Package: hupwatch
Simple utility for graceful reloading of services.General usage bases on my other projectianitor.hupwatchis a
simple shell command that wraps process and can be simply used in your
existing process/service supervision tool likesupervisord,circus,runitetc.why?Reloading of web application code gracefully is a today’s must. Every
DevOps should tell you that already. Many of Python web servers allow
such scenario but handle this differently. Usually you end up with only
few reasonable options:gunicornallows to do this
by simply sending UnixHUPsignal to the gunicorn process. It
will gracefully stop existing workers and start new ones with new
version of code. Other choice is to sendUSR2signal that will
spawn new master gunicorn process with set of udpated workers. You
are then able touWSGIgives few options and the
best one is very similar in effect to sendingUSR2togunicorn. uWSGI provides very verbose documentation on that. One
could say “too verbore”. Good luck on understanding it!You may depend on load balancing on the higher level your
architecture stack. Still you need to be able to finish current
requests that are being processed during the deployment. So you
require a web server that can at least gracefully shutdown its
workers.Graceful reloading implementations provided by gunicorn and uWSGI are
both neat but fail in reality when you try to automate things or use it
with any of the popular tools for process/service supervision:Gunicorn’s HUP-reload will fail if you switch your codebase using
symlinks. This is a very popular technique in many organizations.
Gunicorn will also it will not update gunicorn its own configuration
onHUPbut will only restart workers with a new code. This
approach will work fine if you simply replace the codein situand
do not need to tune configuration options like worker class or
concurrency settings. Still, I wish you a good luck with that if you
use Django and you did a lot of updates to templates with your latest
release. Old gunicorn workers will “gracefuly” shutdown but may also
“gracefully” respond with server errors if you replaced code in
place. Gunicorn accepts a--chdirargument that should fix that
but it again does not play well with some popular deploment solutions
like buildout.The second approach (spawning new master server process) seems to be
a better way and generally works well with symlinks but does not play
well with many process supervision tools. The most popular one in
Python world - supervisord - must spawn a subprocess by itself in
order to be able to control it. Once gunicorn (afterUSR2signal)
or uWSGI (after whatever it requires) spawns (re-execs) a new master
process it has no longer the supervisor as its parent. In short, it
will eventually become the child process ofinitand there is no
way to “adopt” such process by supervisor. Supervisor will try to
spawn it again and you end up with twice more workers than you
expected. If you won’t notice that and perform many subsequent
reloads in that manner you may eventually run out of resources.Generally handling updates with the help of load balancer seems like
a safer solution because you can simply restart web servers once they
are removed from your stack. This is unfortunately a lot harder to
automate and adds additional level of complexity to your whole
operations.You may of course work around these issues by providing some custom
integration for whatever process supervision tool you are using or do
some crazy next/current instance switching and mantain doubled
configuration of services only for this single purpose. This will make
your solution either non-portable to other tools or make the whole
solution harder to automate in a reliable way.how does it work?hupwatchprovides a single solution that can be easily integrated
with virtually any supervision tool and allows to reload whole web
servers with only single command that is available on any POSIX system.
It iskill:# on terminal or inside supervisord config:
$ hupwatch -- gunicorn myapp:application --bind unix:/tmp/myapp.sock
=> HUP watch [INFO    ]: Starting HUP watch (92808)
=> HUP watch [INFO    ]: Child process 92809 started
=> HUP watch [INFO    ]: Pausing for signal
[2016-01-27 17:24:13 +0100] [92809] [INFO] Starting gunicorn 19.4.5
[2016-01-27 17:24:13 +0100] [92809] [INFO] Listening at: unix:/tmp/myapp.sock (92809)
[2016-01-27 17:24:13 +0100] [92809] [INFO] Using worker: sync
[2016-01-27 17:24:13 +0100] [92812] [INFO] Booting worker with pid: 92812

# issued on any other terminal:
kill -HUP <hupwatch_pid>

# continued result in the hupwatch stdout:
[...]
=> HUP watch [DEBUG   ]: HUP: >>>
=> HUP watch [DEBUG   ]: HUP: Waiting for process (92955) to warm up
=> HUP watch [DEBUG   ]: HUP: Sending SIGTERM to old process (92809)
=> HUP watch [DEBUG   ]: HUP: Waiting for process (92809) to quit...
[2016-01-27 17:24:46 +0100] [92809] [INFO] Handling signal: term
[2016-01-27 17:24:46 +0100] [92955] [INFO] Starting gunicorn 19.4.5
[2016-01-27 17:24:46 +0100] [92955] [INFO] Listening at: unix:/tmp/myapp.sock (92955)
[2016-01-27 17:24:46 +0100] [92955] [INFO] Using worker: sync
[2016-01-27 17:24:46 +0100] [92964] [INFO] Booting worker with pid: 92964
[2016-01-27 17:24:58 +0100] [92812] [INFO] Worker exiting (pid: 92812)
[2016-01-27 17:24:58 +0100] [92809] [INFO] Shutting down: Master
=> HUP watch [DEBUG   ]: CHLD: >>>
=> HUP watch [INFO    ]: CHLD: Child process quit
=> HUP watch [DEBUG   ]: CHLD: <<<
=> HUP watch [INFO    ]: HUP: Old process quit with code: 0
=> HUP watch [DEBUG   ]: HUP: <<<
=> HUP watch [INFO    ]: Pausing for signalhupwatchwill start anything that you have provided after the--characters as its own subprocess (usingsubprocess.Popen()) and
listens for incoming Unix signals. Whenever it gets aHUPsignal it
starts a new process with the same arguments and sendsTERMsignal
to the process that was started previously so it can shutdown gracefuly.This make the whole realoading process very easy to automate. No need to
execute multiple commands and mantain any state between them. Simply
HUP’n’go! This is a good news forfabricenthusiasts - don’t need
to worry about lost shh connection during whole reload procedure because
it takes only one step (at least if you use symlinks). There is nothing
to interrupt!Rolling back the update is also painless: simply change project symlink
and issue anotherHUPsignal to the same hupwatch pid. Auto rollback
should be also easy to implement and we are open to any contributions!If you paid attention then you should already notice that this requires
only two things to make it working as a solution for graceful reload:You need to use Unix sockets instead of ports so both old and new
processes can bind to the same addressYour web server needs to perform a graceful shutdown when it receivesTERMsignal. Gunicorn already does that. uWSGI is not supported
yet.anything else?See the usage withhupwatch--helpfor more information on possible
configuration options:There is also some details important detail of handling failures and
what to do whenhupwatchreceives other signals (e.g.KILL,TERM,INT). By default it assumes that you want to have have
your process working no matter what happens with the parent (hupwatch).
So in case of failure it leaves it as it is - spawned process will
become a child ofinit. If you that this happened you can clean up
the mess manually without interrupting the process of serving web
requests. This behaviour can be changed with--kill-at-exitflag.status of this project?This is more a proof of concept than a battle-tested tool. Anyway, there
are only few lines of code that actually do any work. Most of the code
in this package is extensive logging and parsing of arguments. This
state of this package will eventually change in a near future, because
it solves a real problem that we have in my organization. So give it a
try at least in your staging/testing environment.Contributions are really welcome!