Application Load Balancer (ALB)

Overview

• It functions at the application layer (the seventh layer of the OSI model) (HTTP).

• After the load balancer receives a request, it evaluates the listener rules in priority order to determine which rule to apply, and then selects a target from the target group for the rule action.

• You can configure listener rules to route requests to different target groups based on the content of the application traffic.

• Routing is performed independently for each target group, even when a target is registered with multiple target groups.

• You can configure the routing algorithm used at the target group level.

Components

• Load balancer

  • Serves as the single point of contact for clients.

  • The load balancer distributes incoming application traffic across multiple targets.

  • You add one or more listeners to your load balancer.

• Listener

  • You must add one or more listeners.

  • It checks for connection requests from clients, using the protocol and port that you configure.

  • Rules determine how the load balancer routes requests to its registered targets.

• Target group

  • The target group could contain:

    • An IP address.

    • EC2 Instances.

    • Lambda functions.

  • You can register a target with multiple target groups.

  • When you create each listener rule, you specify a target group and conditions.

  • When a rule condition is met, traffic is forwarded to the corresponding target group.

  • You can create different target groups for different types of requests.

    • E.g. you can create one target group for general requests and another target group for requests to the microservices for your application.

Support For

• Path conditions.

  • You can configure rules for your listener that forward requests based on the URL in the request.

    • This enables you to structure your application as smaller services, and route requests to the correct service based on the content of the URL.

• Host conditions.

  • You can configure rules for your listener that forward requests based on the host field in the HTTP header.

    • This enables you to route requests to multiple domains using a single load balancer.

• Routing based on fields in the request, such as HTTP header conditions and methods, query parameters, and source IP addresses.

• Redirecting requests from one URL to another.

• Returning a custom HTTP response.

HTTP headers

• HTTP requests and HTTP responses use header fields to send information about the HTTP messages.

• ALBs support the following X-Forwarded headers:

  • X-Forwarded-For

  • X-Forwarded-Proto

  • X-Forwarded-Port