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Update (October 2024): In response to numerous requests from external developers, we have open-sourced a version of our Enzyme to React Testing Library (RTL) conversion tool. You can now find it on npm, along with detailed instructions on how to integrate and use it in your projects. In the world of frontend development, one thing remains certain: change is the only constant. New frameworks emerge

Cron scripts are responsible for critical Slack functionality. They ensure reminders execute on time, email notifications are sent, and databases are cleaned up, among other things. Over the years, both the number of cron scripts and the amount of data these scripts process have increased. While generally these cron scripts executed as expected, over time the reliability of their execution has occ

We are heavy users of Amazon Compute Compute Cloud (EC2) at Slack — we run approximately 60,000 EC2 instances across 17 AWS regions while operating hundreds of AWS accounts. A multitude of teams own and manage our various instances. The Instance Metadata Service (IMDS) is an on-instance component that can be used to gain an insight to the instance’s current state. Since it first launched over 10 y

Slack handles billions of inbound network requests per day, all of which traverse through our edge network and ingress load balancing tiers. In this blog post, we’ll talk about how a request flows — from a Slack’s user perspective — across the vast ether of the network to reach AWS and then Slack’s internal services. Let’s dive in! How packets flow Our edge network consists of a set of globally-di

Summary In recent years, cellular architectures have become increasingly popular for large online services as a way to increase redundancy and limit the blast radius of site failures. In pursuit of these goals, we have migrated the most critical user-facing services at Slack from a monolithic to a cell-based architecture over the last 1.5 years. In this series of blog posts, we’ll discuss our reas

Did you know that ground stations transmit signals to satellites 22,236 miles above the equator in geostationary orbits, and that those signals are then beamed down to the entire North American subcontinent? Satellite radios today serve hundreds of channels across 9,540,000 square miles. Unless you’re working at a secret military facility, deep underground, you can enjoy satellite radio everywhere

A Look Into Ways We Use Terraform to Build Our Infrastructure At Slack, we use Terraform for managing our Infrastructure, which runs on AWS, DigitalOcean, NS1, and GCP. Even though most of our infrastructure is running on AWS, we have chosen to use Terraform as opposed to using an AWS-native service such as CloudFormation so that we can use a single tool across all of our infrastructure service pr

Building load test infrastructure is tricky and poses many questions. How can we identify performance regressions in newly deployed builds, given the overhead of spinning up test clients? To gather the most representative results, should we load test at our peak hours or when there’s a lull? How do we incentivize engineers to invest time in load testing, especially when environment setup is time c

On September 30th 2021, Slack had an outage that impacted less than 1% of our online user base, and lasted for 24 hours. This outage was the result of our attempt to enable DNSSEC — an extension intended to secure the DNS protocol, required for FedRAMP Moderate — but which ultimately led to a series of unfortunate events. The internet relies very heavily on the Domain Name System (DNS) protocol. D

A deep dive on how we crafted an order of magnitude change in our spend (10x reduction compared to baseline growth) over the last two years with iterative understanding and changes in Slack’s Continuous Integration (CI) infrastructure. Slack is an integral part of where work happens for teams across the world, and our work in the Core Development Engineering department supports engineers throughou

Saurabh Sahni Sr. Staff Software Engineer, Developer Platform More than five years ago, we launched the Slack Platform, giving developers an easy way to build apps in Slack and publish them in our App Directory. Today, millions of users bring their work into Slack, and those apps built by over 885,000 active developers on the platform are key to further improving collaboration in Slack. Over the y

At Slack, we believe that designing an optimal keyboard experience is key to delivering a best-in-class product for all our customers. However, despite our design system components being individually accessible, we heard from keyboard users that we were still missing focus transitions in their end-to-end user experience. Non-sighted users who relied on a screenreader constantly found themselves co

How Slack models and tests with incredibly large groups of users Complex systems are difficult to reason about at scale; we often can’t accurately extrapolate system behavior and performance, so we need to derive that data empirically. We use load testing to do just that: find the limits of our systems and weed out bugs at a large scale in a controlled environment. Slack is a pretty complex system

Maria Sabastian Senior Software Engineer, Mobile Build & Release Artwork courtesy of the Jenkins project. At Slack we manage a sophisticated Jenkins infrastructure to continuously build and test our mobile apps before release. We have hundreds of jobs running in a variety of different environments. One day something very odd happened — our Jenkins UI stopped working although the jobs continued to

Ariane van der Steldt Staff Software Engineer, Site Reliability Slack has a global customer base, with millions of simultaneously connected users at peak times. Most of the communication between users involves sending lots of tiny messages to each other. For much of Slack’s history, we’ve used HAProxy as a load balancer for all incoming traffic. Today, we’ll talk about problems we faced with HAPro

And now we welcome the new year. Full of things that have never been. — Rainer Maria Rilke January 4th 2021 was the first working day of the year for many around the globe, and for most of us at Slack too (except of course for our on-callers and our customer experience team, who never sleep). The day in APAC and the morning in EMEA went by quietly. During the Americas’ morning we got paged by an e

Konstantin Savransky Senior Software Engineer, Frontend In the first installment of the article, we examined why we built a React analytics library. We also looked at how we use the library to share data efficiently, log smarter impressions, and simplify event logging. In this second part of the article, we will focus on how we abstracted the library for use by any team at Slack, along with powerf

Rafael Chacón Staff Software Engineer, Infrastructure From the very beginning of Slack, MySQL was used as the storage engine for all our data. Slack operated MySQL servers in an active-active configuration. This is the story of how we changed our data storage architecture from the active-active clusters over to Vitess — a horizontal scaling system for MySQL. Vitess is the present and future of Dat

Like many applications, the Slack desktop app logs how users interact with it. For example, it may log when a user views a screen or clicks on a button. Product Managers and Data Scientists analyze the logs, hoping to discover actionable insights to drive product refinements. Slack’s desktop application is written in React. We built a React analytics logging library to: 1) increase developer veloc

This story describes the technical details of the problems that caused the Slack downtime on May 12th, 2020. To learn more about the process behind incident response for same outage, read Ryan Katkov’s post, “All Hands on Deck”. On May 12, 2020, Slack had our first significant outage in a long time. We published a summary of the incident shortly after, but this story is an interesting one, and we’

Deploys require a careful balance of speed and reliability. At Slack, we value quick iteration, fast feedback loops, and responsiveness to customer feedback. We also have hundreds of engineers who are trying to be as productive as possible. Keeping to these values while growing as a company means continual refinement of our deployment system. We had to invest in greater visibility and reliability

Most new things in technology turn out to be fads: patterns of talking and doing that come and go without leaving a permanent mark. Microkernels; EPIC architectures like IA-64; object request brokers; and 1990s’-style neural nets are gone, and will not return. Sorry for the deep throwbacks; only time proves which things are fads, so for uncontroversial examples we have to reach pretty far back. Wh

“What is the easiest way to securely connect tens of thousands of computers, hosted at multiple cloud service providers in dozens of locations around the globe?” If you want our answer, it’s Nebula, but I recommend that you read the rest of this short post before clicking that shiny link. At Slack, we asked ourselves this very question a few years ago. We tried a number of approaches to this probl

Two years ago, I wrote a post about Libslack, Slack’s shared C++ client library. That post described how Slack used the Libslack library in its mobile applications to encapsulate shared business logic, and to handle syncing and caching of data. In the intervening time, we decided to move away from using a shared C++ library in our clients, but we haven’t discussed that decision publicly. We were s

This post was co-written with Zack Sultan, Lead Product Designer at Slack In 2016, Slack was two years old and already used by millions of people. Our codebase had grown rapidly, and like many companies that focused on product/market fit, our code was built in a way that favored time-to-market over maintainability, consistency, or reusability. We never encountered a single breaking point in our us

Service Workers at Slack: Our Quest for Faster Boot Times and Offline Support Using a CDN-cached HTML file, a persisted Redux store, and a Service Worker, we reduced Slack boot time by more than 50%. We recently rolled out a new version of Slack on the desktop, and one of its headlining features is a faster boot time. In this post, we’ll take a look back at our quest to get Slack running quickly,

Slack is a large and complex piece of software that’s been added to and changed many times over the last five years. We added features, grew to 10,000,000 DAUs, and made major architectural changes. We made assumptions and tested them with processes that often resembled science. Whenever we launch features or make changes, we test the fault tolerance of that new code. Unfortunately, we seldom get