Core Concepts
- Node: A fixed-size chunk of secure, network-attached RAM, which is the smallest building block of an ElastiCache deployment. Each node runs the engine (Redis or Memcached) you select and has its own DNS name and port.
- Cluster: A collection of one or more nodes.
- Caching Strategies: Caching is most effective when:
- Data is expensive or slow to acquire from the primary database.
- Data is accessed very frequently.
- Data is relatively static, or some degree of staleness is acceptable.
- Common Use Cases: Database caching, web session storage, real-time analytics, leaderboards, and message queueing.
Engine Comparison: Redis vs. Memcached
Choosing the right engine is the most important decision when starting with ElastiCache.
| Feature | Redis | Memcached |
|---|---|---|
| Primary Use Case | Complex caching, session stores, leaderboards, pub/sub, geospatial data. | Simple object caching for reducing database load. |
| Data Structures | Supports advanced data structures: Strings, Lists, Sets, Sorted Sets, Hashes, Bitmaps. | Supports simple key-value pairs (strings, objects). |
| Performance Model | Primarily single-threaded. | Multi-threaded, can offer better performance on multi-core nodes for simple get/set operations. |
| High Availability | Yes. Supports Multi-AZ with automatic failover from a primary to a read replica. | No native automatic failover. Must handle node failures in application logic. |
| Data Persistence | Yes. Can create backups and snapshots of the cache data. Can be used as a primary data store. | No. Purely an in-memory cache. Data is lost on restart or failure. |
| Replication | Yes. Supports read replicas to scale read traffic and improve availability. | No. Cannot replicate data across nodes. |
| Publish/Subscribe | Yes. Built-in Pub/Sub messaging capabilities. | No. |
| Transactions | Yes. Supports transactions for executing multiple commands as a single atomic operation. | No. |
| Geospatial Support | Yes. Supports indexing and querying of geospatial data. | No. |
| Security | Supports Encryption in-transit & at-rest, and AUTH token for authentication. | No encryption. Supports SASL authentication. |
Rule of Thumb: Start with Redis unless you have a specific reason to use Memcached (e.g., existing Memcached environment or need for the simplest possible key-value cache). Redis offers far more features and flexibility.
Redis In-Depth
Redis Modes: Cluster vs. Non-Clustered
Redis on ElastiCache can run in two modes. The primary difference is the ability to partition (shard) data across multiple nodes for horizontal scaling.
| Feature | Redis (Cluster Mode Disabled) | Redis (Cluster Mode Enabled) |
|---|---|---|
| Shards (Node Groups) | Always 1 | 1 to 500 |
| Data Partitioning | No (all data on a single primary node) | Yes (data is spread across multiple shards) |
| Scale Up (Vertical) | Yes | No |
| Multi-AZ Failover | Yes (Optional) | Yes (Required) |
| Promote Replica to Primary | Yes | No (failover is managed by the cluster) |
Key Redis Features
- High Availability: With Multi-AZ enabled, if the primary node fails, ElastiCache automatically promotes one of the up-to-5 read replicas to be the new primary.
- Data Persistence: You can enable automatic snapshots and create manual backups of your Redis cluster, allowing you to restore data and providing durability.
- Complex Data Structures:
- Sorted Sets: Guarantees uniqueness and ordering of elements, making it perfect for building real-time leaderboards.
- Hashes: Maps that store field-value pairs, ideal for representing objects.
- Pub/Sub: A messaging paradigm where publishers send messages to channels, and subscribers receive messages from the channels they are subscribed to, without direct knowledge of each other.
Security
Encryption (Redis)
- In-Transit Encryption: Encrypts data as it moves between your application and the ElastiCache cluster, and between nodes within the cluster. Enabling this can have a minor performance impact.
- At-Rest Encryption: Encrypts on-disk data during sync operations and when creating backups/snapshots.
- Both encryption features are only available for Redis clusters running in a VPC on supported versions (e.g., 3.2.6+).
Authentication
- Redis AUTH: You can require clients to provide a token (password) using the
AUTHcommand before they are allowed to execute commands. This improves data security by preventing unauthorized access.
Network Security
- ElastiCache clusters should be launched in an Amazon VPC for network isolation.
- Access to the cluster nodes is controlled via Security Groups. You must open the appropriate TCP port to allow your application servers to connect.
- Default Redis Port:
6379 - Default Memcached Port:
11211
Sources
- Default Redis Port: