AWS placement strategies — Cluster, Partition, Spread

🔹 1. Cluster Placement Group

Goal: Achieve low-latency, high-throughput communication between instances.

✅ Use this when:

• You need fast communication between instances.

• Your application is performance-sensitive and needs tightly-coupled computing, such as:

  • High Performance Computing (HPC)
  • Machine Learning Training
  • Financial trading systems
  • Video rendering
  • Scientific simulations

🧠 How it works:

• All instances are placed very close together in the same Availability Zone (AZ) — same rack or neighboring racks.
• This gives high bandwidth and low network latency.

⚠️ Limitation:

• If there is a hardware failure or AZ issue, many instances may be affected — higher risk of correlated failure.

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🔹 2. Partition Placement Group

Goal: Reduce risk of failure by isolating groups of instances on different hardware.

✅ Use this when:

• You're running large, distributed systems that replicate data across nodes.

• Examples include:

  • Big Data tools: Hadoop, Spark
  • Databases like Cassandra
  • Streaming platforms like Kafka

🧠 How it works:

• Instances are split into logical partitions.
• Each partition is physically isolated — different racks, power sources, and network paths.
• No two partitions share the same hardware.

🔄 Benefits:

• If one partition fails, others remain unaffected.
• Allows for data replication and fault tolerance.

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🔹 3. Spread Placement Group

Goal: Maximize high availability by putting each instance on completely separate hardware.

✅ Use this when:

• You have a small number of critical instances and want to minimize risk of simultaneous failure.

• Ideal for:

  • Web servers
  • Load balancers
  • Domain controllers
  • Application layer with few but crucial VMs

🧠 How it works:

• AWS ensures each instance is on different hardware, possibly across different racks or even AZs.
• Maximum of 7 instances per AZ in a Spread group.

🛡️ Strongest failure isolation.

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🔍 Quick Decision Guide

Scenario	Use this placement strategy
Fast communication between nodes	Cluster
Distributed app with fault tolerance needs	Partition
Few critical instances that must not fail together	Spread

Feature	Spread Placement Group	Dedicated Tenancy
Goal	Distribute instances across separate hardware	Use hardware dedicated to one customer (no sharing with other AWS accounts)
Prevents nodes from sharing same hardware?	✅ Yes (each instance on different hardware)	❌ No (your instances can still share hardware with each other)
Use case	High availability, fault isolation	Compliance, licensing, isolation from other tenants
Cost	No extra cost	Much higher (dedicated hardware billing)