Many of the binaries provided by systemd indeed have a non-public interface between them, but they are all compile-time optional, with the exception of journald

Many of the binaries provided by systemd indeed have a non-public interface between them, but they are all compile-time optional, with the exception of journald. Would you say that the people who claim they are migrating to FreeBSD because of systemd are all misguided because FreeBSD, having all of its kernel and userland plumbing in a single repository and tightly coupled, is a monolithic system by design? Here's a bug on Debian/kFreeBSD about a FreeBSD 10 kernel ABI change breaking ifconfig: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=740509#51 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=740509#51 Would you call postfix monolithic too, given that you cannot replace any of its binaries with one from e.g. qmail because the interfaces are different? Is postfix any better than sendmail in this regard? > They can seem like unnecessary work in the beginning, but it is going to be hell maintaining that mess of code in the long run. Public interfaces aren't without cost - putting public interfaces where they don't belong is one of the best ways to add accidental complexity to your project and make it much harder to maintain. If you find some minor design flaw in a non-public interface, you think about what the ideal interface would do, and then you just change the existing interface because you have control of both client and provider of the interface. If it's a public interface it has to be versioned and you never can remove the implementation of older versions because you have no idea who is using it and what would break, and introducing a new version adds enough extra work that some minor design flaws in the interface will never be fixed because the effort-to-benefit trade-off is different if you have to keep compatibility in mind. See this article from another successful monolithic project for some good reasons to have internal-only interfaces. https://www.kernel.org/doc/Documentation/stable_api_nonsense.txt https://www.kernel.org/doc/Documentation/stable_api_nonsense... If monolithic systems are so obviously inferior, why are micro-kernel implementations of UNIX mostly a historic curiosity?