EDIT: what you're saying appears to be untrue. According to http://0pointer.de/blog/projects/the-biggest-myths.html, they provide configure switches for enabling or disabling many things, not unlike how the Linux kernel allows you to enable or disable features.
You know, there are so many people here bashing on Lennart, I'm wondering whether any of you have done your homework at all or because you just bash Lennart for the sake of it. I'm not even using systemd and I found these things in 5 minutes.
- Certain guarantees and consistencies disappear. Instead of having a system that you know you can rely on, it suddenly becomes entirely dependent on the configuration options. While it sounds nice if any part can be enabled, disabled, moved or swapped, then the system's predictability goes down. Furthermore, some combinations may be incompatible because abstractions are inherently leaky. Good luck finding out whether you're suffering from a compatibility problem or not. You can compare this to the many complaints about Android fragmentation. Because everybody can customize Android, writing an app that works on all Android devices becomes extremely difficult.
- Installation complexity goes way up. It's much easier to install a system if it states "I need this, this and this", instead of "I can use this, or this, or this, if it's configured in X, Y and Z way".
- Certain features cannot be implemented in a simple manner because you have to cater to the lowest common denominator. XFree86 was like that. Because it had to be portable, it cannot assume any kernel capabilities. And as a result XFree86 came with its own mode setting code, its own ELF binary loader, had to run as root, etc etc.
The issue here is that the Linux kernel developers are starting to make decisions that assume systemd is being used. For example, the (proposed?) new cgroups API is being built with the idea that systemd will manage it, and anything else that wants to interact with it will go through systemd.
> Unprivileged access to the cgroup hierarchy will be strongly discouraged; the hope is to have a single, privileged process handling all of the cgroup management tasks. That process will, in turn, provide some sort of higher-level interface to the rest of the system.
and that:
> This hierarchy becomes private property of systemd. systemd will set it up. Systemd will maintain it. Systemd will rearrange it. Other software that wants to make use of cgroups can do so only through systemd's APIs.
That is, systemd will implement a manager on top of the new cgroups API, and all cgroups users will be assumed to go through systemd.
True, except that the systemd developers are in charge of defining this API and have no reason to consult with other cgroups manager developers when defining it. This is, to make an analogy, equivalent to allowing Microsoft to unilaterally define all web standards from now on. The competitors are always going to be playing catch-up, and there's going to be tons of edge cases where the competitors don't quite work the same.
It's not at all like the kernel does it. With Linux, there are multiple competing implementations of most features, pushed by people with competing interests. There is a lot more room for debate and experimentation.
If by "monolithic" you mean the lack of multiple competing implementations, then lots and lots of things are monolithic. Apache and Nginx would be monolithic by your definition.
Apache2 and Nginx are modular. Let me tell you the story of a small web application.
Once upon a time, requests were hitting an Apache server, dispatched to mod_python, and an embedded interpreter was keeping itself warm running Python code to answer the requests.
As the site grew, and the state of the art evolved, a programmer wrapped the application in a wsgi container, and gave it to a mod_wsgi container so that it would run at its own pace.
One day, the programmer brought home an nginx server it had found freezing in the tundra, and gave it a job serving static content in front of Apache. Nginx brought his uwsgi friend and they took turns running the site, nginx serving images and scripts, uwsgi replacing mod_wsgi.
This is what a competitive space looks like. Apache has competing modules, there are http servers and wsgi servers, some of them embedded, some of them external, http servers can collaborate or replace each other when a better thing comes along.
You don't see an http server deciding it will be the sole http server on the system, swallowing a wsgi server and waging war to all others.
You can replace pretty much everything. You need only systemd, systemd-udev and systemd-journald. Virtually everything else is optional. It also allows you to use syslog with journald. It allows you use use crond instead of the timer units. You could use ConsoleKit instead of systemd-logind as session manager... and so on and so forth.
You have to use all of systemd, or none of it. You can't replace bits of it, or use just parts of it.