Gain insights into Redis's capabilities as a database, cache, and message broker. Delve into commands, transactions, and clustering. Explore flexibility, security, and data structure support with Redis.

Redis.tar.gz

Redis is an open-source, in-memory data structure store used as a database, cache, and message broker. Unlike Memcached, it supports multiple data types, giving a lot of flexibility to the user. Redis is easy to configure and provides clustering support to store data in multiple servers.

In this course, you will learn about the various commands that you can use to store different types of data structures in Redis. Some of those commands include utility, insertion, and modification commands.

You will also become familiar with different aspects of Redis, such as transactions, security, partitioning, and clustering. In all, this is your go-to guide to Redis.

Complete Guide to Redis

In the previous section of this course, we discussed many concepts regarding replication and partitioning. Although the concepts seemed simple, it is difficult to handle sharding if the nodes are constantly being added or removed. The Redis cluster is a way to do all of this automatically. 

After over four years of development, Redis Cluster was released with the 3.0.0 release of Redis on April 1, 2015. “According to the founder, it is: 

 >    “…basically a data sharding 
     strategy, with the ability to 
     reshard keys from one node to 
     another while the cluster is 
     running, together with a failover 
     procedure that makes sure the 
     system is able to survive certain 
     kinds of failures.””


The two major advantages of Redis cluster are:
* The ability to automatically split the dataset among multiple nodes.
* The ability to continue operations when a subset of the nodes are experiencing failures or are unable to communicate with the rest of the cluster.

## Redis cluster working
In Redis cluster, each node has two TCP connections open:
1. The normal port which is used for client communication, such as 6379. This connection is available to all clients as well as those cluster nodes that need to use client connection for key migration.
2. The cluster bus port, which is used by other cluster nodes for failure detection, configuration updates, and fail-over authorization. This channel uses less bandwidth by utilizing a binary protocol for data exchange. This port is always 10000 + the normal port. So if the normal port is 6379, then the cluster bus port will be 16379.

### Hash slots
Redis cluster uses **hash slots** to shard the keys. A Redis cluster has 16,383 slots. These slots are distributed among the servers. So, if there are 3 servers in a cluster, then: 
* Server A contains hash slots from 0 to 5,500.
* Server B contains hash slots from 5,501 to 11,000.
* Server C contains hash slots from 11,001 to 16,383.
> It is not necessary for the slots to be equally distributed among the servers.

Whenever a new server is added or deleted, these slots are redistributed. Moving hash slots from a node to another does not require you to stop operations. Thus, adding and removing nodes, or changing the percentage of hash slots held by nodes does not require any downtime.

When a key is inserted in Redis, the hash slot is found using the CRC-16 hash function to convert the key into an integer. Then modulo 16383 of that integer is calculated to get the **hash slot** for this key.
```
HASH_SLOT = CRC16(key) mod 16383
```

### Hash tags
The Redis cluster allows multiple key operations, if the keys are on the same node and have the same hash slot. The user can force multiple keys to be part of the same hash slot by using **hash tags**.

  In **hash tags**, a sub-string within {} is added between the key. The hash is then calculated using this sub-string. Thus, all the keys return the same hash. Let's say we are storing a few keys in Redis, and we want them to end up in the same node. We will insert a random substring(let's say abcd) in between our keys, e.g., 23464{abcd}2344, relat{abcd}ed, pre34{abcd}pared. When these keys are inserted, the hash slot will be calculated using the string, **abcd**. Thus, all the keys will have the same hash slot.



# Redis cluster working
In Redis cluster, each node has two TCP connections open:
1. The normal port which is used for client communication, such as 6379. This connection is available to all clients as well as those cluster nodes that need to use client connection for key migration.
2. The cluster bus port, which is used by other cluster nodes for failure detection, configuration updates, and fail-over authorization. This channel uses less bandwidth by utilizing a binary protocol for data exchange. This port is always 10000 + the normal port. So if the normal port is 6379, then the cluster bus port will be 16379.

## Hash slots
Redis cluster uses **hash slots** to shard the keys. A Redis cluster has 16,383 slots. These slots are distributed among the servers. So, if there are 3 servers in a cluster, then: 
* Server A contains hash slots from 0 to 5,500.
* Server B contains hash slots from 5,501 to 11,000.
* Server C contains hash slots from 11,001 to 16,383.
> It is not necessary for the slots to be equally distributed among the servers.

Whenever a new server is added or deleted, these slots are redistributed. Moving hash slots from a node to another does not require you to stop operations. Thus, adding and removing nodes, or changing the percentage of hash slots held by nodes does not require any downtime.

When a key is inserted in Redis, the hash slot is found using the CRC-16 hash function to convert the key into an integer. Then modulo 16383 of that integer is calculated to get the **hash slot** for this key.
```
HASH_SLOT = CRC16(key) mod 16383
```

## Hash tags
The Redis cluster allows multiple key operations, if the keys are on the same node and have the same hash slot. The user can force multiple keys to be part of the same hash slot by using **hash tags**.

  In **hash tags**, a sub-string within {} is added between the key. The hash is then calculated using this sub-string. Thus, all the keys return the same hash. Let's say we are storing a few keys in Redis, and we want them to end up in the same node. We will insert a random substring(let's say abcd) in between our keys, e.g., 23464{abcd}2344, relat{abcd}ed, pre34{abcd}pared. When these keys are inserted, the hash slot will be calculated using the string, **abcd**. Thus, all the keys will have the same hash slot.



Learn about Redis cluster in this lesson.

Let's Begin

Redis Commands

Redis Advanced Concepts

Redis Cluster

Appendix

Redis Cluster Introduction

Redis cluster working