System Design 101 - Scalability

System Design 101 - Scalability

Understanding Scalability - How to Build Systems That Can Handle High Traffic


In our previous post, we explored the key system design concepts that every software engineer should know, from database design and caching strategies to network protocols and load-balancing techniques. However, designing a system that can handle high traffic and scale efficiently is not just about implementing these concepts - it's about understanding how they work together to build a scalable architecture.

Scalability is the ability of a system to handle the increased load, whether it's due to a growing user base, a sudden spike in traffic, or other factors. Building a scalable system is critical for businesses that want to grow, as it enables them to handle more traffic, serve more users, and process more data without compromising performance or reliability.

In this post, we'll explore the key principles and techniques that can help you design and build scalable systems, from horizontal scaling and vertical scaling to partitioning and sharding. We'll also look at real-world examples of outages and incidents that occurred due to scalability issues, and how they could have been prevented.

What is Scalability?

Scalability is the ability of a system to handle increasing loads by adding resources such as CPU, memory, storage, and network bandwidth. There are two main types of scalabilities: horizontal scalability and vertical scalability.

Vertical Scaling

Vertical scaling, also known as scaling up, involves increasing the resources available to a single server or node to handle increased traffic. This can include adding more memory, CPU, or storage capacity to a server. Vertical scaling is often simpler and more cost-effective than horizontal scaling, as it requires less infrastructure to manage. However, there are limits to how much a server can be vertically scaled, and it may not be feasible for systems that require high levels of scalability.

Horizontal Scaling

Horizontal scaling, also known as scaling out, involves adding more servers or nodes to a system to handle increased traffic. For example, a web application that uses load balancing to distribute traffic across multiple servers can be horizontally scaled by adding more servers to the cluster.

Horizontal scaling is often a more flexible and scalable solution than vertical scaling, as it allows for easier expansion as traffic increases. However, horizontal scaling also presents unique challenges, including the need for load balancing, data consistency, and communication between nodes.

Partitioning and Sharding

Partitioning and sharding are techniques used to distribute data across multiple servers or databases to improve scalability and performance.

Partitioning is the process of dividing a large database or dataset into smaller, more manageable partitions or shards. Each shard can be stored on a separate server or database, allowing queries to be distributed across multiple nodes and improving overall performance.

Sharding is a specific type of partitioning that involves distributing data based on a specific key or attribute. For example, a social networking site may shard user data based on their location or interests, allowing queries to be directed to the appropriate shard and improving response times.

Load Balancing

Load balancing is the process of distributing traffic across multiple servers or nodes to improve performance, reliability, and scalability. Load balancing is critical for horizontal scaling, as it involves distributing traffic across multiple servers or nodes to improve performance and prevent any one server from becoming overloaded.

Load balancing can be achieved through hardware or software solutions, and it's important to carefully manage load balancing to ensure that traffic is properly distributed and that nodes are evenly utilized.

Data Consistency

Ensuring data consistency across multiple nodes can be challenging when horizontally scaling a system. This can include ensuring that data is updated across all nodes, handling conflicts when multiple nodes attempt to update the same data simultaneously and maintaining data integrity across the entire system.

Communication between Nodes

Horizontal scaling also requires effective communication between nodes to ensure that data is properly distributed and that nodes are aware of each other's status. This can include implementing effective messaging systems, handling network latency and congestion, and ensuring that nodes can handle failures and recover quickly.

Horizontal Scaling vs Vertical Scaling

While horizontal scaling offers greater flexibility and scalability than vertical scaling, it also presents unique challenges that can make it more difficult to implement. These challenges include:

  • Increased complexity: Adding more servers or nodes to a system increases its complexity, making it more difficult to manage and maintain.

  • Communication overhead: Communication between nodes can create additional overhead and latency, making it more difficult to ensure that data is consistent and that nodes are aware of each other's status.

  • Load balancing: Proper load balancing is critical for horizontal scaling, but it can also be complex and difficult to manage effectively.

Real-world Examples

Scalability issues can lead to outages and incidents that can impact businesses and users. For example, in 2017, the cloud-based storage provider Amazon S3 experienced an outage that affected many popular websites and services, including Airbnb, Slack, and Trello. The outage was caused by a simple typo in a command that was entered during maintenance, which caused a large number of servers to be taken offline.

Another example is the incident that occurred with the video conferencing app Zoom in March 2020, when the usage of the app surged due to the COVID-19 pandemic. The app's servers were overwhelmed by the sudden increase in traffic, causing many users to experience connection issues or be unable to join meetings. Zoom quickly responded by increasing its server capacity and implementing new features to improve scalability, such as virtual waiting rooms and password-protected meetings.

These examples demonstrate the importance of designing and building scalable systems, as even a small mistake or unexpected surge in traffic can cause significant disruptions. However, with the right techniques and principles in place, it's possible to build systems that can handle high traffic and provide reliable, high-performance services to users.


Scalability is a critical factor to consider when designing and building software systems. Whether you're building a web application, a database, or a distributed system, understanding the principles and techniques of scalability can help you build systems that can handle increased traffic and scale efficiently.

By implementing techniques such as horizontal scaling, vertical scaling, partitioning, caching, and load balancing, you can build systems that can handle high traffic and provide reliable, high-performance services to users. While horizontal scaling presents unique challenges, it also offers greater flexibility and scalability than vertical scaling.

If you're a software engineer or developer, taking the time to understand scalability and how it can be applied to your systems can help you build more robust, reliable, and scalable applications.