Implementing priority ordering for ALB listener rules

Question

Pulumi · Accepted Answer

Priority ordering for Application Load Balancer (ALB) listener rules in AWS is a way to control the order in which rules are evaluated, and hence, the order in which requests are routed to different target groups based on the conditions specified in the rules.

Each rule associated with a listener has a priority value, which dictates the order of rule evaluation. AWS ALB processes rules in priority order, from the lowest value to the highest. The default rule (which routes requests that do not match any other rule) always has a priority of `default`, and it's processed last.

When creating or updating listener rules in Pulumi using the AWS provider, you need to specify a priority for your rules. Rules with lower priority values are evaluated before rules with higher values. AWS ALB supports priorities from 1 to 50000.

Let's create a simple Pulumi program in TypeScript that sets up an ALB with two listener rules with different priorities. We'll be using the `aws.alb.ListenerRule` resource from the AWS provider to accomplish this.

You'll need the following before running this program:
1. AWS credentials configured for Pulumi using the AWS CLI or Pulumi config.
2. An existing VPC with at least two subnets.

Here's a Pulumi program that defines an ALB, a listener, and two listener rules with priorities:

```typescript
import * as pulumi from "@pulumi/pulumi";
import * as aws from "@pulumi/aws";

// Initiate the ALB. You would substitute vpcId and subnetIds with your existing AWS VPC ID and Subnet IDs.
const loadBalancer = new aws.lb.LoadBalancer("myLoadBalancer", {
    internal: false,
    loadBalancerType: "application",
    securityGroups: [], // Add security group(s) appropriate for your context
    subnets: [], // Provide at least two subnet IDs from your VPC
    // For more configurations, refer to the Pulumi documentation: https://www.pulumi.com/registry/packages/aws/api-docs/lb/loadbalancer/
});

// Create a Target Group for routing traffic.
const targetGroup = new aws.lb.TargetGroup("myTargetGroup", {
    port: 80,
    protocol: "HTTP",
    targetType: "instance",
    vpcId: "yourVpcId", // Replace with your VPC ID
    // Additional configurations can be added here.
});

// Add a listener to the ALB.
const listener = new aws.lb.Listener("myListener", {
    loadBalancerArn: loadBalancer.arn,
    port: 80,
    defaultActions: [{
        type: "forward",
        targetGroupArn: targetGroup.arn,
    }],
    // See more options at https://www.pulumi.com/registry/packages/aws/api-docs/lb/listener/
});

// Define the first listener rule with a lower priority value.
const listenerRule1 = new aws.lb.ListenerRule("myListenerRule1", {
    listenerArn: listener.arn,
    priority: 100, // Lower priority number to be evaluated first
    actions: [{
        type: "forward",
        targetGroupArn: targetGroup.arn,
    }],
    conditions: [{
        pathPattern: {
            values: ["/path1"],
        },
    }],
    // More configurations can be found at https://www.pulumi.com/registry/packages/aws/api-docs/lb/listenerrule/
});

// Define a second listener rule with a higher priority value.
const listenerRule2 = new aws.lb.ListenerRule("myListenerRule2", {
    listenerArn: listener.arn,
    priority: 200, // Higher priority number to be evaluated second
    actions: [{
        type: "forward",
        targetGroupArn: targetGroup.arn,
    }],
    conditions: [{
        pathPattern: {
            values: ["/path2"],
        },
    }],
    // Similar to the first one, more configurations if needed.
});

// Export the DNS name of the load balancer for access.
export const loadBalancerDnsName = loadBalancer.dnsName;
```

This program sets up an application load balancer with the necessary subnets and security groups from your context. It then creates a target group to route traffic to, adds a listener on port 80, and establishes two listener rules. The first listener rule, with priority 100, will match traffic going to `/path1`, and the second rule, with priority 200, will match traffic going to `/path2`. Rules with lower numbers take precedence. The default rule (implicitly created with a priority of `default`) will be applied if no other rules match. Finally, the DNS name of the ALB is exported so you can access it from outside AWS.

To apply this configuration, you would use Pulumi's CLI by calling `pulumi up`, which will create these resources in your AWS account as per the configuration you've provided. Remember that this code is for illustration purposes; for production usage, you would need to substitute placeholder values with actual data, add necessary security configurations, and handle other specific requirements according to your infrastructure.