Caching is a critical component in modern software systems, serving as an effective means to reduce latency and improve system performance. Asynchronous caching is a caching mechanism that allows data to be stored and retrieved in a non-blocking, asynchronous manner. This means that when data is requested, the system doesn’t wait for the cache to provide the data before continuing with its operations.

Understanding the Cache Stampede Problem

Consider a scenario where a web application caches the results of a resource-intensive database query to reduce response time. When the cache expires, multiple users simultaneously request the same data. Without adequate safeguards, the system may:

• Execute the expensive query multiple times, causing unnecessary load on the database server.
• Experience a surge in request processing time, leading to poor user experience.
• Encounter service disruptions or downtime due to resource exhaustion.

To overcome these challenges, asynchronous caching mechanisms are employed, ensuring that cache refresh and population activities are handled efficiently without causing bottlenecks.

Asynchronous Caching Mechanisms

Stale-While-Revalidate (SWR)

Stale-While-Revalidate is a popular technique that allows cached data to be served to users while asynchronously refreshing it in the background. This mechanism helps mitigate cache stampede problems by ensuring that expired cache entries continue to provide service until fresh data is available.

Example:

Consider a news website where articles are cached. When a user requests an article, the stale cache is served while a background process fetches and updates the latest article data.

Background Cache Population

Background cache population involves preloading cache entries before they expire. This approach is beneficial for regularly accessed or critical data to ensure that fresh cache is always available, reducing the likelihood of stampedes.

Example:

In an e-commerce application, product information is preloaded into the cache before expiration, preventing simultaneous requests from hitting the backend

Scheduled Cache Refresh

Scheduled cache refresh involves periodically refreshing cache entries, reducing the chances of simultaneous requests for expired data. This method is suitable for data that changes infrequently.

Example:

A weather application refreshes its cache of weather data every hour, ensuring users consistently receive up-to-date information.

Cache Write-Behind

Cache write-behind is a technique where data is updated in the cache asynchronously after it is updated in the backend, particularly in scenarios where write-intensive workloads could otherwise lead to performance bottlenecks or delays. This method helps maintain cache consistency and reduces the risk of stampedes.

Example:

A social media platform updates the cache with new user posts in the background after they are published.

Caching Proxies or CDNs

Caching proxies or Content Delivery Networks (CDNs) sit between clients and the application server, caching and serving content. They handle cache expiration and updates, reducing the impact of stampedes on the application server.

Example:

A CDN serving static assets like images and videos caches content close to users, reducing the load on the origin server.

Diagram #4: CDN Diagram

Queue-Based Cache Population

Queue-based cache population involves using message queues to asynchronously update cache entries and adding, updating, or evicting cache entries. This approach decouples cache population tasks from the main application, which allows for efficient handling of cache updates and reduces contention, making it more efficient, scalable, and resilient

Example:

An e-commerce platform uses a message queue to update product availability information in the cache.

Rate Limiting and Throttling

Rate limiting and throttling mechanisms control the rate at which requests are allowed to access the cache. By limiting the rate of incoming requests, they prevent stampedes and ensure fair access to cached resources.

Example:

A video streaming service limits the rate at which users can request video metadata to prevent excessive cache requests.

Distributed Caching

Distributed caching systems store data across multiple servers, improving scalability and fault tolerance. These systems can employ various asynchronous mechanisms to handle cache updates efficiently.

Example:

A global e-commerce platform uses a distributed caching system to serve product catalog data across regions

Cache Eviction Policies

Cache eviction policies determine which cache entries are removed when the cache reaches its capacity. Proper eviction policies are essential for maintaining cache efficiency and mitigating stampedes.

Example:

An LRU (Least Recently Used) eviction policy ensures that the least recently accessed cache entries are removed first, allowing fresh data to be cached

Conclusion

The cache stampede problem is a common challenge in modern software systems, but with the right asynchronous caching mechanisms in place, it can be effectively mitigated. Whether it’s using techniques like Stale-While-Revalidate, Background Cache Population, Scheduled Cache Refresh, Cache Write-Behind, or leveraging Caching Proxies, CDNs, Queue-Based Cache Population, Rate Limiting, Throttling, Distributed Caching, or Cache Eviction Policies, there are various strategies to ensure that cached data remains up-to-date and available, even in the face of high concurrent requests.