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Designing a system that supports millions of users is challenging, and it is a journey that requires continuous refinement and endless improvement. In this chapter, we build a system that supports a single user and gradually scale it up to serve millions of users. After reading this chapter, you will master a handful of techniques that will help you to crack the system design interview questions.
A journey of a thousand miles begins with a single step, and building a complex system is no different. To start with something simple, everything is running on a single server. Figure 1 shows the illustration of a single server setup where everything is running on one server: web app, database, cache, etc.
To understand this setup, it is helpful to investigate the request flow and traffic source. Let us first look at the request flow (Figure 2).
1. Users access websites through domain names, such as api.mysite.com. Usually, the Domain Name System (DNS) is a paid service provided by 3rd parties and not hosted by our servers.
2. Internet Protocol (IP) address is returned to the browser or mobile app. In the example, IP address 22.214.171.124 is returned.
3. Once the IP address is obtained, Hypertext Transfer Protocol (HTTP)  requests are sent directly to your web server.
4. The web server returns HTML pages or JSON response for rendering.
Next, let us examine the traffic source. The traffic to your web server comes from two sources: web application and mobile application.
GET /users/12 – Retrieve user object for id = 12
"streetAddress":"21 2nd Street",
With the growth of the user base, one server is not enough, and we need multiple servers: one for web/mobile traffic, the other for the database (Figure 3). Separating web/mobile traffic (web tier) and database (data tier) servers allows them to be scaled independently.
You can choose between a traditional relational database and a non-relational database. Let us examine their differences.
Relational databases are also called a relational database management system (RDBMS) or SQL database. 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. . 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:
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.
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 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.
A load balancer evenly distributes incoming traffic among web servers that are defined in a load-balanced set. Figure 4 shows how a load balancer works.
As shown in Figure 4, users connect to the public IP of the load balancer directly. With this setup, web servers are unreachable directly by clients anymore. For better security, private IPs are used for communication between servers. A private IP is an IP address reachable only between servers in the same network; however, it is unreachable over the internet. The load balancer communicates with web servers through private IPs.
In Figure 4, after a load balancer and a second web server are added, we successfully solved no failover issue and improved the availability of the web tier. Details are explained below:
Now the web tier looks good, what about the data tier? The current design has one database, so it does not support failover and redundancy. Database replication is a common technique to address those problems. Let us take a look.
Quoted from Wikipedia: “Database replication can be used in many database management systems, usually with a master/slave relationship between the original (master) and the copies (slaves)” .
A master database generally only supports write operations. A slave database gets copies of the data from the master database and only supports read operations. All the data-modifying commands like insert, delete, or update must be sent to the master database. Most applications require a much higher ratio of reads to writes; thus, the number of slave databases in a system is usually larger than the number of master databases. Figure 5 shows a master database with multiple slave databases.
Advantages of database replication:
In the previous section, we discussed how a load balancer helped to improve system availability. We ask the same question here: what if one of the databases goes offline? The architectural design discussed in Figure 5 can handle this case:
Figure 6 shows the system design after adding the load balancer and database replication.
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