Why I should configure a AWS ECS Service with two or more Subnets?

Question

Why I should configure an AWS ECS Service or an EC2 Instance with two or more Private Subnets from the same VPC? What would be the benefits of doing such thing instead of configuring it within just one Subnet? Would it be because of availability? I've read the documentation but it was not clear about it.

Reference: https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Subnets.html

Answer 1

This is generally to distribute your ECS service across multiple availability zones, allowing your service to maintain high availability.

A subnet is bound to a single AZ, so it is assumed each subnet is in a different AZ.

By splitting across multiple subnets, during an outage load can be shifted to launch containers entirely in other subnets (assuming they're in different AZs).

This is generally encouraged for all services that support multiple availability zones.

More information on Amazon ECS Availability best practices are available from the blog.