Write-Back Cache

What is write-back cache?

Write-back cache is a caching strategy that enhances system performance by temporarily storing data in a high-speed medium (typically memory) and deferring updates to the primary storage (typically disk). Unlike other caching strategies, write-back prioritizes speed by first writing data to the cache and synchronizing with the main storage asynchronously. This strategy reduces latency for write operations but requires careful management to ensure data consistency.

This resource will explore different caching strategies, compare write-back with other approaches, discuss its benefits and challenges, and provide guidance on when to use it. Whether you’re an application developer or architect, understanding write-back cache can help you optimize performance and scalability in your systems.

• Caching strategies
• Write-back vs. write-through
• Write-back cache benefits and challenges
• What about write-back data loss risks?
  
• Write-back cache use cases
  
• Choosing between write-back and write-through cache
• Key takeaways and resources
  

Caching strategies

Caching is the practice of temporarily storing copies of data for faster retrieval. The most common example is RAM+disk. RAM is typically faster than disk but is also more expensive and limited. Using RAM to cache frequently accessed data can improve performance. Different caching strategies suit different use cases, balancing speed, consistency, and complexity.

Write-back cache

Write-back cache first stores data in the cache and queues it to be written to the primary storage at a later time. When a write occurs, it’s immediately considered successful as long as the data is stored in the cache, not waiting for the disk to be updated. The system asynchronously updates the main storage. Subsequent reads pull from memory, which provides another performance benefit. Write-back is particularly useful for applications requiring high throughput. Of course, there is a risk that the write to disk will fail. There are many ways to reduce that risk (we’ll explore that later), although mathematically, it will always be a risk.

Write-through cache

In a write-through cache, data is written to both the cache and the primary storage “simultaneously” (through a transaction/lock mechanism). This approach enforces data consistency across all storage layers at the cost of higher latency for write operations.

Write-around cache

Write-around cache bypasses the cache entirely for write operations, storing data directly in the primary storage. The cache is only updated when data is read. This strategy minimizes the overhead of writing to the cache but may lead to cache misses for frequently updated data. Write-around cache is well suited for scenarios with infrequent data updates, or situations where the data being written won’t be accessed immediately. Overall, write-around cache is used less frequently than write-back and write-through.

Write-back vs. write-through

Write-back and write-through caching represent two ends of the spectrum in terms of speed and consistency.

• Write-back caching prioritizes performance by deferring updates to primary storage, which reduces write latency. However, the risk of data loss increases if the cache fails before synchronizing with storage.
• Write-through caching emphasizes data consistency by ensuring every write operation updates both the cache and the main storage. The trade-off is increased latency and potentially higher resource usage.

Choosing between the two depends on your application’s tolerance for latency and consistency.

Write-back cache benefits and challenges

Benefits

Enhanced write performance: Writing data to cache is faster than writing to slower primary storage.
Reduced storage traffic: As writes to the primary storage are batched or delayed, overall I/O (input/output) traffic decreases, reducing strain on storage systems.
Improved read performance: Frequently accessed data remains in the cache, speeding up read operations.

Challenges

Data consistency risks: Data may be lost if the cache fails before synchronizing with storage.
Complex cache management: Ensuring that cache and storage remain synchronized requires robust error handling and monitoring, especially if you integrate two different data systems (a database and a separate key-value cache store, for instance).
Durability: Applications requiring immediate persistence might find write-back unsuitable unless there are ways to mitigate risk (which a caching system like Couchbase provides, for instance).

What about write-back data loss risks?

Couchbase provides a durable and distributed architecture to reduce the risk of data loss. The default setting in the Couchbase SDK is for writes to be completely asynchronous, meaning you risk losing data if a server fails. However, by simply increasing the durability level to “majority,” the operation becomes synchronous, reducing the risk of data loss (data loss would result from multiple servers failing simultaneously during the operation). Further, durability requirements can be increased to “majorityAndPersistActive” and “persistToMajority.” These make data loss even less likely (widespread server failure and disk loss during the operation would have to occur for data loss). In any of the above situations, data loss would only occur during the failure event. With increased durability, the risk still exists mathematically, in the same way that winning the lottery is possible.

These settings also increase latency, but in a complex system, some operations benefit more from performance, and some require more durability. Write-back caching can prioritize certain data types (e.g., purchases need the highest durability, and steady-state log data is a lower priority). Couchbase’s write-back system and durability options give you the flexibility that write-through doesn’t.

Write-back cache use cases

Write-back cache is well suited for scenarios where write performance is critical and occasional delays in consistency are acceptable. Use cases include:

• Gaming and user session management: It can be used for multiplayer games and web applications that store session or player data to provide fast experiences with minimal latency.
• E-commerce systems: Shopping cart, browsing, user preferences, and other e-commerce operations are cached for speed, while less frequent but more critical purchases can use increased durability.
• Video streaming platforms: It can be used to cache metadata, such as watch history or recommendations, for faster access.
• Social media: Couchbase is a core technology of LinkedIn’s caching architecture, serving up profiles and social media content faster.

A properly built caching system with a write-back approach, like Couchbase, is well suited for both performance and data reliability.

Choosing between write-back and write-through cache

The decision to use write-back or write-through caching depends on your application’s requirements. Consider the following:

• Performance vs. durability: Write-back is ideal when write speed is a priority and risks can be reduced (e.g., Couchbase’s durability options). Write-through can be adequate for systems where read operations far outweigh write operations.
• Failure tolerance: Systems with limited tolerance for data loss should avoid write-back unless additional redundancy mechanisms are in place (e.g., Couchbase’s distributed architecture).
• Scalability: Write-back caching is valuable in architectures where scalability is crucial. By reducing write loads to primary storage, systems can handle more concurrent users and improve responsiveness.

Key takeaways and resources

• Write-back cache provides superior write performance by delaying synchronization with primary storage, but it comes with risks to data consistency that a distributed system with durability options can address.
• Write-through cache ensures data integrity by writing simultaneously to cache and storage, making it suitable for read-heavy applications where flexibility isn’t necessary.
• Choosing the right caching strategy requires understanding your system’s performance needs, consistency requirements, and tolerance for risk.

Suggested next steps

• Explore Couchbase’s memory-first architecture, which implements caching strategies like write-back for enhanced performance.
• Learn more about durable writes to mitigate risks associated with caching.
• Review our blog and concepts hub to keep learning about topics related to caching.