REDIS

When somebody asks us to create a web application, the software requirements will be Apache, Mysql, PhP(LAMP where L stands for Linux). Imagine an application which stores data in main memory instead of disk, this will increase your application performance tremendously. One such application is Redis. Redis stores data on memory instead of disk and will periodically sync with the disks(if necessary).

Why Redis?:

• Redis will be faster as it keeps data on memory. I read somewhere Memory is like disk and disk is like tape for redis.
• Redis allows lot of data structures. Basically its a NOSQL database(http://en.wikipedia.org/wiki/NoSQL) . They dont support table or database schema we use traditionally. For them everything is key value pair as in hashmap. Redis allows a key to have values of types- string, set, sorted set, list, hashmap. To understand each data structure and the command they support, have a look at http://simonwillison.net/static/2010/redis-tutorial/
• Lot of  web apps will be easier to code, as complex operations like sorting, union, aggregation can be done by redis itself

Do Redis follow ACID?

• As of now, if redis server is crashed and data is not synced with the disk then consistency of data will be an issue
• Redis server is a single threaded program, so no concurrency issues.
• If you want a client to make a transaction, multi and exec commands in redis will take care of that. Unlike other databases there is no locking mechanisms involved during transaction. When a client performs exec, server wont get commands from other clients till all command queued for this transaction are done.
• Single thread can reduce scalability of Redis. Redis supports slaves where a client can read from master and slaves but writes should first affect the master and then synced across the clients. So time consuming operations like sort can be moved to the slaves.

Some Cool Commands to Try Out:

• BLPOP and BRPOP are some good operations that one always do on queues. Say I am waiting on a queue for a representative to pick up my call. So once a representative comes available, BLPOP will return the representative to the guy who waits for a longer time. No application logic is actually needed
• Multi and Exec, they support transactions

Where To start?:

• Install redis (apt-get or yum) or get a free redis instance at http://redis4you.com
• Almost all languages support redis client coding(for Java download Jedis jar, for Ruby try redis-rb gem). For complete list check at http://redis.io

Start developing web apps without database if your data will fit in memory