uWSGI: --master with --emperor spawns two emperors

Question

I can see that if I start uwsgi like this:

sudo /usr/local/bin/uwsgi --emperor /etc/uwsgi/vassals --uid www --gid www

it creates one emperor copy. But if I start it with --master additionally (as recommended here), it creates two emperor copies. Does it make sense to use --master with --emperor? I would say no, but if I run it without that option I get this warning:

*** WARNING: you are running uWSGI without its master process manager ***

Answer 1

I'd like to add some specific information for using harakiri mode for vassals running under emperor. If master is not also in the config for the vassals harakiri will have no effect regardless of whether master/harakiri is specified in the emperor config. Given an example emperor config:

[uwsgi]
emperor = ...
daemonize = ...
emperor-pidfile = ...
vassal-set = enable-metrics=1
emperor-stats = 127.0.0.1:6000

The vassals will need the following for harakiri to work:

[uwsgi]
strict
processes = 4
stats = 127.0.0.1:5000
memory-report
daemonize = ...
pidfile = ...
close-on-exec
py-tracebacker = /tmp/tbsocket
master
harakiri = 5
harakiri-verbose

Note that master and harakiri are both present in the vassal and setting them in emperor config would have no effect on the vassals for uwsgi version 2.0.12

Answer 2

I disagree - the documentation says it is not a good idea NOT to use it, in production anyway; I guess the double negative could be written more clearly.

Therefore it would appear it IS a good idea to use it, hence the warning.

Answer 3

Here is what official documentation says:

The emperor should generally not be run with --master, unless master features like advanced logging are specifically needed.

If you're wondering what master option does, here is the answer:

master

uWSGI’s built-in prefork+threading multi-worker management mode, activated by flicking the master switch on. For all practical serving deployments it’s not really a good idea not to use master mode.

So, to summarize:

• Use --master for usual uWSGI instance,
• Do not use --master for uWSGI Emperor.