redis

Synopsis

Redis (“REmote DIctionary Service”) is an open source (BSD licensed), 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 and geospatial indexes with radius queries.

Redis Data Types

## Various Keys:

____________________________
|     "just add abcd"      |  -> String
|__________________________|

____________________________
| 011011010110111101101101 |  -> Bitmap
|__________________________|

____________________________
|  {23334}{6634728}{916}   |  -> BitField
|__________________________|

____________________________
|  {a: "hello", b:"world"} |  -> Hash
|__________________________|

____________________________
|    [A -> B -> C -> D]    |  -> List
|__________________________|

____________________________
|       { A < B < C }      |  -> Set
|__________________________|

____________________________
|    {A:1, B:2, C:3}       |  -> Sorted Set
|__________________________|

____________________________
|     {A:(50.1,0,5)}       |  -> Geospatial
|__________________________|

____________________________
|     01101101 01101101    |  -> Hyperlog
|__________________________|

_______________________________________
| {id1=time1.seq((a:"foo",b:"bar"))}  |  -> Streams
|_____________________________________|

Why is Redis Useful

Redis uses a design where the data is always read and modified from the random access memory (RAM) on a computer. It can also be stored on a disk— in an inappropriate format for random data access— but only to reconstruct the data in memory after the system restarts.

his is why Redis can be considered both a store and a cache. We can use it to decrease data access latency, increase throughput, and ease the load off our relational or NoSQL database and application.

Other benefits of Redis include simple authentication: designed to be only accessed by trusted clients, and can be restricted to certain interfaces.

Redis Use Cases

Redis as a cache instance

____________________________________________________
|                                                   |
|            ____________                           |
|            |   Redis  |                           |
|            |__________|                           |
|          / ↗️          ↖️ \                      |
| < 1-5   / /              \ \                      |
|  secs  / /                \ \                     |
|       ↙️ /                \ ↘️                   |
|  ____________             ____________ Postgres   |
|  |  Web App |  -------➡️ | Database |  MySQL      |
|  |__________| ⬅️-------- |__________|  Microsoft  |
|              30 or 60 secs             SQL Server |
|___________________________________________________|

Let’s say we have a web application that takes too long too retrieve data from a database server. Suppose iit takes from 30 to 60 seconds to perform that operation.

This waiting time is a terrible experience for the user. To fix this problem we can leverage a Redis instance. Now, instead of directly querying the database and waiting 30 to 60 seconds, we can store data in a Redis cache instance and just query that instance running in the system memory (RAM).

The web server will first check if Redis has the data that it requires. If it doesn’t have the data, this will be considered a cache miss. The web server will then query the database and retrieve the data that it needs. After that it will populate the Redis instance with that data— so that the next time it needs to retrieve that data, it will get it from the Redis instance. We call this a cache hit.

This way we will bring down the total wait time to less than 5 seconds or even less than 1 second.

Redis as a multi-model database

Redis can also be used as a fully fledged multi-model database that can be used to store and and keep multiple data formats. Let’s look at one case where we would make a lot of things simpler and easier by implementing a database with it.

We have a complex social media application that uses multiple microservices and different types of databases including Relational, Graph and Document type databases, and also a cache.

As we can see all these data services need to be maintained and deployed which means it takes a lot of work, energy and experience to keep them in a good and running state. Another thing which we will face in the future with this architecture is scaling: when scaled all of the mentioned data services will have different requirements which can be an additional challenge for our team.

A good workaround for these problems is to use cloud platforms which comes with a drawback: we will have to pay for each managed data service separately. On the backend, we also face a problem: the application code gets very complex because we have to talk to multiple data services, and for each we need a separate connector and logic. This causes quite a few difficulties like higher latency due to each connection step between the services. This will add some latency to our application, and because of this, testing our application becomes challenging as well.

To solve all of these issues we can Redis as multi-model database. We will have only one data service so our microservices will need to talk to only a single data store, we will have less latency, less complicated code in our backend, we will be able to store multiple type of databases into one. Redis can also additionally act as a cache to improve the performance of our web application.

Redis Basic Commands

Strings

Strings are the most basic data type in Redis. They are used to store simple data like numbers, strings, and so forth. They are also used to store the values of keys in Redis.

Lists

Lists are simply lists of strings, sorted by insertion order. You can add elements to a Redis list using LPUSH, RPUSH, or using the generic command PUSH. You can also add multiple elements to a list using LPUSHX, RPUSHX, or using the generic command PUSHX.

Sets

Sets are an unordered collection of strings. Redis Sets are similar to Python Sets. You can add elements to a Redis set using SADD. You can also add multiple elements to a set using SADD. You can also remove elements from a set using SREM.

Sorted Sets

Sorted sets are similar to regular sets with the addition of a score, which is used to sort the elements in the set. You can add elements to a Redis sorted set using ZADD. You can also add multiple elements to a sorted set using ZADD. You can also remove elements from a sorted set using ZREM.

Hashes

Hashes are maps between string fields and string values, so they are the perfect data type to represent objects (eg. A User with a number of fields like name, surname, age, and so forth). You can add elements to a Redis hash using HSET. You can also add multiple elements to a hash using HMSET. You can also remove elements from a hash using HDEL.

HyperLogLog

HyperLogLog is a probabilistic data structure used to estimate the cardinality of a set. It is very useful when you need to count the number of unique elements in a very large set. You can add elements to a Redis HyperLogLog using PFADD. You can also count the number of unique elements in a HyperLogLog using PFCOUNT.

Transactions

Transactions are a way to bundle multiple commands together and execute them atomically. If any command fails, the whole transaction is aborted and the connection is reset to its initial state. You can use MULTI to start a transaction, and then use EXEC to execute all the commands issued after MULTI. You can also use DISCARD to discard all the commands issued after MULTI.

Pub/Sub

Publish/Subscribe is a pattern where senders (publishers) send messages to a channel, without knowing who the receivers (subscribers) are. Receivers can subscribe to a channel and receive messages sent to it. You can use PUBLISH to publish a message to a channel, and use SUBSCRIBE to subscribe to a channel.

Scripting

Scripts are Lua programs that are executed by the Redis server. You can use EVAL to evaluate a Lua script server side. You can also use EVALSHA to evaluate a Lua script server side using its SHA1 digest.

Connection

Connection commands allow you to inspect the state of the connection. You can use AUTH to authenticate to the server. You can also use ECHO to echo the given string. You can also use PING to ping the server. You can also use QUIT to close the connection.

Server

Server commands allow you to inspect and modify the Redis server’s configuration.

Cluster

Cluster is a way to group multiple Redis instances together to form a single logical Redis server. Cluster commands allow you to inspect and modify the Redis cluster’s configuration.

Sentinel

Sentinel is a system for monitoring and failover of Redis instances. Sentinel is a distributed system composed of multiple independent processes that monitor multiple Redis instances.

Client

Client is a special category of commands that are not related to Redis per se, but are used to inspect and control the client connection.