Building a Durable In-Memory Database with Amazon MemoryDB for Redis - Unifying Cache and Primary Data Store
Learn how to operate a Redis-compatible in-memory database with MemoryDB, understand its durability mechanisms, and decide when to use it versus ElastiCache.
Overview of MemoryDB
MemoryDB for Redis is a durable in-memory database service compatible with Redis 7.x, capable of storing up to hundreds of terabytes of data across up to 500 nodes. While ElastiCache for Redis is specialized for caching (volatile data), MemoryDB guarantees write durability through a Multi-AZ transaction log, making it suitable as a primary data store.
Durability and When to Choose Over ElastiCache
MemoryDB synchronously records writes to a Multi-AZ transaction log, ensuring no data loss during node failures. ElastiCache uses asynchronous replication, which means recent writes may be lost during failover. MemoryDB is well suited for use cases that need in-memory speed without risking data loss, such as session stores, user profiles, and leaderboards. ElastiCache is better suited as a front-end cache for RDS or DynamoDB, where some data loss is acceptable.
How Durability Works and Practical Usage
MemoryDB persists its transaction log across multiple Availability Zones and fully restores data after node failures. While ElastiCache is designed for caching (where data loss is tolerable), MemoryDB provides the durability needed to serve as a primary database. With microsecond read latency and millisecond write latency, it is ideal as a primary data store for session stores, leaderboards, and real-time analytics. You can take full advantage of Redis data structures (Sorted Sets, Hashes, Streams) and position MemoryDB as a data layer that complements DynamoDB or RDS. To deepen your knowledge of database design, specialized books on Amazon can be a valuable resource.
MemoryDB Pricing
A db.r7g.large MemoryDB node costs approximately $0.463 per hour (about $333 per month). This is roughly 20% more expensive than an equivalent ElastiCache node, but includes transaction log storage for durability. Data transfer within the same region is free, while cross-region transfer follows standard data transfer pricing. Snapshot storage beyond the free tier (equal to the node's memory size) costs approximately $0.085 per GB per month. If your use case is caching only, ElastiCache is the more cost-effective option.
Summary
MemoryDB is a Redis-compatible in-memory database with guaranteed durability. It achieves write persistence through a Multi-AZ transaction log, and unlike ElastiCache (which is designed for caching), it can serve as a primary database. With microsecond read latency, it is an excellent data layer for session stores, leaderboards, and real-time analytics.