Caching

Cache Invalidation

Write-through cache:

Under this scheme data is written into the cache and the corresponding database at the same time.

Every write operation must be done twice before returning success to the client

Pros:

• (Data Consistency) The cached data allows for fast retrieval, and since the same data gets written in the permanent storage, we will have complete data consistency between cache and storage.

• (Fault Tolerance) This scheme ensures that nothing will get lost in case of a crash, power failure, or other system disruptions. It minimizes the risk of data loss.

Cons:

• (Higher Latency) higher latency for write operations

Write-around cache:

This technique is similar to write through cache, but data is written directly to permanent storage, bypassing the cache.

Pros:

• This can reduce the cache being flooded with write operations that will not subsequently be re-read,

Cons:

• but has the disadvantage that a read request for recently written data will create a “cache miss” and must be read from slower back-end storage and experience higher latency.

Write-back cache:

Under this scheme, data is written to cache alone, and completion is immediately confirmed to the client.

The write to the permanent storage is done after specified intervals or under certain conditions.

Pros:

• (Low Latency & High Throughput) This results in low latency and high throughput for write-intensive applications,

Cons:

• (Data Loss) however, this speed comes with the risk of data loss in case of a crash or other adverse event because the only copy of the written data is in the cache.

Cache Invalidation Schemes Comparison

	Write-through cache	Write-around cache	Write-back cache
Description
Pros
Cons

Cache Eviction Policies

Cache Eviction Policies	Description
First In First Out (FIFO)	The cache evicts the first block accessed first without any regard to how often or how many times it was accessed before.
Last In First Out (LIFO)	The cache evicts the block accessed most recently first without any regard to how often or how many times it was accessed before.
Least Recently Used (LRU)	Discards the least recently used items first.
Most Recently Used (MRU)	Discards, in contrast to LRU, the most recently used items first.
Least Frequently Used (LFU)	Counts how often an item is needed. Those that are used least often are discarded first.
Random Replacement (RR)	Randomly selects a candidate item and discards it to make space when necessary.