gdb, although an incredibly powerful tool for debugging single programs, doesn’t work so well for distributed systems.

The crucial difference between a single program and a distributed system is that distributed computation revolves around network messages. Distributed systems spend much of their time doing nothing more than waiting for network messages. When they receive a message, they perform computation, perhaps send out a few network messages of their own, and then return to their default state of waiting for more network messages.

Because there’s a network separating the nodes of the distributed system, you can’t (easily) pause all processes and attach gdb. And, in the words of Armon Dadgar, “even if you could, the network is part of your system. Definitely not going to be able to gdb attach to that.”

Suppose that you decide to attach gdb to a single process in the distributed system. Even then, you’ll probably end up frustrated. You’re going to spend most of your time waiting on a select or receive breakpoint. And when your breakpoint is triggered, you’ll find that most of the messages won’t be relevant for triggering your bug. You need to wait for a specific message, or even a specific sequence of messages, before you’ll be able to trace through the code path that leads to your bug.

Crucially, gdb doesn’t give you the ability to control network messages, yet network message are what drive the distributed system’s execution. In other words, gdb operates at a level of abstraction that is lower than what you want.

Distributed systems need a different kind of debugger. What we need is a debugger that will allow us to step through the distributed system’s execution at the level of network messages. That is, you should be able to generate messages, control the order in which they arrive, and observe how the distributed system reacts.

Shameless self-promotion: STS supports an “Interactive Mode” that takes over control of the (software) network separating the nodes of a distributed system. This allows you to interactively reorder or drop messages, inject failures, or check invariants. We need something like this for testing and debugging general distributed systems.