Ruby Concurrency: What Actually Happens

Since I wrote about async Ruby and patched Solid Queue to support fibers, people keep asking the same questions. What happens when a fiber blocks? Don’t you still need threads? What about database transactions? What about Ractors? This post answers all of it. From the ground up. The four primitives Ruby gives you four concurrency primitives: processes, threads, fibers, and Ractors. They nest. Every process has an implicit “main Ractor” where your code runs by default, so you never have to think about Ractors unless you explicitly create one. Without Ractors, the hierarchy is simply process – threads – fibers. With Ractors, it becomes: graph TD P[Process] --> R1["Ractor 1 (GVL 1)"] P --> R2["Ractor 2 (GVL 2)"] R1 --> T1[Thread 1] R1 --> T2[Thread 2] R2 --> T3[Thread 3] T1 --> F1[Fiber A] T1 --> F2[Fiber B] T2 --> F3[Fiber C] T3 --> F4[Fiber D] T3 --> F5[Fiber E] style P fill:#4a90a4,color:#fff style R1 fill:#c084fc,color:#fff style R2 fill:#c084fc,color:#fff style T1…