path:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
listener.add_targets("Example.Com Fleet",
    priority=10,
    conditions=[
        ListenerCondition.host_headers(["example.com"]),
        ListenerCondition.path_patterns(["/ok", "/path"])
    ],
    port=8080,
    targets=[asg]
)

A target with a condition contains either pathPatterns or hostHeader, or both. If both are specified, both conditions must be met for the requests to be routed to the given target. priority is a required field when you add targets with conditions. The lowest number wins.

Every listener must have at least one target without conditions, which is where all requests that didn’t match any of the conditions will be sent.

Convenience methods and more complex Actions

Routing traffic from a Load Balancer to a Target involves the following steps:

  • Create a Target Group, register the Target into the Target Group

  • Add an Action to the Listener which forwards traffic to the Target Group.

Various methods on the Listener take care of this work for you to a greater or lesser extent:

  • addTargets() performs both steps: automatically creates a Target Group and the required Action.

  • addTargetGroups() gives you more control: you create the Target Group (or Target Groups) yourself and the method creates Action that routes traffic to the Target Groups.

  • addAction() gives you full control: you supply the Action and wire it up to the Target Groups yourself (or access one of the other ELB routing features).

Using addAction() gives you access to some of the features of an Elastic Load Balancer that the other two convenience methods don’t:

  • Routing stickiness: use ListenerAction.forward() and supply a stickinessDuration to make sure requests are routed to the same target group for a given duration.

  • Weighted Target Groups: use ListenerAction.weightedForward() to give different weights to different target groups.

  • Fixed Responses: use ListenerAction.fixedResponse() to serve a static response (ALB only).

  • Redirects: use ListenerAction.redirect() to serve an HTTP redirect response (ALB only).

  • Authentication: use ListenerAction.authenticateOidc() to perform OpenID authentication before serving a request (see the @aws-cdk/aws-elasticloadbalancingv2-actions package for direct authentication integration with Cognito) (ALB only).

Here’s an example of serving a fixed response at the /ok URL:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
listener.add_action("Fixed",
    priority=10,
    conditions=[
        ListenerCondition.path_patterns(["/ok"])
    ],
    action=ListenerAction.fixed_response(200,
        content_type=elbv2.ContentType.TEXT_PLAIN,
        message_body="OK"
    )
)

Here’s an example of using OIDC authentication before forwarding to a TargetGroup:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
listener.add_action("DefaultAction",
    action=ListenerAction.authenticate_oidc(
        authorization_endpoint="https://example.com/openid",
        # Other OIDC properties here
        # ...
        next=ListenerAction.forward([my_target_group])
    )
)

Defining a Network Load Balancer

Network Load Balancers are defined in a similar way to Application Load Balancers:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import aws_cdk.aws_ec2 as ec2
import aws_cdk.aws_elasticloadbalancingv2 as elbv2
import aws_cdk.aws_autoscaling as autoscaling

# Create the load balancer in a VPC. 'internetFacing' is 'false'
# by default, which creates an internal load balancer.
lb = elbv2.NetworkLoadBalancer(self, "LB",
    vpc=vpc,
    internet_facing=True
)

# Add a listener on a particular port.
listener = lb.add_listener("Listener",
    port=443
)

# Add targets on a particular port.
listener.add_targets("AppFleet",
    port=443,
    targets=[asg]
)

One thing to keep in mind is that network load balancers do not have security groups, and no automatic security group configuration is done for you. You will have to configure the security groups of the target yourself to allow traffic by clients and/or load balancer instances, depending on your target types. See Target Groups for your Network Load Balancers and Register targets with your Target Group for more information.

Targets and Target Groups

Application and Network Load Balancers organize load balancing targets in Target Groups. If you add your balancing targets (such as AutoScalingGroups, ECS services or individual instances) to your listener directly, the appropriate TargetGroup will be automatically created for you.

If you need more control over the Target Groups created, create an instance of ApplicationTargetGroup or NetworkTargetGroup, add the members you desire, and add it to the listener by calling addTargetGroups instead of addTargets.

addTargets() will always return the Target Group it just created for you:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
group = listener.add_targets("AppFleet",
    port=443,
    targets=[asg1]
)

group.add_target(asg2)

Using Lambda Targets

To use a Lambda Function as a target, use the integration class in the @aws-cdk/aws-elasticloadbalancingv2-targets package:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import aws_cdk.aws_lambda as lambda
import aws_cdk.aws_elasticloadbalancingv2 as elbv2
import aws_cdk.aws_elasticloadbalancingv2_targets as targets

lambda_function = lambda.Function(...)
lb = elbv2.ApplicationLoadBalancer(...)

listener = lb.add_listener("Listener", port=80)
listener.add_targets("Targets",
    targets=[targets.LambdaTarget(lambda_function)],

    # For Lambda Targets, you need to explicitly enable health checks if you
    # want them.
    health_check=HealthCheck(
        enabled=True
    )
)

Only a single Lambda function can be added to a single listener rule.

Configuring Health Checks

Health checks are configured upon creation of a target group:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
listener.add_targets("AppFleet",
    port=8080,
    targets=[asg],
    health_check={
        "path": "/ping",
        "interval": cdk.Duration.minutes(1)
    }
)

The health check can also be configured after creation by calling configureHealthCheck() on the created object.

No attempts are made to configure security groups for the port you’re configuring a health check for, but if the health check is on the same port you’re routing traffic to, the security group already allows the traffic. If not, you will have to configure the security groups appropriately:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
listener.add_targets("AppFleet",
    port=8080,
    targets=[asg],
    health_check={
        "port": 8088
    }
)

listener.connections.allow_from(lb, ec2.Port.tcp(8088))

Using a Load Balancer from a different Stack

If you want to put your Load Balancer and the Targets it is load balancing to in different stacks, you may not be able to use the convenience methods loadBalancer.addListener() and listener.addTargets().

The reason is that these methods will create resources in the same Stack as the object they’re called on, which may lead to cyclic references between stacks. Instead, you will have to create an ApplicationListener in the target stack, or an empty TargetGroup in the load balancer stack that you attach your service to.

For an example of the alternatives while load balancing to an ECS service, see the ecs/cross-stack-load-balancer example.

Protocol for Load Balancer Targets

Constructs that want to be a load balancer target should implement IApplicationLoadBalancerTarget and/or INetworkLoadBalancerTarget, and provide an implementation for the function attachToXxxTargetGroup(), which can call functions on the load balancer and should return metadata about the load balancing target:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
attach_to_application_target_group(target_group, ApplicationTargetGroup)LoadBalancerTargetProps
    target_group.register_connectable(...)return {
        "target_type": TargetType.Instance | TargetType.Ip,
        "target_json": {"id": , ..., "port": , ...}
    }

targetType should be one of Instance or Ip. If the target can be directly added to the target group, targetJson should contain the id of the target (either instance ID or IP address depending on the type) and optionally a port or availabilityZone override.

Application load balancer targets can call registerConnectable() on the target group to register themselves for addition to the load balancer’s security group rules.

If your load balancer target requires that the TargetGroup has been associated with a LoadBalancer before registration can happen (such as is the case for ECS Services for example), take a resource dependency on targetGroup.loadBalancerDependency() as follows:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
# Make sure that the listener has been created, and so the TargetGroup
# has been associated with the LoadBalancer, before 'resource' is created.
resourced.add_dependency(target_group.load_balancer_dependency())