EC2 INSTANCE PLACEMENT STRATEGY

PLACEMENT GROUPS

• Logical grouping of instances within an availability zone.
• Types of placement groups - 
• Cluster - instances bucketed into a low-latency group in a single Availability Zone.
• Same rack
• Same AZ
• Same hardware
• Pro: Provides a great network (10 Gbps speed between instances)
• Con: If the rack fails, all instances will suffer downtime.
• Use-case
• Big Data jobs that need to complete very fast
• Apps requiring extremely low latency and high network throughput
• Risk appetite of failure is acceptable to the use cases.

• Spread - instances are spread across different hardware (max 7 instances per group per AZ)
• Minimises the failure risk
• EC2 instances are placed on different hardware.
• Pro: Reduced risk of simultaneous failure
• Pro: spans across AZ
• Con: Limited to 7 instances per AZ
• Use Case : 
• Apps requiring high availability
• Critical apps where each instance must be isolated from failure of another instance.

• Partition - spreads instances across many different partitions which rely on different sets of racks of hardware within an AZ. Capable of scaling to 100s of instances per group (Hadoop, Cassandra, Kafka)
• instances spread across partitions in multiple AZs
• can have up to 7 partitions per AZ
• each partition can have many EC2 instances
• each partition represents a rack
• Pro : safe from rack failure
• Pro : allows 100s of EC2 instances to be set up
• Pro : instances in a partition do not share hardware racks with other partitions
• partition information available as metadata to the EC2 instances
• Use-Cases : Partition aware apps like Big Data apps : HDFS, HBase, Cassandra, Kafka