ElastiCache is a fully managed in-memory data store service provided by Amazon Web Services (AWS) that supports two popular open-source in-memory key-value stores - Redis and Memcached. The data storage mechanism differs between the two engines supported by ElastiCache.
For Redis, the data is stored in memory and optionally written to disk using persistence mechanisms like RDB (Redis Database) or AOF (Append Only File). The Redis data can be persisted in one of the following ways:
RDB snapshotting: Redis creates point-in-time snapshots of the dataset at specified intervals. These snapshots are stored as an RDB file on disk, which can be used to recover the database in case of failures.
AOF persistence: Redis logs all write operations to an append-only file, which can be used to reconstruct the dataset when Redis is restarted. Redis provides different options to configure AOF, such as always appending every command received, or writing to disk after a specified number of write commands.
For Memcached, the data is stored only in memory and not persisted to disk. Therefore, a restart or failure will result in loss of the cached data.
In summary, ElastiCache stores data in memory for both Redis and Memcached engines. However, Redis can optionally persist data to disk using RDB or AOF mechanisms. It is important to note that the persistence mechanisms increase durability at the cost of performance and memory usage, and therefore should be chosen carefully based on use cases and workload requirements.