If you work in tech, you probably often find yourself in Slack. You may even be in an ongoing Slack conversation right now.

Slack is one of the most popular, successful, and interesting companies to emerge in the past few decades. Created with the simple goal of improving team communication, it has grown from a simple messaging app to a globally adopted organizational communication platform. The company went public in June 2019 through a direct listing. In December 2020, Salesforce announced its intention to acquire Slack for $27.7 billion, a deal that was finalized in July 2021.

All that success is well and good. But what is the history of Slack, particularly its technology stack? How did this tech stack serve as a foundation for success and continuity as Slack went through multiple funding rounds, going public, and an acquisition? What can software engineers and architects learn from a software development company perspective?

Let’s explore the technical evolution of Slack, sharing how Slack’s tech stack has evolved over time while detailing key decisions made by founders and technical leaders.

Slack’s Inception & History: Founding, Initial Vision, and Technical Foundation

Slack was founded in 2009 by Stewart Butterfield, Eric Costello, Cal Henderson, and Serguei Mourachov. The group built Slack as a chat channel for Tiny Speck, the company behind the online game Glitch. When Glitch didn’t gain traction and was shut down in 2012, the team recognized the potential of their internal communication platform and pivoted to develop Slack, which launched publicly in 2013.

The founding team’s technical backgrounds significantly influenced Slack’s early architecture. Cal Henderson, Slack’s CTO, spent years as the lead engineer at Flickr, where he built web apps similar to those he would go on to build and manage for Slack. His understanding of PHP and cloud-based infrastructure guided early technical decisions. Similarly, Serguei Mourachov’s expertise in back-end systems ensured the platform was robust and reliable even as the user base grew.

Their collective experience in building consumer-scale internet products allowed them to create an intuitive and highly adaptable platform for enterprise environments. This technical foundation laid the groundwork for Slack’s rapid growth and eventual dominance in workplace communication.

Evolution of Slack’s Tech Stack

Slack was first built on the LAMP stack—a combination of Linux, Apache HTTP Server, MySQL, and PHP. This was a popular choice for web applications because it allowed developers to build and deploy quickly. It met Slack’s needs at the time, offering good usability and integration with other apps. The team also used Smarty, a template engine for PHP, which made front-end work relatively easy. Amazon EC2 provided hosting as it was scalable and affordable.

Slack grew exponentially with new users joining every day. The foundational architecture began to buckle. The LAMP stack made it difficult to scale. It was hard to manage the system, especially since the team was working to deploy new features constantly. Additionally, the growing volume of data traffic stressed the MySQL database, impacting response times and overall platform stability. A key figure that highlights this challenge was the rapid increase in daily active users, pushing the limits of Slack’s early infrastructure.

Transition to Hacklang: A Better PHP

To address these challenges, Slack transitioned its back end technology stack from PHP to Hack, a programming language developed by Facebook. There were many reasons for the migration. Hack introduced static typing, which made it easier to detect errors and maintain code. It was also somewhat compatible with PHP. This made the transition somewhat straightforward.

Hack ran on the HipHop Virtual Machine (HHVM), which seemed a better fit.It also helped reduce costs by managing resources more efficiently. By using Hack, Slack’s stack would not only perform better, but also not miss out on speed or reliability. Moreover, static typing improved developer productivity. Moving to Hack made Slack more reliable, especially as they began to sell into the enterprise market.

Backend and Infrastructure Scaling

As Slack grew and sold more to enterprise clients, its monolithic architecture no longer worked. The development process was not as agile as the organization, either.

Because of this, the team modularized the architecture, disassembling components into loosely coupled modules. This transition to a more modular system improved scalability and maintainability. Different teams could now work on separate components.

To support this change, Slack took advantage of Amazon Web Services (AWS) and other software. Some of the key integrations included:

• Amazon S3 for scalable storage
• AWS Elastic Load Balancing (ELB)
• AWS Identity and Access Management (IAM)

By integrating with these new tools, Slack could scale, no matter how traffic spiked. This approach was also more flexible and cost-effective.

Bringing Slack’s Mobile Apps into the Future

As more users and companies flocked to Slack, ensuring that its mobile apps could keep up became increasingly important. While Slack’s mobile apps were functional, the team knew they needed a major overhaul. They aimed to boost performance, simplify maintenance, and ensure consistency across iOS and Android.

iOS and Android Improvements

Slack’s original iOS app was built using Objective-C, which became increasingly difficult to maintain as the codebase grew. To modernize the application, Slack completely rewrote its iOS app in Swift, leveraging the language’s modern syntax, safety features, and improved performance. This transition enhanced app responsiveness and simplified maintenance, allowing developers to implement new features more efficiently.

Similarly, the Android app underwent a substantial re-architecture. Slack adopted modern design patterns like MVVM (Model-View-ViewModel) and utilized Jetpack components to improve modularity and maintainability. This shift improved the separation of concerns, enabling teams to work autonomously and accelerating development cycles. The modernization also enhanced app stability and performance, delivering a consistent user experience.

Managing and Overcoming Technical Debt

As companies evolve, they inevitably accumulate technical debt—Slack was no exception. After years of rapid iteration and growth, Slack had to confront and address its technical debt in its mobile apps.

Rewriting the iOS app and re-architecting the Android app required careful planning to avoid disrupting usability. Slack used a modular strategy, breaking the system into smaller, more manageable components. This approach reduced technical debt and allowed different teams to collaborate. Engineers could work concurrently on various components.

Maintaining a consistent user experience across iOS and Android wasn’t easy. The inherent differences in design paradigms and development frameworks made developing and deploying a challenge. As a result, Slack adopted a design system approach, ensuring a unified look and feel across platforms while respecting each platform’s attributes.

Through strategic rewrites and architectural improvements, Slack successfully modernized its mobile products, enhancing performance, maintainability, and user experience, setting the stage for continued growth and innovation.

Important Decisions and Lessons Learned

As the organization grew from a small startup to a global company, its tech needed to evolve. Slack made several strategic architectural decisions to support its explosive growth and lay the foundation for where Slack is today.

Tech teams worldwide can learn from Slack about how it serves clients and how it improves collaboration and deployment. Consider these takeaways, no matter where your company is in its growth journey:

• Stability and innovation go hand in hand. Slack embraced new technologies to enhance user experience and back-end efficiency while prioritizing stability to maintain user trust. This required strategic versioning, feature rollouts, and backward compatibility.
• Automate whenever possible. Slack was successful because of their focus on automation as they transitioned to new tools that would better serve them. When migrating from Jenkins to GitHub Actions (GHA), Slack built tools to convert complex Jenkins pipelines into GHA workflows, minimizing disruption.
• Modularization for large-scale platforms. Slack’s growth underscored the power of modularization. Decoupling components and adopting modular design patterns reduced technical debt, improved maintainability, and enabled rapid iteration without sacrificing stability.

A Blueprint for Scaling Success

What began as an internal chat tool is now a household name. Slack’s growth came from strategic tech choices—moving from PHP to Hack, modularizing its architecture, and modernizing its mobile apps. These decisions set it up for long-term success.

Slack’s journey shows the balance between innovation and stability, the power of automation, and the value of modularization. For teams navigating growth, it offers a blueprint for building a scalable, resilient, and future-ready tech stack.