While at work I was reading through the exq library I stumbled upon an issue that discussed the perils of running a Supervisor under a GenServer. Specifically Ben Wilson calls out the following:
- "There is less guarantee provided by the supervision tree as the process might exit and the supervisor will not have terminated its children."
- "This can lead to problems if the supervisors children are named because the named children might still exist when a restart occurs higher up the tree (above the process calling start_link in its init/1"
- "you lose some advanced OTP features, like code reloading, as process modules are discovered by walking the supervision tree"
To my surprise the codebase I was working in, was using this pattern. Thankfully I was able to hunt down some resources that help to show how you should be modelling your process tree. The gist of it is this:
- Your process tree is composed of either worker nodes or supervisor nodes
- Nodes with children must be supervisors
- Leaf nodes must be workers (in our case GenServers)
Here's a nice graphic that was taken from little-otp that helps to visualize everything from above.
