It's not black and white, but primarily: - Libraries are suitable for providing building blocks and common functionality, that you'll build your system on top

It's not black and white, but primarily: - Libraries are suitable for providing building blocks and common functionality, that you'll build your system on top of, that systems will depend more heavily and execute more frequently. - Services are a strong choice when there's a need to abstract high level operations and architectural complexity. Your system can then focus on making high-level calls without the burden of operating and managing all components and dependencies involved.