Once you completed the Amazon ECS Task Definition, you are ready to create an Amazon ECS Service.
Select the ECS cluster that you created earlier, click the Services tab and then Create button.
In the Create Service wizard, follow the below configuration (make sure you select FARGATE in the Launch type).
In the network configuration, select the VPC that you created earlier and specify your subnets and ECS-Tasks-SG in the security group.
Select Application Load Balancer in the load balancer type, and choose the load balancer that you created earlier.
Click Add to load balance to add the container name:port
In the Service discovery (optional) section, uncheck the “Enable service discovery integration” and press [Next step]
In Auto Scaling configuration, select Configure Service Auto Scaling and specify the minimum, desired, maximum number of tasks.
In the Automatic task scaling policies, set the scaling policy type to Target Tracking, provide name of the scaling policy (e.g. Requests-policy), select the service metric (e.g. ALBRequestCountPerTarget) and then set the Target value (e.g. 300).
You can repeat that for different service metric (e.g. CPU and Memory utilization).
Finally, Review and click Create Service to create the Amazon ECS Service.
Once the Service status is in Active state and all the tasks are in the Running state, browse to the target web site using the loadbalancer DNS.
You might also need to configure autoscaling for the ECS nodes, check the this guide for more details
In case you encounter any issues, Please check Amazon ECS Troubleshooting guide