AWS recently introduced spread placement groups, an important piece for high availability for certain types of smaller deployments.
Let’s say your workload required 4 instances and you’ve placed 2 instances Availability Zone A and 2 in Availability Zone B within a single region. With a normal deployment, it would be possible for the instances in the same AZ to placed on the same host. In the event of a host failure, half of your capacity would be lost.
However, Spread Placement Groups ensure that 2 instances in the same Availability Zone are not on the same host.
I will note that Spread Placement Groups are a bit inflexible at this time…you can’t move existing instances into them (without creating an AMI, then placing it into the spread placement group at launch).