Slack’s Incident on 2-22-22

By Laura Nolan, with contributions from Glen D. Sanford, Jamie Scheinblum, and Chris Sullivan. Assessing conditions Slack experienced a major incident on February 22 this year, during which time many users were unable to connect to Slack, including the author — which certainly made my role as Incident Commander more challenging! This incident was a…

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Laura Nolan
•15 min read•advanced•
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Overview

The article discusses a significant incident that occurred at Slack on February 22, 2022, which resulted in many users being unable to connect to the platform. It details the complex systems failure that led to this incident, the contributing factors, and the steps taken to mitigate the issues.

What You'll Learn

1

How to analyze complex system failures in distributed applications

2

Why throttling requests can mitigate overload during incidents

3

When to implement caching strategies to improve performance

Prerequisites & Requirements

  • Understanding of distributed systems and caching mechanisms
  • Experience with incident response in software engineering(optional)

Key Questions Answered

What caused the Slack incident on February 22, 2022?
The incident was triggered by complex interactions between the application, Vitess datastores, caching systems, and service discovery mechanisms during a maintenance rollout of the Consul agent fleet, which led to a cascading failure scenario.
How did Slack mitigate the overload during the incident?
Slack mitigated the overload by throttling client boot requests, which reduced the load on the database and allowed users with booted clients to experience more normal service. This approach was necessary to manage the high query load on the database.
What role did caching play in the incident?
Caching was critical as the client boot process relied on cached data. When cache misses occurred, it led to inefficient scatter queries that overwhelmed the database, causing timeouts and further exacerbating the incident.

Technologies & Tools

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Database
Vitess
Used for horizontal scaling of MySQL databases at Slack.
Caching
Memcached
Serves as the caching tier for low-latency access to frequently-used data.
Service Discovery
Consul
Used for managing service discovery and configuration in the caching architecture.

Key Actionable Insights

1
Implementing throttling mechanisms during peak loads can help maintain service availability.
By controlling the rate of incoming requests, Slack was able to stabilize its services during the incident, allowing users with active sessions to continue using the platform.
2
Regularly review and optimize caching strategies to ensure high availability.
The incident highlighted the importance of having a warm cache to prevent overload scenarios, emphasizing that caching strategies should be resilient to changes in system architecture.
3
Conduct thorough testing of system changes in a controlled environment before deployment.
The cascading failure was partly due to the maintenance rollout of the Consul agent. Testing changes can help identify potential issues before they affect users.

Common Pitfalls

1
Failing to account for the impact of system changes on existing infrastructure can lead to cascading failures.
In this incident, the maintenance on the Consul agent fleet triggered a series of failures due to the interaction with the caching layer, highlighting the need for careful planning and testing.