Durability options
ElastiCache for Valkey offers two durability options: synchronous and asynchronous writes.
With synchronous writes, successful write operations are durably stored in the Multi-AZ transactional log before returning to clients. This incurs single-digit millisecond write latency and ensures that no acknowledged write operations will be lost in the event of a failure.
With asynchronous writes, successful write operations are returned to clients before being durably stored in the Multi-AZ transactional log. Since the write operations don't wait to be durably stored in the Multi-AZ transactional log, the write operation latency is equivalent to ElastiCache without durability. However, up to the last 10 seconds of successful write operations may be lost in the event of a failure.
To understand the potential data loss with asynchronous writes, consider the concept of a durability buffer. The durability buffer represents the maximum age of any write that has been accepted by the primary node but not yet persisted to the Multi-AZ transactional log. The primary node tracks the age of the oldest unacknowledged write. As long as this age remains under 10 seconds, the node continues accepting new writes normally. If the age of the oldest unacknowledged write grows beyond 10 seconds, the primary node will reject all incoming write commands until it catches up. Read operations continue to be served at microsecond latency during this period. Once the pending writes are persisted, the node resumes accepting writes automatically. This ensures that potential data loss is limited to 10 seconds worth of writes in the event of a failure.
When configuring your client to send traffic to an asynchronous durable cluster, ensure that the
client automatically retries with exponential backoff any write commands that are rejected with the
cluster down error message. For guidance on configuring your clients to handle this and other transient
errors, see Best
practices: Valkey/Redis OSS clients and Amazon ElastiCache
Choosing a durability option
Use synchronous writes when your application cannot tolerate any data loss during failures. With synchronous writes, you can use ElastiCache for a broader set of use cases beyond caching where data loss is not acceptable, such as knowledge bases for RAG applications, AI agent memory, AI agent workflow state, payment tokenization, streaming metadata, gaming player state, and real-time inventory management.
Use asynchronous writes when your application prioritizes write performance and can tolerate the potential loss of up to 10 seconds of uncommitted data during a failure. This option is ideal for workloads such as application data caching, session stores, gaming leaderboards, and real-time analytics.
