How to use caching in Rails?

Caching is a technique employed in Rails (and many other frameworks) to store precomputed results and thereby reduce the time and computational effort required for future requests. By retaining frequently accessed data in a location quicker to retrieve than its source, caching can greatly improve a Rails application’s performance.

Rails provides several layers of caching:

1. Page Caching: This is the most straightforward form of caching. Entire rendered HTML pages are stored on disk and delivered directly by the web server, bypassing the Rails app entirely. This method is super-fast but is a bit outdated and can be inappropriate for dynamic content.

2. Action Caching: Unlike page caching, action caching allows the execution of filters (like authentication) before the cache is served. This provides a bit more granularity in controlling what content gets cached and when it should be delivered.

3. Fragment Caching: Sometimes, only parts of the view need caching, rather than the entire page or action. Fragment caching lets you specify these portions, ensuring only that fragment is stored for reuse. With the introduction of Russian Doll Caching, when a single item changes, only that item’s fragment needs to be recreated, and not the entire structure.

4. Low-Level Caching: Rails also provides more granular caching mechanisms using the `Rails.cache` interface, where you can manually read and write data to the cache.

5. SQL Caching: Activated by default, this caches the results of SQL queries, so if the same query is called again in the same request, the result is returned from cache rather than hitting the database again.

To effectively use caching in Rails:

1. Choose the Right Store: Rails supports multiple cache stores like `MemoryStore`, `FileStore`, `MemCacheStore`, and others. Depending on the scenario, you might want to pick a different store.

2. Expire Caches: Stale or outdated cached data can be detrimental. Set up mechanisms to expire and refresh caches when data changes.

3. Consider External Tools: Tools like Redis and Memcached are excellent companions for Rails caching strategies, offering powerful and scalable caching solutions.

Caching is a potent tool in Rails development, accelerating application responsiveness and reducing server load. However, it requires careful planning and consideration to ensure accuracy and freshness of the cached data.