Accelerating developers at MongoDB

Since 2021, MongoDB has relied on the Antithesis platform to test critical components like their core server database, WiredTiger storage engine, and sharded clusters.

First purchased as a targeted tool, Antithesis quickly became a foundational component in MongoDB’s testing stack. MongoDB now puts eight different network topologies under test within Antithesis, ranging from multi-sharded clusters to replica sets, and including complex operations like version upgrades and downgrades within clusters.

This expansion was driven not by mandate, but by outcomes. Their results consistently surfaced valuable bugs, enabled faster resolution, and improved release confidence, prompting MongoDB to increase their annual investment in Antithesis testing by over 3x to date.

Over the last 4 years, this strategic partnership has enabled MongoDB to:

• Find and resolve 100+ critical bugs, and use the lessons learned to improve internal tooling.
• Reduce mean time-to-resolution (MTTR) by 47%.
• Involve fewer engineers per bug, reducing team load by 54%.
• Boost confidence across leadership, while empowering junior engineers.

The result?

An estimated ROI of over 10x, driven by faster resolution times, reduced risk, and freeing engineers to do more feature work and less debugging.

This level of impact is made possible by how MongoDB integrates Antithesis into its core testing workflows, to simulate the unpredictable and uncover the “unknown unknowns.” Let’s dig into it.

A deep testing culture

MongoDB is an open-source document database built on a horizontal scale-out architecture that uses a flexible schema. Initially founded in 2007 as a high-performance NoSQL database, MongoDB has evolved into a modern data platform used by over 46,000 organizations, from Fortune 500 companies to global government agencies.

Today, MongoDB powers cutting-edge solutions across AI, edge computing, and serverless development, backed by a thriving developer community and a well-earned reputation for reliability.

They’ve built one of the most advanced homegrown testing infrastructures in the industry – a world-class system maintained by a 20+ person team, running tens of thousands of tests in parallel across every build. They’ve been working on this since day 1, investing dozens of engineer/years in its development. Safe to say, testing is deeply embedded in the company’s engineering culture: it’s how they ship fast, innovate continuously, and still meet the demands of mission-critical production environments. As systems grow more complex, however, so do the bugs, and even with thousands of tests running daily, some issues are simply bound to slip through.

In fact, long before deterministic simulation testing (DST) gained traction in the industry, MongoDB recognized a critical truth: no matter how comprehensive your test suite, some bugs only emerge under the kinds of chaotic, unpredictable conditions found in the real world. Rather than wait to be surprised in production, they became the first major database company to adopt this next-generation approach to reliability.

Finding what other tests don’t

Antithesis has had a concrete influence on our release schedule for our annual release. This is the kind of bug we’d often take a month+ to solve with all hands on deck, getting there in a week improves our release confidence and saves me 3+ weeks of some of my most expensive engineers. I’m impressed, really well done.

– SVP of Engineering, MongoDB

Thanks to Antithesis’s continuous testing and its guided fault injection, MongoDB is able to find novel and difficult bugs quickly, enabling them to be triaged and fixed right away.

Since their testing cadence began, MongoDB has surfaced an average of one bug per 2,500 hours of testing on Antithesis’s platform – a reflection of a core principle we’ve seen time and again: the longer you test, the deeper the bugs you uncover.

Crucially, 75% of the bugs that Antithesis surfaced evaded even MongoDB’s extensive internal testing.

The majority of these bugs were categorized as a “Major” or “Critical” priority, with nearly 40% severe enough to block a release, highlighting the danger posed by “unknown unknowns” that are extremely unlikely to be found through other, more manual methods.

When a bug is found, the Antithesis’ detailed reports and robust artifacting make even subtle issues actionable. Bugs that might have taken days of log-diving to root cause, or weeks to repro, can now be resolved in a fraction of the time: the MTTR on Antithesis bug tickets is under 3 days. Given the complexity and potential severity of the issues, the ability to catch and fix issues quickly before they impact staging or production is no small feat, and has consequently become a core driver of MongoDB’s engineering velocity.

Keeping releases on track

While roughly 95% of MongoDB bugs found using Antithesis’s platform can be identified and fixed with the information in the Antithesis triage report, some especially difficult and rare bugs need additional details.

For example: when Antithesis found a critical data corruption bug as an annual release was rapidly approaching, MongoDB engineers needed insight and answers fast. Enter the Antithesis bug report.

The bug report uses Antithesis’s deterministic platform to analyze a multiverse of program states, discovering how likely the bug is to arise from various points in the history leading up to the moment the bug is detected. The unique ability of Antithesis to perform this analysis means engineers can not only see the moment a failure was reported, but also work backwards to find when the problem truly occurred.

This critical data corruption bug was detected 370 seconds into a particular scenario. However, the figure below shows that its likelihood of occurring rose sharply nearly ten seconds before it manifested, leading engineers to investigate the program state around that time – without wasting time investigating around the time of manifestation, as conventional observability tooling would have required.

Without Antithesis’s intelligent exploration capabilities, this bug might not have been found until after the release, perhaps resulting in data corruption for a MongoDB customer. But thanks to Antithesis’ analysis, detailed artifacts, and perfect reproduction, MongoDB was able to quickly qualify and triage this issue, keeping their release schedule on track.

A strategic partnership

Collaborations like this one have strengthened the partnership between MongoDB and Antithesis, resulting in many benefits to both companies.

As an instrumental design partner, MongoDB has helped to mature the Antithesis offering in countless ways over the past several years. In addition to having deep expertise in distributed computing, consensus protocols, replication, and developer productivity, MongoDB engineers are obsessive about correctness. This combination of domain knowledge on the one hand and dedication to quality on the other has enabled the dozens of MongoDB engineers that work with Antithesis to provide valuable feedback and recommendations on Antithesis features and functionality.

“Antithesis was an incredible partner to work with. They continue to improve how their testing explores paths to maximize coverage. But the other (arguably larger) half of what makes Antithesis great is the company’s commitment to their customers’ success.”

– Principal Staff Engineer, MongoDB

Antithesis, in turn, has become a key part of MongoDB’s release qualification process. It’s now fully integrated into the company’s testing workflows and continues to expand across more areas of the product. MongoDB’s journey with Antithesis demonstrates the power of intelligent, autonomous testing to accelerate development, reduce risk, and raise the bar for software quality, redefining what “release-ready” means, at scale.

Want to accelerate testing, reduce risk, and ship with confidence like MongoDB? Try Antithesis yourself!