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Terraform

AWS ElastiCache Cluster vs Replication Group: Key Differences

By Filip on 11/13/2024

This article explains the key differences between AWS ElastiCache Cluster and Replication Group, helping you choose the right option for your caching needs.

AWS ElastiCache Cluster vs Replication Group: Key Differences

Table of Contents

Introduction

Amazon ElastiCache is a popular choice for adding caching layers to applications, offering both Memcached and Redis engines. While both enhance performance, understanding their deployment models in AWS is crucial. This guide clarifies the differences between ElastiCache's Memcached clusters, Redis replication groups, and Redis global datastores, ensuring you choose the right approach for your needs. We'll cover key AWS CLI commands and their Terraform equivalents, highlighting potential points of confusion.

Step-by-Step Guide

  1. ElastiCache offers two main caching engines: Memcached and Redis.
  2. For Memcached, you use "cache clusters." 
    aws elasticache create-cache-cluster ...
  3. For Redis, you typically use "replication groups." These provide high availability and data redundancy. 
    aws elasticache create-replication-group ...
  4. A replication group consists of multiple Redis nodes (clusters). One is the primary for read/write, while others are read-only replicas.
  5. Redis has two modes: Cluster Mode Enabled and Disabled. Replication groups are for Cluster Mode Disabled.
  6. For Redis Cluster Mode Enabled, you use "global datastores." These span multiple AWS regions for even greater scalability and resilience.
  7. Don't confuse replication groups with Redis clustering. Replication groups are for high availability within a single region.
  8. You can't mix and match commands. Use create-cache-cluster for Memcached and create-replication-group for Redis (Cluster Mode Disabled).
  9. Terraform also distinguishes between these. Use aws_elasticache_cluster for Memcached and aws_elasticache_replication_group for Redis (Cluster Mode Disabled).

Code Example

The code provides examples of creating Memcached and Redis cache clusters on AWS using both the AWS CLI and Terraform. It covers creating a Memcached cluster, a Redis replication group with cluster mode disabled, and a Redis global datastore with cluster mode enabled. The examples demonstrate the basic commands and resource configurations for each scenario, highlighting key parameters and differences between CLI and Terraform approaches. The description also mentions the need for customization based on specific requirements and points out the limitation of Terraform in directly creating Redis clusters, suggesting the use of 'aws_elasticache_global_replication_group' resource instead.

1. Creating a Memcached Cache Cluster:

aws elasticache create-cache-cluster \
    --cache-cluster-id my-memcached-cluster \
    --engine memcached \
    --cache-node-type cache.t3.micro \
    --num-cache-nodes 1

2. Creating a Redis Replication Group (Cluster Mode Disabled):

aws elasticache create-replication-group \
    --replication-group-id my-redis-group \
    --replication-group-description "My Redis Replication Group" \
    --primary-cluster-id my-redis-primary \
    --cache-subnet-group-name my-cache-subnet-group \
    --engine redis \
    --engine-version 6.x \
    --cache-node-type cache.t3.micro \
    --num-cache-clusters 2

3. Creating a Redis Global Datastore (Cluster Mode Enabled):

aws elasticache create-global-replication-group \
    --global-replication-group-id my-global-redis \
    --primary-replication-group-id my-redis-group \
    --global-replication-group-description "My Global Redis Datastore"

Terraform Examples

1. Creating a Memcached Cluster:

resource "aws_elasticache_cluster" "example" {
  cluster_id           = "my-memcached-cluster"
  engine               = "memcached"
  node_type            = "cache.t3.micro"
  num_cache_nodes      = 1
  parameter_group_name = "default.memcached1.6"
}

2. Creating a Redis Replication Group (Cluster Mode Disabled):

resource "aws_elasticache_replication_group" "example" {
  replication_group_id          = "my-redis-group"
  replication_group_description = "My Redis Replication Group"
  engine                        = "redis"
  engine_version               = "6.x"
  node_type                     = "cache.t3.micro"
  num_cache_clusters           = 2
  subnet_group_name             = aws_elasticache_subnet_group.example.name
}

resource "aws_elasticache_subnet_group" "example" {
  name       = "my-cache-subnet-group"
  description = "Subnet group for Redis replication group"
  subnet_ids = [aws_subnet.example.id]
}

Note: These are basic examples. You'll need to adjust parameters like instance types, security groups, and subnet groups based on your specific needs.

Remember that you cannot directly create a Redis cluster (Cluster Mode Enabled) using the aws_elasticache_cluster or aws_elasticache_replication_group resources in Terraform. You would need to use the AWS provider's aws_elasticache_global_replication_group resource to achieve that.

Additional Notes

  • Think of "clusters" as the basic building blocks. For Memcached, a cluster is your caching layer. For Redis (Disabled mode), a cluster is part of a larger replication group.
  • Redis replication groups are for high availability within an AWS region. For cross-region disaster recovery, you'll need multiple replication groups and data synchronization strategies.
  • Global datastores (Redis Cluster Mode Enabled) are powerful but come with considerations:
    • Increased management complexity.
    • Latency implications for cross-region data access.
    • Cost implications due to data replication across regions.
  • Always refer to the AWS documentation for the most up-to-date information on commands and parameters. The examples provided are simplified for illustration.
  • Security is paramount: Configure security groups, encryption at rest and in transit, and IAM policies to secure your ElastiCache deployments.
  • Monitoring and performance tuning are essential: Utilize CloudWatch metrics, performance insights, and other tools to optimize your cache for your workload.
  • Consider your use case carefully: Memcached is simpler for basic caching, while Redis offers more advanced data structures and features if your application needs them.
  • Experiment and test: Spin up different configurations in a development environment to understand the performance and cost implications before deploying to production.

Summary

Feature Memcached Redis (Cluster Mode Disabled) Redis (Cluster Mode Enabled)
Deployment Unit Cache Cluster Replication Group Global Datastore
AWS CLI Command create-cache-cluster create-replication-group N/A (use Global Datastore API)
Terraform Resource aws_elasticache_cluster aws_elasticache_replication_group N/A (use Global Datastore resource)
High Availability Configurable Built-in with replicas Built-in, spans multiple regions
Data Redundancy Configurable Built-in with replicas Built-in, spans multiple regions
Scalability Limited Within a single region Across multiple regions

Key Points:

  • Choose the right deployment method: Use cache clusters for Memcached, replication groups for Redis (Cluster Mode Disabled), and global datastores for Redis (Cluster Mode Enabled).
  • Don't mix commands: Use the appropriate command/resource for each engine and mode.
  • Replication groups are for high availability within a region, while global datastores provide cross-region resilience.

Conclusion

In conclusion, Amazon ElastiCache provides flexible and powerful caching solutions for your applications. By understanding the differences between Memcached and Redis, and their respective deployment models – cache clusters, replication groups, and global datastores – you can optimize your caching layer for performance, availability, and scalability. Remember to choose the right deployment method and commands for your chosen engine and mode, and always prioritize security and performance monitoring. By carefully considering your use case, experimenting with different configurations, and leveraging the insights provided in this guide, you can effectively implement ElastiCache to enhance your application's performance and user experience.

References

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