How to solve RESOURCE:ENI error when creating an ECS task on EC2 server instance?

Supposing there are two ECS services with ``awsvpc`` networking on a ``m5.large`` EC2 instance, each service has two target tasks, and now you are adding a new service with the same settings. It is expected to see the below error under Tasks tab. service was unable to place a task because no container instance met all of its requirements. The closest matching container-instance XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX encountered error "RESOURCE:ENI". For more information, see the Troubleshooting section. For ``RESOURCE:ENI`` errors, it means that there are not enough elastic network interface (ENI) attachment points in your cluster. By running the below command, you can see that the maximum network interface for each m5 types. aws ec2 describe-instance-types --filters Name=instance-type,Values=m5.* --query "InstanceTypes[].{Type: InstanceType, MaxENI: NetworkInfo.MaximumNetworkInterfaces, IPv4addr: NetworkInfo.Ipv4AddressesPerInterface}" For ``m5.large``, the ``maxENI`` is ``3``. From [the official documentation](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-networking.html), it states that Each Amazon ECS task that uses the awsvpc network mode receives its own elastic network interface (ENI), which is attached to the Amazon EC2 instance that hosts it. There is a default limit to the number of network interfaces that can be attached to an Amazon EC2 instance, and the primary network interface counts as one. For example, by default a c5.large instance may have up to three ENIs attached to it. The primary network interface for the instance counts as one, so you can attach an additional two ENIs to the instance. Because each task using the awsvpc network mode requires an ENI, you can typically only run two such tasks on this instance type. For more information on the default ENI limits for each instance type, see IP addresses per network interface per instance type in the Amazon EC2 User Guide for Linux Instances. Hence, let's do the math. Since there are two EC2 instances, the maxENI is 3 * 2 = 6 for this ECS cluster. We need 1 primary network interface for each instance, which means now we only have 4 available ENI to use. A service has two target tasks, each task takes 1 ENI. Therefore, two services take 4. Hence, before adding a new service, the available ENI is actually ``6 - 1 - 1 - 2 - 2 = 0``. Therefore, when we try to add a new service, even with one target task, it will still fail as there is no available ENI. Therer are several solutions. - You can choose a different instance type. For the number, you can run ``describe-instance-types`` to check it. - You can change the task count to free some ENI. - You can raise the limit by using Elastic network interface trunking.