#Application Load Balancer

Application Load Balancers can be used to re-route requests when certain traffic patterns are met. While traffic can be routed to services such as EC2 it can also be routed to Lambda functions which can in turn be used process incoming requests.

The Serverless Framework makes it possible to setup the connection between Application Load Balancers and Lambda functions with the help of the alb event.

#Event definition

functions:
  albEventConsumer:
    handler: handler.hello
    events:
      - alb:
          listenerArn: arn:aws:elasticloadbalancing:us-east-1:12345:listener/app/my-load-balancer/50dc6c495c0c9188/
          priority: 1
          conditions:
            path: /hello

#Using different conditions

functions:
  albEventConsumer:
    handler: handler.hello
    events:
      - alb:
          listenerArn: arn:aws:elasticloadbalancing:us-east-1:12345:listener/app/my-load-balancer/50dc6c495c0c9188/
          priority: 1
          conditions:
            host: example.com
            path: /hello
            method:
              - POST
              - PATCH
            host:
              - example.com
              - example2.com
            header:
              name: foo
              values:
                - bar
            query:
              bar: true
            ip:
              - fe80:0000:0000:0000:0204:61ff:fe9d:f156/6
              - 192.168.0.1/0

#Enabling multi-value headers

By default when the request contains a duplicate header field name or query parameter key, the load balancer uses the last value sent by the client.

Set the multiValueHeaders attribute to true if you want to receive headers and query parameters as an array of values.

functions:
  albEventConsumer:
    handler: handler.hello
    events:
      - alb:
          listenerArn: arn:aws:elasticloadbalancing:us-east-1:12345:listener/app/my-load-balancer/50dc6c495c0c9188/
          priority: 1
          multiValueHeaders: true
          conditions:
            path: /hello

When this option is enabled, the event structure is changed:

module.exports.hello = async (event, context, callback) => {
  const headers = event.multiValueHeaders;
  const queryString = event.multiValueQueryStringParameters;

  ...

  return {
    statusCode: 200,
    statusDescription: '200 OK',
    isBase64Encoded: false,
    multiValueHeaders: {
      'Content-Type': ['application/json'],
      'Set-Cookie': ['language=en-us', 'theme=rust']
    }
  };
};

The handler response object must use multiValueHeaders to set HTTP response headers, headers would be ignored.

#Prepending a prefix to generated target group names

By default, target group names are strings generated by hashing a combined string of the function name, alb's id and whether multi-value headers are enabled. This produces a fixed length, unique id for target groups.

If you need target group names to have a common predictable prefix, you may preset the prefix with the provider.alb.targetGroupPrefix setting. Note maximum length of this prefix is 16 chars.

provider:
  alb:
    targetGroupPrefix: my-prefix

functions:
  albEventConsumer:
    handler: handler.hello
    events:
      - alb:
          listenerArn: arn:aws:elasticloadbalancing:us-east-1:12345:listener/app/my-load-balancer/50dc6c495c0c9188/
          priority: 1
          conditions:
            path: /hello