Amazon EC2 Placement Groups are a feature of Amazon Elastic Compute Cloud (EC2) that enable you to influence the placement of instances within the AWS infrastructure.

• on a specific Availability Zone
• A specific rack
• or Zone based placement etc

The placement group you choose for your instances can significantly impact the performance, availability, and fault tolerance of your applications.

There are three (3) types of EC2 Placement Groups:

1. Cluster Placement Group.
2. Spread Placement Group.
3. Partition Placement Group

Cluster Placement Group:

Instances are placed on the same hardware, meaning instances in a cluster placement group are placed in close proximity to each other, providing low-latency communication between them.

Use Cases: Ideal for applications that require low-latency, high-throughput communication between instances. Commonly used for high-performance computing (HPC) workloads.

Pros:

1. Low Latency: Instances in a cluster placement group share the same hardware, leading to low-latency communication.

2. High Throughput: Ideal for high-performance computing (HPC) workloads that require high throughput and minimal communication overhead.

3. Performance Optimization: Well-suited for applications that benefit from instances being close to each other, minimizing network latency.

Cons:

1. Limited Fault Tolerance: All instances are on the same hardware, making them susceptible to correlated failures.

2. Instance Type Limitations: Not all instance types are supported in cluster placement groups.

3. No Cross-Region Support: Cluster placement groups cannot span multiple AWS regions.

Spread Placement Group:

Instances in a spread placement group are placed on distinct underlying hardware to reduce the risk of simultaneous failures.

Use Cases: Useful for applications that require a small number of critical instances to be kept separate for fault tolerance.

1. Fault Tolerance: Offers high fault tolerance, as instances are placed on distinct underlying hardware.
2. Isolation: Provides high isolation between instances, reducing the risk of correlated failures.
3. Availability Zone Support: Can span multiple Availability Zones, enhancing availability.

Cons:

1. Communication Overhead: Instances in a spread placement group may experience higher communication overhead due to being on separate hardware.

2. Limited Instance Type Support: Not all instance types are supported in spread placement groups.

3. Limited Number of Instances: There is a limit on the number of instances per Availability Zone in a spread placement group.
(Maximum of 7 instances per group per Availability Zone.)

Partition Placement Group:

Instances are spread across logical partitions, each with its own set of racks.

Use Cases: Suitable for large distributed and replicated workloads, such as big data and database systems.
Pros:

1. Fault Tolerance: Offers better fault tolerance than cluster placement groups, as instances are spread across different logical partitions.

2. Scalability: Can handle a larger number of instances compared to cluster placement groups.

3. Isolation: Provides moderate isolation between instances due to partitioning.

Cons:

1. Communication Overhead: Instances in different partitions may have higher communication overhead compared to those in a cluster placement group.

2. Instance Type Limitations: Instances in a partition placement group must be of the same type.

3. Partition Limitations: There is a limit on the number of partitions per Availability Zone.

How to create placement group

1. Sign in to your AWS console and navigate to the EC2 Dashboard, on the left navigation section, select Placement Groups, under Network & Security.
2. On the top right hand corner, select the "create placement group" button
3. Input the Name, select your placement group strategy, then click the "create group button", on the low right corner. To implement the created placement group in an instance.
4. Upon creation of an Instance, scroll down to the advanced details section, scroll down to placement group and select the created placement group.

Conclusion:

EC2 Placement Groups provide a way to optimize the placement of your instances based on your specific workload requirements, whether it's maximizing performance, achieving fault tolerance, or enhancing isolation between instances.