If you have multiple replicas in your service i.e., multiple containers, and when you need to update your service, for example change an image or eNV variable then DuploCloud will make this change one container at a time i.e., it will bring down the first container and bring up the new one on that host with the updated config. If the new container fails to start or the health check URL does not return Http 200 status, then DuploCloud will pause the upgrade of remaining containers. A user must intervene by fixing the issue typically by updating the service with a newer image that has a fix. If no health check URL is specified, then DuploCloud only checks for the new container to be running. To specify health check, go the ELB menu and you will find the health check URL suffix.