add readme

README.md

@@ -0,0 +1,47 @@
+# systemd-ml
+
+`systemd-ml` provides `libsystemd`-like functionality for interacting with the systemd service
+manager, in self-contained ocaml code (with a bit of C). `systemd-ml` is integrated with `eio`,
+which provides for use in `eio`-based daemons
+
+Unfortunately, `systemd-ml` currently assumes usage of the `eio_linux` backend. It's hard to make
+code backend-agnostic because not enough functionality around Unix domain sockets is exposed by the
+high-level `eio` APIs, necessitating use of the low-level backend APIs. Therefore, it does not
+currently work on `eio_posix`, though future support is possible. Support for `eio_windows` is not
+planned, since there is no systemd port for windows.
+
+Additionally, some functionality that would be useful for systemd integration is currently not
+possible without library patches. This currently includes functionality for responding to memory
+pressure events.
+
+
+- `systemd-ml` with entry module `Systemd` is the main library
+- `systemd-ml.xlog` with entry module `Systemd_xlog` configures `xlog` to send log messages to
+  journald
+- `systemd-ml.main` with entry module `Systemd_main` provdes a convenience function
+  `Systemd_main.run` which handles common boilerplate operations for `Type=notify` or
+  `Type=notify-reload` daemons with `xlog` output to journald
+
+Also note that per the license field in the package info, this package is not currently open source.
+If you have questions about this you should contact me
+
+## Features
+
+- detect
+  [systemd](https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#%24SYSTEMD_EXEC_PID)
+  and
+  [journald](https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#%24JOURNAL_STREAM)
+  presence
+- [rich journald output](https://systemd.io/JOURNAL_NATIVE_PROTOCOL/) using `xlog`
+- convenience functions to retrieve [directories passed by
+  systemd](https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#%24RUNTIME_DIRECTORY)
+- [service notifications](https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html),
+  with wrapper functions for common notification types
+- [fdstore](https://systemd.io/FILE_DESCRIPTOR_STORE/) functionality, and [socket
+  activation](https://www.freedesktop.org/software/systemd/man/latest/systemd.socket.html)
+- [watchdog](https://www.freedesktop.org/software/systemd/man/latest/sd_watchdog_enabled.html)
+  notifications
+
+features that would be nice to have but unfortunately require patches (eio does not currently
+support polling for POLLPRI)
+- [memory pressure](https://systemd.io/MEMORY_PRESSURE/)