Joe Leo
Founder & CEO, Def Method
I founded Def Method in 2014 with a simple premise: the most valuable engineering work happens when the stakes are high and getting it wrong isn't an option.
I've helped engineering teams at companies like Citi, Walmart, and Paxos modernize systems that couldn't afford to break. Revenue-critical platforms. Regulated environments. Operationally sensitive infrastructure.
I'm also the co-author of The Well-Grounded Rubyist (now in its 4th edition) and co-host of The Ruby AI Podcast, where we explore the intersection of Ruby and artificial intelligence.
Speaking & Podcast
I regularly speak at conferences about Ruby, Rails, and engineering leadership. I'm also the co-host of The Ruby AI Podcast.
The Ruby AI Podcast
Biweekly conversations exploring Ruby in the age of AI.](/content/podcast/index.html)
RailsConf 2025
Silent Killers: Lessons from the Brink](https://www.youtube.com/watch?v=OIOU7p4HUQM)
Latest Insights
[May 26, 2026 **The Guardrail Is the Architecture**
A movement is forming around guardrails for agentic coding. The questions being asked are the right ones. But the answers, so far, are mostly tooling dressed up as architecture.](/content/essential-complexity/the-guardrail-is-the-architecture/index.html) [May 11, 2026 **Your Platform Has a Monolith, Too**
GitHub published two incident reports in a single week. Both failures share the same structure: a system designed for a different scale, operating under conditions it wasn't built for.](/content/essential-complexity/your-platform-has-a-monolith-too/index.html) [April 27, 2026 **Already Inside**
The Vercel breach had nothing to do with AI-generated bugs. The attacker moved through doors that were already open. Containment has to extend beyond code boundaries.](/content/essential-complexity/already-inside/index.html)
The Well-Grounded Rubyist
4th Edition — Co-authored with David A. Black
The Well-Grounded Rubyist is a comprehensive guide to Ruby that has helped thousands of developers master the language. Now in its 4th edition, it covers Ruby 3.x and modern Ruby idioms while maintaining the deep, thorough approach that made it a classic.
Whether you're new to Ruby or looking to deepen your understanding, this book provides the foundation you need to write confident, idiomatic Ruby code.
Want to work together?
If you're facing a Rails modernization challenge where getting it wrong isn't an option, let's talk.