List of things that let you write a single program which performs a distributed task

These are problem areas where you'd normally use multiple separate programs/configurations/whatevers. But with these things, you can use a single program instead. Problem areas where you'd normally write a single program, but with so-and-so tools, you can instead write multiple programs (for locality and performance, usually) are also interesting (transforming a shell script using ssh into multiple communicating programs; BPF; XDP; the storage equivalent of XDP; uploading code into a service in general; etc.), but not eligible for this list since they're the exact opposite.