Merge pull request #11771 from ndeloof/elb

Document custom load balancer usage

Author: usha-mandya

engine/context/ecs-integration.md

@@ -376,7 +376,44 @@ of a VPC when deploying a Compose application.
 - Use `x-aws-loadbalancer` as a top-level element in your Compose file to set
 the ARN of an existing LoadBalancer.
 
-- Use `external: true` inside a network definition in your Compose file for
+The latter can be used for those who want to customize application exposure, typically to
+use an existing domain name for your application:
+
+1. Use the AWS web console or CLI to get your VPC and Subnets IDs. You can retrieve the default VPC ID and attached subnets using this AWS CLI commands:
+
+```console
+$ aws ec2 describe-vpcs --filters Name=isDefault,Values=true --query 'Vpcs[0].VpcId' 
+
+"vpc-123456"
+$ aws ec2 describe-subnets --filters Name=vpc-id,Values=vpc-123456 --query 'Subnets[*].SubnetId'
+
+[
+    "subnet-1234abcd", 
+    "subnet-6789ef00", 
+]
+```
+1. Use the AWS CLI to create your load balancer. The AWS Web Console can also be used but will require adding at least one listener, which we don't need here.
+
+```console
+$ aws elbv2 create-load-balancer --name myloadbalancer --type application --subnets "subnet-1234abcd" "subnet-6789ef00"
+
+{
+    "LoadBalancers": [
+        {
+            "IpAddressType": "ipv4", 
+            "VpcId": "vpc-123456", 
+            "LoadBalancerArn": "arn:aws:elasticloadbalancing:us-east-1:1234567890:loadbalancer/app/myloadbalancer/123abcd456", 
+            "DNSName": "myloadbalancer-123456.us-east-1.elb.amazonaws.com", 
+...            
+```
+1. To assign your application an existing domain name, you can configure your DNS with a
+CNAME entry pointing to just-created loadbalancer's `DNSName` reported as you created the loadbalancer.
+
+1. Use Loadbalancer ARN to set `x-aws-loadbalancer` in your compose file, and deploy your application using `docker compose up` command.
+
+Please note Docker ECS integration won't be aware of this domain name, so `docker compose ps` command will report URLs with loadbalancer DNSName, not your own domain.
+
+You also can use `external: true` inside a network definition in your Compose file for
 Docker Compose CLI to _not_ create a Security Group, and set `name` with the
 ID of an existing SecurityGroup you want to use for network connectivity between
 services: