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* **The command line interface** of `systemd`, `systemctl`, `loginctl`, `journalctl`, and all other command line utilities installed in `$PATH` and documented in a man page.
We will make sure that scripts invoking these commands will continue to work with future versions of systemd.
Note however that the output generated by these commands is generally not included in the promise, unless it is documented in the man page.
Example: the output of `systemctl status` is not stable, but that of `systemctl show` is, because the former is intended to be human readable and the latter computer readable, and this is documented in the man page.
* **The D-Bus interfaces of the main service daemon and other daemons**. We try to always preserve backwards compatibility, and intentional breakage is never introduced.
Nevertheless, when we find bugs that mean that the existing interface was not useful, or when the implementation did something different than stated by the documentation and the implemented behaviour is not useful, we will fix the implementation and thus introduce a change in behaviour.
But the API (parameter counts and types) is never changed, and existing attributes and methods will not be removed.
* **The set of states of the various state machines used in systemd**, e.g. the high-level unit states inactive, active, deactivating, and so on, as well (and in particular) the low-level per-unit states.
One of the main goals of systemd is to unify basic Linux configurations and service behaviors across all distributions.
Systemd project does not contain any distribution-specific parts.
Distributions are expected to convert over time their individual configurations to the systemd format, or they will need to carry and maintain patches in their package if they still decide to stay different.
What does this mean for you? When developing with systemd, don't use any of the latter interfaces, or we will tell your mom, and she won't love you anymore.
You are welcome to use the other interfaces listed here, but if you use any of the second kind (i.e. those where we don't yet make a stability promise), then make sure to subscribe to our mailing list, where we will announce API changes, and be prepared to update your program eventually.
Note that this is a promise, not an eternal guarantee.
These are our intentions, but if in the future there are very good reasons to change or get rid of an interface we have listed above as stable, then we might take the liberty to do so, despite this promise.
However, if we do this, then we'll do our best to provide a smooth and reasonably long transition phase.
This list is intended to be useful for distribution and OS developers who are interested in maintaining a certain level of compatibility with the new interfaces systemd introduced, without relying on systemd itself.
In general it is our intention to cooperate through interfaces and not code with other distributions and OSes.
That means that the interfaces where this applies are best reimplemented in a compatible fashion on those other operating systems.
To make this easy we provide detailed interface documentation where necessary.
That said, it's all Open Source, hence you have the option to a) fork our code and maintain portable versions of the parts you are interested in independently for your OS, or b) build systemd for your distro, but leave out all components except the ones you are interested in and run them without the core of systemd involved.
We will try not to make this any more difficult than necessary.
Patches to allow systemd code to be more portable will be accepted on case-by-case basis (essentially, patches to follow well-established standards instead of e.g. glibc or linux extensions have a very high chance of being accepted, while patches which make the code ugly or exist solely to work around bugs in other projects have a low chance of being accepted).
**Portability to OSes:** systemd is not portable to non-Linux systems.
It makes use of a large number of Linux-specific interfaces, including many that are used by its very core.
We do not consider it feasible to port systemd to other Unixes (let alone non-Unix operating systems) and will not accept patches for systemd core implementing any such portability (but hey, it's git, so it's as easy as it can get to maintain your own fork...).
APIs that are supposed to be used as library code are exempted from this: it is important to us that these compile nicely on non-Linux and even non-Unix platforms, even if they might just become NOPs.
**Portability to Distributions:** It is important to us that systemd is portable to all Linux distributions.
However, the goal is to unify many of the needless differences between the distributions, and hence will not accept patches for certain distribution-specific work-arounds.
Compatibility with the distribution's legacy should be maintained in the distribution's packaging, and not in the systemd source tree.
**Compatibility with Specific Versions of Other packages:** We generally avoid adding compatibility kludges to systemd that work around bugs in certain versions of other software systemd interfaces with. We strongly encourage fixing bugs where they are, and if that's not systemd we rather not try to fix it there.
(There are very few exceptions to this rule possible, and you need an exceptionally strong case for it).
systemd's APIs are available everywhere where systemd is available.
Some of the APIs we have defined are supposed to be generic enough to be implementable independently of systemd, thus allowing compatibility with systems systemd itself is not compatible with, i.e. other OSes, and distributions that are unwilling to fully adopt systemd.
| API | Type | Covered by Interface Stability Promise | Fully documented | Known External Consumers | Reimplementable Independently | Known Other Implementations | systemd Implementation portable to other OSes or non-systemd distributions |
Items for which "systemd implementation portable to other OSes" is "partially" means that it is possible to run the respective tools that are included in the systemd tarball outside of systemd.
Note however that this is not officially supported, so you are more or less on your own if you do this.
If you are opting for this solution simply build systemd as you normally would but drop all files except those which you are interested in.
Of course, it is our intention to eventually document all interfaces we defined.
If we haven't documented them for now, this is usually because we want the flexibility to still change things, or don't want 3rd party applications to make use of these interfaces already.
That said, our sources are quite readable and open source, so feel free to spelunk around in the sources if you want to know more.
