About
In computer science, a data cache is a component that aims to:
- improve performance
- reduce load on the server.
The cache will:
- store transparently a request response
- and use it to for later requests until a cache time limit has been reached
The data that is stored within a cache might be:
- values that have been computed earlier
- or duplicates of original values that are stored elsewhere.
Properties
Cache Request:
- Data Cache - Time to Live (TTL) or hop limit - maximal timespan in the cache
Cache Store
- Algorithm such as Cache - LRU (Least Recently Used)
Goal
Which information is loaded into cache depends on algorithms and certain assumptions about programming code. The goal of the cache system is to ensure that the application has the next bit of data it will need already loaded into cache by the time it goes looking for it (also called a cache hit).
Cache hit, cache miss
If requested data is contained in the cache, a cache hit occurs and this request can be served by simply reading the cache, which is comparably faster.
Otherwise, a cache miss occurs and the data has to be recomputed or fetched from its original storage location, which is comparably slower.
Hence, the more requests can be served from the cache the better the overall system performance is.
A cache is transparent
As opposed to a buffer, which is managed explicitly by a client, a cache stores data transparently: This means that a client who is requesting data from a system is not aware that the cache exists, which is the origin of the name cache (from French “cacher”, to conceal).
Validity Cache Time vs Business Level
This section is about data processing cache where data needs to be aggregated.
| Architecture Level | Data Latency |
|---|---|
| Raw Source | Immediate |
| Real Time Cache | Seconds |
| Business Process | Minutes |
| Business Management | Day |
| Business Lead | Week, Month, Year |
Key
Whenever a cache receives a request, it needs to decide:
- whether it has a copy of this exact request already saved and can reply with that,
- if it needs to forward the request to the server.
Caches tackle this problem using the concept of cache keys that identifies uniquely the resource.
See HTTP cache key
Implemententation
- memcached Free & open source, high-performance, distributed memory object caching system
Cachedir tag to identify a cache directory
To avoid backing up, archiving, copying, or moving cache directories, you can add the file CACHEDIR.TAG to identify a directory as containing cache data.
Example: All CACHEDIR.TAG have the same content
Signature: 8a477f597d28d172789f06886806bc55
# This file is a cache directory tag created by (application name).
# For information about cache directory tags, see:
# http://www.brynosaurus.com/cachedir/