Slack did not publicly disclose the root cause of its Wednesday outage beyond status page updates. However, Spencer Kimball, co-founder and CEO of Cockroach Labs, has offered analysis of what may have contributed to the incident.

The messaging platform experienced widespread service degradation beginning at 10:47 a.m. Eastern time, with engineers requiring approximately nine hours to restore functionality. The company serves users across more than 150 countries, including 77 of the Fortune 100 companies. Slack's user base encompasses organizations such as Airbnb, Target, Uber, and the U.S. Department of Veterans Affairs.

During the incident, Slack's status updates referenced problems with corrupted database shards. At 4:04 p.m. EST, the company stated: "Remediation work involves repairing affected database shards, which are causing feature degradation issues. This has become a diligent process to ensure we're prioritizing the database replicas with the most impact." By 7:42 p.m. EST, Slack reported restoring full functionality to messaging, workflows, threads, and API-related features. Recovery efforts continued into Thursday, with the company noting that events created during the downtime remained queued and paused.

The Problem With MySQL

Slack has relied on MySQL as its storage engine since the platform's 2013 launch. Beginning in 2017, the company initiated a migration to Vitess, an open-source database scaling system built on MySQL. According to a December 2020 post on Slack's engineering blog, the company faced recurring scaling and performance challenges as it expanded. The authors—Rafael Chacón, Arka Ganguli, Guido Iaquinti, and Maggie Zhou—noted that "our application performance teams were regularly running into scaling and performance problems and having to design workarounds for the limitations of the workspace sharded architecture."

The decision to adopt Vitess rather than fundamentally rearchitect reflected a practical constraint: Slack's deep integration with MySQL. The engineering team explained: "At the time there were thousands of distinct queries in the application, some of which used MySQL-specific constructs. And at the same time we had years of built-up operational practices for deployment, data durability, backups, data warehouse ETL, compliance, and more, all of which were written for MySQL." Consequently, the company ruled out alternatives such as NoSQL systems like DynamoDB or Cassandra, as well as NewSQL options including Spanner or CockroachDB.

Kimball suggested that Slack's continued reliance on a sharding model may have created conditions for Wednesday's outage. He explained the mechanics: "basically what you do is you have a lot of customers, a lot of data, way too much to put into one single, monolithic database. So you create lots and lots of databases, and you call them shards. And so you say, 'OK, well, customers one through 100 are on shard one, and 100 through 200 are in shard two,' and so forth, right? Problem is, you're kind of in the position of managing 100 databases."

This distributed approach introduces tradeoffs. While losing a single shard affects only a subset of customers, managing hundreds of independent database instances creates operational complexity. Each shard exhibits unique characteristics based on customer usage patterns, compounding management challenges.

Testing What Went Wrong

Kimball, whose company offers CockroachDB—a distributed SQL database designed for horizontal scaling with built-in redundancy—acknowledged his commercial interest in promoting alternative architectures. Nevertheless, he identified structural vulnerabilities in Slack's legacy system. He noted: "They have become essentially a database company as well as a corporate messaging company, because of the thing they've built that accommodates this idea of shards and the resilience on each one of those shards. When you have one of these things, every piece of code you write, every new feature you write, it has to also deal with the underlying, exposed reality of this complex architecture that you've cobbled together, that, by the way, doesn't work together."

For organizations operating sharded MySQL systems, Kimball recommended prioritizing resilience testing. He stated: "Whatever just happened to them, this should be part of their standard testing process." Companies should focus on reducing Recovery Time Objective (RTO)—the metric measuring how quickly services are restored. Slack required approximately nine hours to restore most functionality on Wednesday.

Kimball contextualized Slack's experience within a broader pattern. He observed: "It's not like Slack is anywhere unique. Everywhere saw these outages that have been happening this year. It's insane. From the [Federal Aviation Administration], to Barclays and Capital One, everyone has outages." The key distinction lies in preparation: "But the question is, OK, whatever just happened, let's routinely test that. And when we have our runbooks and we apply them, what can we optimize our RTO to?"

Testing database infrastructure quarterly or biannually, combined with team visibility into resolution timelines, enables organizations to identify performance regressions and quantify downtime costs. Deploying backup databases on different cloud providers than primary systems represents another recommended practice.

The Cost of Resilience

Resilience standards have evolved significantly. When Cockroach Labs was founded a decade ago, surviving individual node failures represented the baseline. Requirements have since escalated to surviving data center failures, entire regional outages, and even complete cloud provider unavailability.

Many organizations neglect regular resilience testing due to cost considerations. Kimball explained: "It's expensive to do these things." Beyond direct expenses, testing and rearchitecting legacy systems demands substantial staff resources that compete with feature development priorities.

Organizations face difficult tradeoffs. Some accept downtime risk in exchange for avoiding expensive modernization efforts. However, this calculus differs by industry. Kimball noted: "For financial services, that's not an option anymore, especially with regulator scrutiny. For Slack, you know, maybe they're going to stay on their thing because it's just too hard to move. Eventually, you have to modernize things, and they'll just find the right time."