Default developer use case to systemd
This moves the developer use case over to systemd, and updates all the
relevant docs to discuss the systemd workflow instead of screen. It
does so by defaulting USE_SCREEN=False, so will not impact people that
set it explicitly.
Change-Id: I6d664612bc2b850eb7f56852afbc841867223ab7
diff --git a/doc/source/systemd.rst b/doc/source/systemd.rst
new file mode 100644
index 0000000..efe79e4
--- /dev/null
+++ b/doc/source/systemd.rst
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+===========================
+ Using Systemd in DevStack
+===========================
+
+By default DevStack is run with all the services as systemd unit
+files. Systemd is now the default init system for nearly every Linux
+distro, and systemd encodes and solves many of the problems related to
+poorly running processes.
+
+Why this instead of screen?
+===========================
+
+The screen model for DevStack was invented when the number of services
+that a DevStack user was going to run was typically < 10. This made
+screen hot keys to jump around very easy. However, the landscape has
+changed (not all services are stoppable in screen as some are under
+Apache, there are typically at least 20 items)
+
+There is also a common developer workflow of changing code in more
+than one service, and needing to restart a bunch of services for that
+to take effect.
+
+Unit Structure
+==============
+
+.. note::
+
+ Originally we actually wanted to do this as user units, however
+ there are issues with running this under non interactive
+ shells. For now, we'll be running as system units. Some user unit
+ code is left in place in case we can switch back later.
+
+All DevStack user units are created as a part of the DevStack slice
+given the name ``devstack@$servicename.service``. This makes it easy
+to understand which services are part of the devstack run, and lets us
+disable / stop them in a single command.
+
+Manipulating Units
+==================
+
+Assuming the unit ``n-cpu`` to make the examples more clear.
+
+Enable a unit (allows it to be started)::
+
+ sudo systemctl enable devstack@n-cpu.service
+
+Disable a unit::
+
+ sudo systemctl disable devstack@n-cpu.service
+
+Start a unit::
+
+ sudo systemctl start devstack@n-cpu.service
+
+Stop a unit::
+
+ sudo systemctl stop devstack@n-cpu.service
+
+Restart a unit::
+
+ sudo systemctl restart devstack@n-cpu.service
+
+See status of a unit::
+
+ sudo systemctl status devstack@n-cpu.service
+
+Operating on more than one unit at a time
+-----------------------------------------
+
+Systemd supports wildcarding for unit operations. To restart every
+service in devstack you can do that following::
+
+ sudo systemctl restart devstack@*
+
+Or to see the status of all Nova processes you can do::
+
+ sudo systemctl status devstack@n-*
+
+We'll eventually make the unit names a bit more meaningful so that
+it's easier to understand what you are restarting.
+
+.. _journalctl-examples:
+
+Querying Logs
+=============
+
+One of the other major things that comes with systemd is journald, a
+consolidated way to access logs (including querying through structured
+metadata). This is accessed by the user via ``journalctl`` command.
+
+
+Logs can be accessed through ``journalctl``. journalctl has powerful
+query facilities. We'll start with some common options.
+
+Follow logs for a specific service::
+
+ journalctl -f --unit devstack@n-cpu.service
+
+Following logs for multiple services simultaneously::
+
+ journalctl -f --unit devstack@n-cpu.service --unit
+ devstack@n-cond.service
+
+or you can even do wild cards to follow all the nova services::
+
+ journalctl -f --unit devstack@n-*
+
+Use higher precision time stamps::
+
+ journalctl -f -o short-precise --unit devstack@n-cpu.service
+
+
+Known Issues
+============
+
+Be careful about systemd python libraries. There are 3 of them on
+pypi, and they are all very different. They unfortunately all install
+into the ``systemd`` namespace, which can cause some issues.
+
+- ``systemd-python`` - this is the upstream maintained library, it has
+ a version number like systemd itself (currently ``234``). This is
+ the one you want.
+- ``systemd`` - a python 3 only library, not what you want.
+- ``python-systemd`` - another library you don't want. Installing it
+ on a system will break ansible's ability to run.
+
+
+If we were using user units, the ``[Service]`` - ``Group=`` parameter
+doesn't seem to work with user units, even though the documentation
+says that it should. This means that we will need to do an explicit
+``/usr/bin/sg``. This has the downside of making the SYSLOG_IDENTIFIER
+be ``sg``. We can explicitly set that with ``SyslogIdentifier=``, but
+it's really unfortunate that we're going to need this work
+around. This is currently not a problem because we're only using
+system units.
+
+Future Work
+===========
+
+log colorizing
+--------------
+
+We lose log colorization through this process. We might want to build
+a custom colorizer that we could run journalctl output through
+optionally for people.
+
+user units
+----------
+
+It would be great if we could do services as user units, so that there
+is a clear separation of code being run as not root, to ensure running
+as root never accidentally gets baked in as an assumption to
+services. However, user units interact poorly with devstack-gate and
+the way that commands are run as users with ansible and su.
+
+Maybe someday we can figure that out.
+
+References
+==========
+
+- Arch Linux Wiki - https://wiki.archlinux.org/index.php/Systemd/User
+- Python interface to journald -
+ https://www.freedesktop.org/software/systemd/python-systemd/journal.html
+- Systemd documentation on service files -
+ https://www.freedesktop.org/software/systemd/man/systemd.service.html
+- Systemd documentation on exec (can be used to impact service runs) -
+ https://www.freedesktop.org/software/systemd/man/systemd.exec.html