Cache Invalidation and the Methods to Invalidate Cache

Cache invalidation is a state where we push away the data from the cache memory when the data present is outdated so do we perform this operation of pushing back/flushing the cache otherwise this still data will result in inconsistency of data.

When cached data gets stale or inaccurate, cache invalidation is the process of removing or updating it.. When the original data changes, the process of invalidating a cache involves deleting or updating cached data. It’s crucial because programs that rely on cached data may experience issues if it becomes outdated or erroneous over time.

Why Cache Invalidation is Important?

By keeping a copy of frequently accessed material in memory or on a disc, the concept of caching allows users to retrieve that data more quickly. However, the cached copy could grow outdated or erroneous if the original data changes. Incorrect results or performance issues may ensue if the application keeps using the cached data. When the original data changes, the process of invalidating a cache involves deleting or updating cached data.

Cache Invalidation Methods

Cache invalidation is an important process for maintaining accurate and up-to-date data in a cache. There are several methods of cache invalidation, each with its own advantages and disadvantages. Here are a few of the most popular techniques:

1. Time-based Cache Invalidation

Time-based invalidation involves setting an expiration time for cached data.  The cached data must be refreshed from the source once the expiration time has passed because it is then deemed invalid. This approach is straightforward to use and useful for data that doesn’t change frequently. However, if the expiration time is set too long or too short, it might result in the use of stale data or unneeded refreshes.

Benefits of Time-based Cache Invalidation

• Simple to implement.
• Effective for data that doesn’t change frequently.

Challenges of Time-based Cache Invalidation

• This may lead to the use of stale data if the expiration time is set too long.
• Unnecessary refreshes if the expiration time is set too short.

2. Key-based Cache Invalidation 

Key-based invalidation involves associating a unique key with each piece of cached data. The associated key is invalidated when the original data is altered, and the cached data is either deleted or updated. This approach can guarantee that the most recent data is always used and is effective for data that changes frequently. It might be trickier to put into practice than time-based invalidation, though, and more key storage space might be needed.

Benefits of Key-based Cache Invalidation

• Effective for data that changes frequently.
• Ensures that the most up-to-date data is always used.

Challenges of Key-based Cache Invalidation

• More complex to implement than time-based invalidation.
• May require additional storage for the keys.

3. Write-through Cache Invalidation 

Write-through invalidation involves updating the original data source first and then updating or removing the cached data. By using this technique, the risk of stale data is decreased and the cached data is always current. The application must wait for the primary data source to be updated before updating the cache, so it can be slower than other approaches.

Benefits of Write-through Cache Invalidation

• Ensures that the cached data is always up-to-date.
• Reduces the risk of stale data.

Challenges of Write-through Cache Invalidation

• Slower than other methods because the application must wait for the original data source to be updated before updating the cache.

4. Write-behind Cache Invalidation 

Write-behind invalidation involves updating the cached data first and then updating the original data source. Due to the lack of a wait time for the application, while the original data source is updated, this approach may be quicker than write-through invalidation. Because the cached data might not always be in sync with the original data source, it can, however, increase the risk of stale data.

Benefts of Write-behind Cache Invalidation

• Can be faster than write-through invalidation.

Challenges of Write-behind Cache Invalidation

• Increases the risk of stale data because the cached data may not always be in sync with the original data source.

5. Purge Cache Invalidation

By using the purge method, cached content for a particular object, URL, or collection of URLs is deleted. When the content has been updated or changed and the cached version is no longer accurate, it is typically used. The cached content is immediately deleted in response to a purge request, and the following request for the content will be fulfilled by the origin server directly.

Benefits of Purge Cache Invalidation

• Ensures that all cached data is removed and the cache is completely cleared.

Challenges of Purge Cache Invalidation

• Can be a slow and resource-intensive process
• Can cause temporary service disruptions if done incorrectly.

6. Refresh Cache Invalidation

Even if there is cached content available, fetches the requested content from the origin server. The cached content is updated with the most recent version from the origin server in response to a refresh request, making sure the content is current. A refresh request, in contrast to a purge, updates the existing cached content with the most recent version rather than erasing it.

Benefits of Refresh Cache Invalidation

• Can be done quickly and easily
• Ensures that the cached data is up-to-date

Challenges of Refresh Cache Invalidation

• This can result in a temporary spike in traffic as clients request the updated resource

7. Ban Cache Invalidation

A URL pattern or header is an example of specific criteria that the ban method uses to invalidate cached content. Any cached content that meets the requirements of a ban request is immediately removed, and any ensuing requests for the content will be fulfilled directly by the origin server.

Benefits of Ban Cache Invalidation

• Allows you to selectively invalidate cached data without removing all cached data. 

Challenges of Ban Cache Invalidation

• Can be complex to implement and can result in additional overhead.

8. Time-To-Live(TTL) expiration Cache Invalidation 

With this technique, cached content is given a time limit after which it becomes stale and needs to be refreshed. The cache checks the time-to-live value when a request for the content is made and only serves the cached content if the value is still valid. The cache gets the most recent copy of the content from the origin server and caches it if the value has expired.

Benefits of Time-To-Live(TTL) expiration Cache Invalidation 

• Allows you to automatically invalidate cached data after a certain amount of time

Challenges of Time-To-Live(TTL) expiration Cache Invalidation 

• This can result in clients receiving stale data if the expiration time is too long.

9. Stale-while-revalidate Cache Invalidation 

Web browsers and CDNs employ this technique to serve out-of-date content while it is being updated in the background. When someone requests a piece of content, the cached copy is delivered right away, and an asynchronous request is sent to the origin server to get the most recent copy. The cached version is updated when the most recent version becomes available. The user is always quickly served content thanks to this technique, even if the cached version is slightly out of date.

Benefits of Stale-while-revalidate Cache Invalidation

• Ensures that clients always have access to some version of the resource, even if it is not the latest version.

Challenges of Stale-while-revalidate Cache Invalidation

• This can result in clients receiving outdated data for a short period of time.