A Placement group is a logical grouping of instances within a single Availability Zone.  Using placement groups enables applications to participate in a low-latency, 10 Gbps network.  Placement groups are recommended for applications that benefit from low network latency, high network throughput, or both.

Use Cases:

Hadoop clusters, Cassandra Nodes

Exam Tips:

• A placement group cannot span multiple Availability Zones
  • Does create a single point of failure.
• The name you specify for a placement group must be unique within your AWS account. (Global for your account)
• Only certain types of instances can be launched in a placement group.  (Compute Optimized, GPU, Memory, Storage Optimized.)
• AWS recommend homogeneous instances within placement groups.
  • Instances of the same size and same family.  (Has to do with reservations…)
• You cannot merge placement groups.
• You cannot move an existing instance into a placement group.  You can create an AMI from your existing instance, and then launch a new instance from the AMI into a placement group.