Amazon ElastiCache Cache Design - Choosing Between Redis and Memcached with Caching Strategies
Explore the criteria for choosing between Redis and Memcached, caching strategies like Lazy Loading and Write-Through, and how to leverage Serverless mode.
Criteria for Choosing Between Redis and Memcached
ElastiCache offers two engines, Redis and Memcached, delivering microsecond latency and handling millions of requests per second. Redis operates on a single-threaded event loop model and provides rich data structures including Strings, Hashes, Lists, Sets, Sorted Sets, Streams, and HyperLogLog. It supports data persistence (AOF, RDB snapshots), read replicas, automatic failover, and Pub/Sub messaging, making it suitable not only for caching but also for session stores, leaderboards, real-time rankings, and message brokers. Memcached features a multi-threaded architecture specialized for simple key-value caching with easy horizontal scaling through node additions. It is well-suited for scenarios where data persistence and replication are unnecessary and you simply want to cache database query results. For new projects, Redis is the predominant choice due to its rich feature set and operational flexibility.
Designing Caching Strategies
Lazy Loading (Cache-Aside) is a strategy that fetches data from the database on a cache miss and writes it to the cache. Since only actually requested data is stored in the cache, memory efficiency is high, but cache misses occur on first access. Write-Through is a strategy that updates the cache simultaneously with database writes. Cache data is always kept up to date, but write latency increases and data that is never read also gets cached. In practice, combining both approaches is effective: use Write-Through to update the cache on writes while setting TTL to automatically expire stale data. TTL values should be determined based on data freshness requirements - short (seconds to minutes) for data requiring real-time accuracy, and long (hours to days) for master data. Continuously monitoring cache hit rates with CloudWatch metrics and periodically reviewing TTL and node sizing is key to effective operations.
Cluster Mode and Sharding
Enabling Redis cluster mode distributes data across 16,384 hash slots, allowing horizontal scaling across multiple shards. Each shard can have a primary and up to 5 read replicas to distribute read load. Clients send requests to the correct shard based on hash slot mapping, and if a slot has moved, a MOVED redirect automatically connects to the new location. For node type selection, memory-optimized r-series instances (cache.r7g series) are well suited for cache workloads, and you adjust the number of shards and replicas according to data volume and throughput requirements. Multi-AZ deployment ensures availability, with automatic failover switching the primary in tens of seconds.
Valkey Compatible Engine and Global Datastore
Valkey is an open-source Redis-compatible engine born as a fork of Redis, now offered as the new default engine for ElastiCache. You can use existing Redis client libraries and command sets as-is, benefiting from a Redis-compatible environment without licensing concerns. ElastiCache Serverless supports both the Valkey engine and Memcached. Global Datastore enables asynchronous cross-region data replication, achieving disaster recovery (DR) and improved global read latency. In the event of a primary region failure, the secondary region can be promoted in minutes, with RPO typically under 1 second.
ElastiCache Serverless and Operations
ElastiCache Serverless, introduced in 2023, eliminates the need to pre-design node types or cluster sizes. Capacity automatically scales based on workload, starting from a minimal configuration and adapting to traffic increases. Pricing is based on pay-per-use for data storage and ElastiCache Processing Units (ECPU). It is particularly well-suited for new applications with unpredictable traffic patterns and development/test environments. On the other hand, for production environments with stable traffic patterns, provisioned configurations with reserved nodes may be more cost-effective. To gain a deeper understanding of ElastiCache, specialized books (Amazon) can be helpful.
ElastiCache Pricing
ElastiCache pricing is based on hourly node charges. A Redis cache.r7g.large costs approximately 0.252 USD per hour (about 181 USD per month). Serverless mode uses pay-per-use pricing for ECPU (ElastiCache Processing Units) at approximately 0.0034 USD per million ECPU, and storage at approximately 0.125 USD per GB per month. Serverless is more cost-effective for intermittent workloads, while provisioned nodes are more advantageous for consistently high-throughput workloads. Reserved nodes offer discounts of up to 55%.
Summary
ElastiCache is a caching service that directly reduces database load and improves application response times. Use Redis as the default choice and design your caching strategy with a combination of Lazy Loading and Write-Through. Ensure scalability with cluster mode sharding and read replicas, and leverage the Valkey compatible engine and Global Datastore for global deployments. An effective approach is to start with Serverless mode for ease of use, then consider migrating to a provisioned configuration once your workload stabilizes.