As I commented here: https://reviews.apache.org/r/34136/
Currently ContainerInfo::Image::Appc is defined as the following
In which the id is a required field. When users specify the image in tasks they likely will not use an image id (much like when you use docker or rkt to launch containers, you often use ubuntu or ubuntu:latest and seldom a SHA512 ID) and we should change it to be optional.
The motivating scenario is that: if the frameworks in the Mesos use something like image=ubuntu:14.04" to run a task and image=ubuntu defaults to image=ubuntu:latest, the operator can swap the latest version for all new tasks requesting image=ubuntu. If they allow users to specify image=ubuntu:live, they can swap the live version under the covers as well. This allows the operator to release important image updates (e.g., security patches) and have it picked up by new tasks in the cluster without asking the users to update their job/task configs.