Redis is a data structure server. It is open-source, networked, in-memory, and stores keys with optional durability.
Redis is an open source, BSD licensed, advanced key-value cache and store. It is often referred to as a data structure server since keys can contain strings, hashes, lists, sets, sorted sets, bitmaps and hyperloglogs.
Varnish is an HTTP accelerator designed for content-heavy dynamic web sites as well as heavily consumed APIs.
Varnish Cache is a web application accelerator also known as a caching HTTP reverse proxy. You install it in front of any server that speaks HTTP and configure it to cache the contents. Varnish Cache is really, really fast. It typically speeds up delivery with a factor of 300 – 1000x, depending on your architecture.
Varnish stores data in virtual memory and leaves the task of deciding what is stored in memory and what gets paged out to disk to the operating system. This helps avoid the situation where the operating system starts caching data while it is moved to disk by the application.
Furthermore, Varnish is heavily threaded, with each client connection being handled by a separate worker thread. When the configured limit on the number of active worker threads is reached, incoming connections are placed in an overflow queue; when this queue reaches its configured limit incoming connections will be rejected.
APC is a great operation code caching system for PHP that can help speed up your site. PHP is a dynamic server-side scripting language that needs to be parsed, compiled and executed by the server with every page request. In many cases though, the requests produce exactly the same results which means that the cloud server has to unnecessarily repeat all these steps for each of them.
This is where APC comes into play. What it does is save the PHP opcode (operation code) in the RAM memory and if requested again, executes it from there. In essence, it bypasses the parsing and compiling steps and minimizes some unnecessary loads on the cloud server.
The Alternative PHP Cache (APC) is a free and open opcode cache for PHP. Its goal is to provide a free, open, and robust framework for caching and optimizing PHP intermediate code. Besides a opcode cache it provides a user cache for storing application data. This module uses the APC user cache as a cache backend for Drupal.
Memcached is a high-performance, distributed memory object caching system, generic in nature, but originally intended for use in speeding up dynamic web applications by alleviating database load
In computing, memcached is a general-purpose distributed memory caching system that was originally developed by Danga Interactive for LiveJournal, but is now used by many other sites. It is often used to speed up dynamic database-driven websites by caching data and objects in RAM to reduce the number of times an external data source (such as a database or API) must be read. Memcached runs on Unix, Linux, Windows and MacOSX and is distributed under a permissive free software license.
Memcached’s APIs provide a giant hash table distributed across multiple machines. When the table is full, subsequent inserts cause older data to be purged in least recently used (LRU) order. Applications using Memcached typically layer requests and additions into RAM before falling back on a slower backing store, such as a database.