uWSGI

Origin uWSGI

How uWSGI is implemented by distributors varies. The uWSGI project itself recommends two methods:

  1. systemd.unit template file as described here One service per app in systemd:

There is one systemd unit template on the system installed and one uwsgi ini file per uWSGI-app placed at dedicated locations. Take archlinux and a searxng.ini as example:

systemd template unit: /usr/lib/systemd/system/uwsgi@.service
        contains: [Service]
                  ExecStart=/usr/bin/uwsgi --ini /etc/uwsgi/%I.ini

SearXNG application:   /etc/uwsgi/searxng.ini
        links to: /etc/uwsgi/apps-available/searxng.ini

The SearXNG app (template /etc/uwsgi/%I.ini) can be maintained as known from common systemd units:

$ systemctl enable  uwsgi@searxng
$ systemctl start   uwsgi@searxng
$ systemctl restart uwsgi@searxng
$ systemctl stop    uwsgi@searxng

  1. The uWSGI Emperor which fits for maintaining a large range of uwsgi apps and there is a Tyrant mode to secure multi-user hosting.

The Emperor mode is a special uWSGI instance that will monitor specific events. The Emperor mode (the service) is started by a (common, not template) systemd unit.

The Emperor service will scan specific directories for uwsgi ini files (also know as vassals). If a vassal is added, removed or the timestamp is modified, a corresponding action takes place: a new uWSGI instance is started, reload or stopped. Take Fedora and a searxng.ini as example:

to install & start SearXNG instance create --> /etc/uwsgi.d/searxng.ini
to reload the instance edit timestamp      --> touch /etc/uwsgi.d/searxng.ini
to stop instance remove ini                --> rm /etc/uwsgi.d/searxng.ini

Distributors

The uWSGI Emperor mode and systemd unit template is what the distributors mostly offer their users, even if they differ in the way they implement both modes and their defaults. Another point they might differ in is the packaging of plugins (if so, compare install packages) and what the default python interpreter is (python2 vs. python3).

While archlinux does not start a uWSGI service by default, Fedora (RHEL) starts a Emperor in Tyrant mode by default (you should have read Pitfalls of the Tyrant mode). Worth to know; debian (ubuntu) follow a complete different approach, read see Debian’s uWSGI layout.

Debian’s uWSGI layout

Be aware, Debian’s uWSGI layout is quite different from the standard uWSGI configuration. Your are familiar with Debian’s Apache layout? .. they do a similar thing for the uWSGI infrastructure. The folders are:

/etc/uwsgi/apps-available/
/etc/uwsgi/apps-enabled/

The uwsgi ini file is enabled by a symbolic link:

ln -s /etc/uwsgi/apps-available/searxng.ini /etc/uwsgi/apps-enabled/

More details can be found in the uwsgi.README.Debian (/usr/share/doc/uwsgi/README.Debian.gz). Some commands you should know on Debian:

Commands recognized by init.d script
====================================

You can issue to init.d script following commands:
  * start        | starts daemon
  * stop         | stops daemon
  * reload       | sends to daemon SIGHUP signal
  * force-reload | sends to daemon SIGTERM signal
  * restart      | issues 'stop', then 'start' commands
  * status       | shows status of daemon instance (running/not running)

'status' command must be issued with exactly one argument: '<confname>'.

Controlling specific instances of uWSGI
=======================================

You could control specific instance(s) by issuing:

    SYSTEMCTL_SKIP_REDIRECT=1 service uwsgi <command> <confname> <confname>...

where:
  * <command> is one of 'start', 'stop' etc.
  * <confname> is the name of configuration file (without extension)

For example, this is how instance for /etc/uwsgi/apps-enabled/hello.xml is
started:

    SYSTEMCTL_SKIP_REDIRECT=1 service uwsgi start hello

uWSGI maintenance

uWSGI setup

Create the configuration ini-file according to your distribution and restart the uwsgi application. As shown below, the Installation Script installs by default:

  • a uWSGI setup that listens on a socket and

  • enables cache busting.

Pitfalls of the Tyrant mode

The implementation of the process owners and groups in the Tyrant mode is somewhat unusual and requires special consideration. In Tyrant mode mode the Emperor will run the vassal using the UID/GID of the vassal configuration file (user and group of the app .ini file).

Without option emperor-tyrant-initgroups=true in /etc/uwsgi.ini the process won’t get the additional groups, but this option is not available in 2.0.x branch (see #2099@uWSGI) the feature #752@uWSGI has been merged (on Oct. 2014) to the master branch of uWSGI but had never been released; the last major release is from Dec. 2013, since the there had been only bugfix releases (see #2425uWSGI). To shorten up:

In Tyrant mode, there is no way to get additional groups, and the uWSGI process misses additional permissions that may be needed.

For example on Fedora (RHEL): If you try to install a redis DB with socket communication and you want to connect to it from the SearXNG uWSGI, you will see a Permission denied in the log of your instance:

ERROR:searx.redisdb: [searxng (993)] can't connect redis DB ...
ERROR:searx.redisdb:   Error 13 connecting to unix socket: /usr/local/searxng-redis/run/redis.sock. Permission denied.
ERROR:searx.plugins.limiter: init limiter DB failed!!!

Even if your searxng user of the uWSGI process is added to additional groups to give access to the socket from the redis DB:

$ groups searxng
searxng : searxng searxng-redis

To see the effective groups of the uwsgi process, you have to look at the status of the process, by example:

$ ps -aef | grep '/usr/sbin/uwsgi --ini searxng.ini'
searxng       93      92  0 12:43 ?        00:00:00 /usr/sbin/uwsgi --ini searxng.ini
searxng      186      93  0 12:44 ?        00:00:01 /usr/sbin/uwsgi --ini searxng.ini

Here you can see that the additional “Groups” of PID 186 are unset (missing gid of searxng-redis):

$ cat /proc/186/task/186/status
...
Uid:      993     993     993     993
Gid:      993     993     993     993
FDSize:   128
Groups:
...