AWS EC2 Placement Groups

Table of Contents

• Table of Contents
• Overview
• Types of placement groups
• Infrastructure as Code
• Real-World Applications
• Advantages and Drawbacks
• Best Practices
• Interview Angle
• Online References
• Summary
• Extra Insights
• Takeaways for Certification

Overview

AWS EC2 Placement Groups are a logical grouping of EC2 instances within a single AWS region that allow you to influence the placement of your instances to meet specific requirements for performance, latency, and fault tolerance.

• From an architect’s perspective, placement groups are a low-level control mechanism that provides granular control over the physical proximity or isolation of your instances on the underlying AWS hardware.
• They are a critical tool for optimizing distributed applications and ensuring high availability.

Types of placement groups

AWS offers three types of placement group strategies, each designed for a different use case:

• Cluster Placement Group:
  • This strategy packs instances close together within a single Availability Zone (AZ) to achieve low network latency and high network throughput.
  • Instances are placed on the same rack or network switch, which is ideal for tightly-coupled, network-intensive applications.
  • Note: However, this also means they share the same underlying hardware, making them vulnerable to correlated hardware failures.
• Spread Placement Group:
  • This strategy places a small number of critical instances on distinct underlying hardware to reduce the risk of simultaneous failures.
  • Each instance in a spread placement group is launched on a separate rack, with its own network and power source.
  • This is perfect for applications that have a few critical instances that must be kept as isolated as possible for maximum fault tolerance.
  • Note: A spread placement group can also span multiple Availability Zones.
    • That is we can configure to spread instances across underlying hardware (max 7 instances per group per AZ).
• Partition Placement Group:
  • This strategy divides a placement group into logical partitions, where each partition is placed on its own set of racks.
  • No two partitions in a group share the same underlying hardware.
  • This provides isolation and fault tolerance, while still allowing for a larger number of instances than a spread placement group.
  • It’s ideal for large-scale distributed and replicated workloads like Apache Hadoop, Apache Cassandra, and Apache Kafka.
  • Summary: Spreads instances across many different partitions (which rely on diferent set of racks) withing an AZ.
    • Scales to 100s of EC2 instances pergroup (Hadoop, Cassandra, Kafka).

Infrastructure as Code

You typically define placement groups and then launch instances into them. Here’s a quick example using the AWS CLI and a run-instances command with a placement group.

# First, create the placement group
aws ec2 create-placement-group --group-name MyClusterGroup --strategy cluster

# Then, launch an instance into the placement group
aws ec2 run-instances --image-id ami-12345 --count 1 --instance-type c5.large --placement-group-name MyClusterGroup

For more complex scenarios, you would use a CloudFormation or Terraform template to define the infrastructure as code.

Diagrams

Cluster Placement Group

Simple EC2 placment

graph TD
    subgraph "Availability Zone 1 (AZ1)"
        subgraph "Rack 1 (R1)"
            A[EC2-A]
            B[EC2-B]
            C[EC2-C]
        end
    end
    A --- B
    B --- C
    A --- C

• Explanation:
  • Instances A, B, and C are all in the same rack, providing very low latency and high throughput.
  • However, a failure of Rack 1 would affect all three instances.

More complex EC2 placement

graph TD
    subgraph "Availability Zone 1 (AZ1)"
      subgraph "Rack (R1)"
        %% We'll use two separate subgraphs to create the top and bottom rows,
        %% and then use invisible links to form the columns.
        direction LR

        subgraph " "
            A[EC2-A] --- B[EC2-B] --- C[EC2-C]
            %% style B fill:#ffc485
        end

        subgraph " "
            D[EC2-D] --- E[EC2-E] --- F[EC2-F]
            %% style D fill:#ffc485
        end

        %% Invisible links to force vertical alignment (columns)
        A --- D
        B --- E
        C --- F

        %% Diagonal connections
        A --- E
        B --- F
        C --- E
        B --- D
      end
    end

Spread Placement Group

A sample single AZ Spread placement

graph TD
    subgraph "Availability Zone 1 (AZ1)"
        subgraph "Rack 1 (R1)"
            A[EC2-A]
        end
        subgraph "Rack 2 (R2)"
            B[EC2-B]
        end
        subgraph "Rack 3 (R3)"
            C[EC2-C]
        end
    end

• Explanation:
  • Each instance (A, B, and C) is placed on a distinct rack, ensuring that a single hardware failure affects only one instance. This provides maximum fault tolerance.

A sample multi AZ Spread placement

graph TD
    subgraph "Placement Groups Spread multi AZ"
        direction LR
        subgraph "us-east-1a"
            direction TB
            subgraph "Hardware 1"
                A[EC2]
            end
            subgraph "Hardware 2"
                B[EC2]
            end
        end

        subgraph "us-east-1b"
            direction TB
            subgraph "Hardware 3"
                C[EC2]
            end
            subgraph "Hardware 4"
                D[EC2]
            end
        end

        subgraph "us-east-1c"
            direction TB
            subgraph "Hardware 5"
                E[EC2]
            end
            subgraph "Hardware 6"
                F[EC2]
            end
        end

        %% Invisible links to force horizontal alignment of Availability Zones
        us-east-1a --- us-east-1b --- us-east-1c
    end

Partition Placement Group

graph TD
    subgraph "Availability Zone 1 (AZ1)"
        subgraph "Partition 1 (P1)"
            subgraph "Rack 1 (R1)"
                A[EC2-A]
            end
            subgraph "Rack 2 (R2)"
                B[EC2-B]
            end
        end
        subgraph "Partition 2 (P2)"
            subgraph "Rack 3 (R3)"
                C[EC2-C]
            end
            subgraph "Rack 4 (R4)"
                D[EC2-D]
            end
        end
    end

• Explanation:
  • Instances are grouped into logical partitions.
    • Partition 1 (with instances A and B) and Partition 2 (with instances C and D) are on completely separate hardware.
  • A rack failure would only affect the instances in that partition, providing a good balance of isolation and scalability for large distributed systems.

Real-World Applications

• Cluster:
  • High-Performance Computing (HPC) clusters, big data processing (e.g., Spark), real-time financial trading platforms, and any application where inter-node communication latency is a primary concern.
• Spread:
  • A few critical microservices that need to be highly available, such as primary and secondary instances of a database or a mission-critical web server, to prevent a single hardware failure from taking down the entire service.
• Partition:
  • Distributed databases like Cassandra, Hadoop (HDFS), and Hbase where data is replicated across different partitions to ensure that a rack failure doesn’t result in data loss.

Advantages and Drawbacks

Trade-offs

The main trade-off is between performance (low latency, high throughput) and fault tolerance.

• If your primary concern is performance, you trade off fault tolerance for speed by using a Cluster placement group.
• If your primary concern is fault tolerance, you trade off maximum performance for isolation by using a Spread or Partition placement group. A Spread group is for a few critical instances, while a Partition group is for large, distributed systems.

Best Practices

• Launch all instances at once:
  • To avoid capacity issues, launch all required instances in a placement group in a single run-instances request.
• Uniformity:
  • Use the same instance type for all instances within a cluster placement group to avoid mismatched network performance.
• Capacity Errors:
  • If you receive a capacity error, you may need to stop and start all instances in the placement group to see if they can be placed on a different set of hardware that has sufficient capacity.

Interview Angle

• Q: What are EC2 Placement Groups and why are they used?
  • Answer:
    • They are a logical grouping of instances to control their physical placement on AWS hardware.
    • They are used to optimize for low latency (Cluster), high fault tolerance (Spread), or a balance of both for large systems (Partition).
• Q: Explain the differences between the three types of placement groups.
  • Answer:
    • Cluster is for tightly-coupled applications needing ultra-low latency, putting instances on the same rack.
    • Spread is for critical, independent instances, putting each on separate hardware for maximum isolation.
    • Partition is for large distributed systems like Hadoop, separating instances into logical partitions on different racks.
• Q: A client has a small, critical application with 3 web servers and a database. They are very concerned about a single hardware failure. Which placement group would you recommend?
  • Answer:
    • A Spread placement group. It’s designed for a small number of instances that need to be on distinct hardware for maximum fault tolerance against correlated failures.

Online References

• AWS EC2 Placement Groups Documentation
• AWS Solutions Architect Associate Exam Training

Summary

Placement groups are a fundamental AWS EC2 feature that provides fine-grained control over instance placement. They are essential for workloads that are either performance-critical (Cluster), fault-tolerant (Spread), or both at a larger scale (Partition). The choice of placement group is a key architectural decision that directly impacts the availability and performance of your application.

Extra Insights

Think of the three placement groups with this mnemonic:

• Cluster -> Close together (Low Latency)
• Spread -> Separate (Max Fault Tolerance)
• Partition -> Partitions of a large system (Scalable Fault Tolerance)

Takeaways for Certification

• Key Notes:
  • Cluster:
    • Remember low latency and high throughput.
    • It is always within a single AZ.
    • Use cases are HPC, big data, and tightly-coupled applications.
  • Spread:
    • Remember maximum fault tolerance.
    • Can span multiple AZs.
    • Use cases are a small number of critical instances.
  • Partition:
    • Remember large distributed systems and fault isolation across logical groups.
    • Supports up to 7 partitions per AZ.
    • Use cases are Hadoop, Cassandra, Kafka.
  • General Rules: You cannot move an existing instance into a placement group. You must stop and start the instance to apply a new placement group. If you get a “capacity” error, it means AWS couldn’t find a contiguous block of hardware to meet your request. The solution is often to try again later or to choose a different instance type.
• Example Question: A company is running a distributed database with 50 nodes and wants to ensure that a single rack failure doesn’t take down the entire system. They also want to be able to query which nodes are on which rack. Which placement group should they use?
  • Answer: A Partition placement group. It’s built for large distributed systems, provides rack-level fault isolation, and gives you visibility into the partition ID of each instance, which can be mapped to a logical rack.

Watch this video to get a hands-on demonstration and in-depth explanation of AWS EC2 Placement Groups. AWS EC2 Placement Groups Hands-on with In-depth Explanation