From AWS documentation I can see that CPU and Memory properties are required in the AWS::ECS::TaskDefinition for Fargate but not in the ContainerDefinition within the resource if Fargate is used.
How does this exactly work? If I do not specify it in the ContainerDefinition it will use as much resources the Task have available? If there is only one container within the task... does it make any sense defining those values? If they are required, it seems pretty redundant and verbose to me.
When you register a task definition, you can specify the total cpu and memory used for the task. This is separate from the cpu and memory values at the container definition level.
If using the Fargate launch type, these task definition fields are required and there are specific values for both cpu and memory that are supported. This will be a hard limit of CPU/Memory to present to the task. For example, if your task is configured to use 1 vCPU and 2 GB of memory, so at the moment the memory limit is 2 GB. If at any moment the task memory utilization exceed the 2 GB, the task will terminate with OutOfMemory error.
You can also specify cpu and memory resource on the container level. This will be the amount of resources to present to the container (a Task can have multiple containers). If your container attempts to exceed the resource specified here, the container is killed. These fields are optional for tasks using the Fargate launch type, and the only requirement is that the total amount of cpu and memory reserved for all containers within a task be lower than the task-level cpu and memory value, if one is specified.
On a container level, the Docker daemon reserves a minimum of 4 MiB of memory for a container, so you should not specify fewer than 4 MiB of memory for your containers.
Standard Container Definition Parameters: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definition_parameters.html#standard_container_definition_params
When a container has no specified limits in a TaskDefinition, the container will use all the available resources for the task, which are mandatory for a Fargate task.
This means that there is no need to define them if there is only one container in the TaskDefinition. They can be specified though they are redundant (if equal to those of the task itself) or even harmful (if they are lower than the amount given to the task).
In case more than once container belongs to the same task, Fargate will distribute evenly the resources among all the containers. This may (or not) be a desired behavior.A