Redis

Redis is a in-memory data structure store, used as a database, cache and message broker.

• It uses disk only for persistence.
• It is written in C.
• All redis operations are atomic

Persistence

Redis provides two persistence modes RDB and AOF, please refer to this for detail.

Data Types

Redis supports 5 data types :

• String, a string value can be at max 512 megabytes in length
• Hash, is map between the string field and string value

  HMSET mykey name "student1" age 20 
  OK 
  redis 127.0.0.1:6379> HGETALL mykey  
  1) "name" 
  2) "student1" 
  3) "age" 
  4) "20" 

• List, is simply list of strings

  redis 127.0.0.1:6379> LPUSH listName value1 
  (integer) 1 
  redis 127.0.0.1:6379> LPUSH listName value2 
  (integer) 2 
  redis 127.0.0.1:6379> LPUSH listName value3 
  (integer) 3 

In the above example, three values are inserted in Redis list named ‘listName’ by command ‘LPUSH’.

• Set, is unordered collection of unique strings, SADD setName keyValue is used to add element to set

• Sorted set, the difference with Set is, every member in it is associated with a score, that is used in order to take the sorted set ordered, from the smallest to greatest

  redis 127.0.0.1:6379> ZADD setName 1 value1 
  (integer) 1 
  redis 127.0.0.1:6379> ZADD setName 2 value2 
  (integer) 1 
  redis 127.0.0.1:6379> ZADD setName 3 value3 
  (integer) 1 
  

  three values are inserted with its score in sorted set name

Commands

Some redis commands as follows:

• SET, set key and value
• GET, get value of the key
• DEL, deletes the key if it exists
• EXPIRE, sets the expiry of the key after the specified time
• TYPE, returns the data type of the value stored in the key
• AUTH password, use password to enter redis

Publish/Subscribe

Pub/Sub implements the messaging system where the senders(called publisher) send the message, while the recivers(subscriber) receive it. The link by which the message is transferred is called channel.

In redis, a client can subscribe any number of channels.

SUBSCRIBE redisChat - one client subscribes a channel named ‘redisChat’.

PUBLISH redisChat "Hello World" - publish message to channed named ‘redisChat’.

Transaction

A group of commands can be executed in a single step.

• All commands in a transaction are sequentially executed as a single isolated operation. It is not possible that a request issued by another client is served in the middle of the execution of a redis transaction.
• It is atomic.

  redis 127.0.0.1:6379> MULTI 
  OK 
  ......list of command
  
  redis 127.0.0.1:6379> EXEC  

Transaction begins with MULTI, ends with EXEC

Backup

Redis backup is done by command SAVE, dump.rdp file will be created in your redis directory.

  127.0.0.1:6379> SAVE  
  OK 

To restore redis data, make sure redis backup file (dump.rdp) is in your redis install directory, then start the server.

Command CONFIG get dir will show your redis directory.

Pipelining

Pipelining means client can send multiple requests to the server without waiting for the replies at all, and finally read the replies in a single step.

Partitioning

It means split data into multiple redis instances.

There are two types:

• Range partitioning, e.g. user 1 -10 will go into instance 1, user 11-20 go into instance 2.
• Hash partitioning, a hash function is used to convert the key into a number and then the data stored in different instances.

Clients

Redis has some clients for Java, such as Jedis, JRedis. Take Jedis for example,

  //Connecting to Redis server on localhost 
  Jedis jedis = new Jedis("localhost");  
  jedis.set("mykey", "Hello World"); 
  System.out.println(jedis.get("mykey")); 

JedisPool pool = new JedisPool(new JedisPoolConfig(), "localhost"), JedisPool is a threadsafe pool.

  Jedis jedis = null;
  try {
      jedis = pool.getResource();
      .....
   } finally {
      // You have to close jedis object. If you don't close then
      // it doesn't release back to pool and you can't get a new
      // resource from pool.
      if (jedis != null) {
          jedis.close();
      }
   }
   /// ... when closing your application:
   pool.close();

Spring Data Redis

It aslo called SDR which is a part of spring data, provides easy configuration and access to redis from spring application. It offers both low level and high level abstractions for interacting with the store.