Redis

Redis is an in-memory data structure store, used as a database, cache and message broker. It supports data structures such as strings, hashes, lists, sets, sorted sets with range queries, bitmaps, hyperloglogs, geospatial indexes with radius queries and streams.

High Performance

• Programming Language(C)
• In-Memory Store and Access
• Single Thread
• I/O Multiplexing Model

Redis I/O Multiplexing Model

• Regular Blocking I/O

• Non-blocking, Multiplexing I/O

  Redis takes great advantage of the non-blocking, multiplexing I/O model in its main processing single thread

  

• Redis Reactor Pattern

  Redis contains a simple but powerful asynchronous event library called ae to wrap different operating system’s polling facilities, such as epoll, kqueue, select, and so on.

  Take the epoll API in Linux as an example. First, you can call epoll_create to tell the kernel that you would like to use epoll. Then, you call epoll_ctl to tell the kernel the file descriptors (FD) and what type of event you’re interested in when an update occurs. After that, epoll_wait gets called to wait for certain events of the FDs you set in epoll_ctl. The kernel will send a notification to you when the FDs get updated. The only thing you have to do is to create handlers for certain events.

  I/O Multiplexing Module

Disadvantage of Redis compared with RDBMS

• Redis is an in-memory store: all your data must fit in memory. RDBMS usually stores the data on disks, and cache part of the data in memory. With a RDBMS, you can manage more data than you have memory. With Redis, you cannot.
• Redis is a data structure server. There is no query language (only commands) and no support for a relational algebra. You cannot submit ad-hoc queries (like you can using SQL on a RDBMS). All data accesses should be anticipated by the developer, and proper data access paths must be designed. A lot of flexibility is lost.
• Redis offers 2 options for persistency: regular snapshotting and append-only files. None of them is as secure as a real transactional server providing redo/undo logging, block checksuming, point-in-time recovery, flashback capabilities, etc.