Dumb-init vs Fleet: What are the differences?
Developers describe Dumb-init as "A minimal init system for Linux containers, by Yelp". dumb-init runs as PID 1, acting like a simple init system. It launches a single process and then proxies all received signals to a session rooted at that child process. Since your actual process is no longer PID 1, when it receives signals from dumb-init, the default signal handlers will be applied, and your process will behave as you would expect. If your process dies, dumb-init will also die, taking care to clean up any other processes that might still remain. On the other hand, Fleet is detailed as "Container management and deployment for your cluster". Fleet is a low-level cluster engine that feels like a distributed init system. With fleet, you can treat your CoreOS cluster as if it shared a single init system.
Dumb-init and Fleet can be categorized as "Container" tools.
Dumb-init and Fleet are both open source tools. Dumb-init with 3.83K GitHub stars and 212 forks on GitHub appears to be more popular than Fleet with 2.45K GitHub stars and 309 GitHub forks.