Write-Behind Cache

The Write-Behind Cache pattern improves performance by allowing operations that modify data to complete quickly, without the latency associated with directly updating persistent storage. Instead of synchronously writing to the database or file system, modifications are first applied to the cache, and a background process handles the eventual persistence of these changes. This decoupling significantly reduces response times for write-heavy operations.

This pattern is especially useful in scenarios where the cost of writing data is high – for example, communicating with a remote database, writing to slow storage, or triggering expensive operations. It depends on the assumption that the cache is sufficiently durable to survive short-term failures, and that eventual consistency is acceptable for the data in question. The cache needs a mechanism to handle potential failures during the background write process, such as retries or logging.

Usage

• High-traffic web applications: Caching user session data, frequently accessed objects, or API responses to reduce database load and improve responsiveness.
• Content Management Systems (CMS): Buffering content updates to minimize the impact on content delivery during peak hours.
• Financial transactions: If immediate consistency isn’t crucial, batching and asynchronously persisting transaction details.
• Event processing systems: Accumulating events in a cache before persisting them to an event store.

Examples

• Redis: A popular in-memory data store often used as a cache. Redis supports various eviction policies, but when used as a write-behind cache, it relies on its AOF (Append Only File) or RDB (Redis Database) persistence mechanisms to be asynchronously written to disk, after the client receives acknowledgment.
• Hibernate (Second Level Cache): The Hibernate object-relational mapping (ORM) framework provides a second-level cache that can be configured to use a write-behind strategy. Instead of immediately writing changes to the database, Hibernate stores them in the cache and periodically flushes them to the database, alleviating load during many updates.
• Operating System Disk Caching: Operating systems extensively use write-behind caching for disk I/O. When an application writes data to a file, the operating system typically caches the changes in memory and writes them to the physical disk in the background. This improves the overall system performance.