Memcached

Memcached is a high-performance, distributed memory object caching system, generic in nature, but originally intended for  use in speeding up dynamic web application by alleviating database load.

Memcached server is an in memory cache that stores anything from binary to text to primitives associated with a key as key-Value pair.  You saves a lot of load of your backend systems, leading to high scalability. since the data is stored in memory, it is generally faster than making an expensive backend call too.

Limitation:
The key in memcached should be less than 255 chars and each value shouldn't exceed 1MB

What it does?
Memcached allows you to take memory from parts of your system where you have more than you need and make it accessible to areas where you have less than you need. With memcached, you can see that all of the servers are looking into the same virtual pool of memory. This means that a given item is always stored and always retrieved from the same location in you entire web cluster.
Before : each node is completely independent(64M+ 64M = 64M)
after using: each node can make use of memory from other nodes(64M+64M=128M)

How to use memcached?
step 1: install

. Install Libevent( Memcached uses the Libevent library for network IO.)
.  Install Memcached

Step 2: Run memcached:
memcached -d -m 512 127.0.0.1 -p 1121
you can test it by : telnet localhost 11211

Step 3: Client
Generally you can use caching wherever there is bottleneck.
MemcachedClient client = new MemcachedClient(new InetSocketAddress("127.0.0.1",11211);
c.set("someKey",3600,someObject);
Object myObject = client.get("someKey");
client.delete("someKey");