# Database

With the growth of the user base, one server is not enough, and we need multiple servers: one for web/mobile traffic, and the other for the database. Separating web/mobile traffic (web tier) and database (data tier) servers allows them to be scaled independently.

<figure><img src="/files/Kg0lPFZaHtqUb1FCKpdo" alt=""><figcaption></figcaption></figure>

### Which databases to use?

You can choose between a traditional relational database and a non-relational database. Let us examine their differences.

Relational databases are also called relational database management systems (RDBMS) or SQL databases. The most popular ones are MySQL, Oracle database, PostgreSQL, etc. Relational databases represent and store data in tables and rows. You can perform join operations using SQL across different database tables.

Non-relational databases are also called NoSQL databases. Popular ones are CouchDB, Neo4j, Cassandra, HBase, Amazon DynamoDB, etc. \[2]. These databases are grouped into four categories: key-value stores, graph stores, column stores, and document stores. Join operations are generally not supported in non-relational databases.

For most developers, relational databases are the best option because they have been around for over 40 years and historically, they have worked well. However, if relational databases are not suitable for your specific use cases, it is critical to explore beyond relational databases. Non-relational databases might be the right choice if:

* Your application requires super-low latency.
* Your data are unstructured, or you do not have any relational data.
* You only need to serialize and deserialize data (JSON, XML, YAML, etc.).&#x20;
* You need to store a massive amount of data.

### Vertical scaling vs horizontal scaling

Vertical scaling, referred to as “scale up”, means the process of adding more power (CPU, RAM, etc.) to your servers. Horizontal scaling, referred to as “scale-out”, allows you to scale by adding more servers into your pool of resources.

When traffic is low, vertical scaling is a great option, and the simplicity of vertical scaling is its main advantage. Unfortunately, it comes with serious limitations.

* Vertical scaling has a hard limit. It is impossible to add unlimited CPU and memory to a single server.
* Vertical scaling does not have failover and redundancy. If one server goes down, the website/app goes down with it completely.

Horizontal scaling is more desirable for large-scale applications due to the limitations of vertical scaling.

In the previous design, users are connected to the web server directly. Users will be unable to access the website if the web server is offline. In another scenario, if many users access the web server simultaneously and it reaches the web server’s load limit, users generally experience slower response or fail to connect to the server. A load balancer is the best technique to address these problems.


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