hupwatch 0.0.2

why?

Graceful reloading of web applications today is a must. Almost every DevOps should tell you that. Many Python web servers allow such scenario but handle this differently. In most cases you end up with two options: * gunicorn allows to do this by simply sending Unix HUP signal to the gunicorn process. It will gracefully stop existing workers and start new ones with new version of code. Other choice is to send USR2 signal that will spawn new master gunicorn process with set of udpated workers. You are then able to * uWSGI gives few options and the best one is very similar in effect to sending USR2 to gunicorn. uWSGI provides very verbose documentation on that. One could say “too verbore”. Good luck on understanding it! * You may depend on load balancing on higher level of architecture abstraction. 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 gracefully shutdown.

Greceful reloading implementations provided by gunicorn and uWSGI are both neat but fail in reality when you try to automate things or use any of popular tools for process supervision:

• Gunicorn’s HUP-reload will fail if you switch your codebase using symlinks. This is a very popular technique in many organizations. And also it will not update gunicorn configuration but only restart workers with a new code. This will work fine if you simply replace the code in situ but good luck with that if you use Django and did a lot of updates to templates with you current release. Old gunicorn workers will “gracefuly” shutdown but may also “gracefully” respond with server errors if you replaced code in place. Gunicorn accepts a --chdir argument that should fix that but it again does not play well with some deploment solutions like buildout.

• The second approach (spewning new master server process) seems better 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 (after USR2 signal) 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 of init and 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 the issue or cleanup this mess you may run out of resources.

You may 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?

hupwatch provides 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: kill

# on terminal or inside supervisord config:
hupwatch -- gunicorn myapp:application --bind unix:/tmp/myapp.sock

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

hupwatch will start anything that you have provided after the -- characters as its own subprocess (using subprocess.Popen()) and listens for incoming Unix signals. Whenever it gets a HUP signal it starts a new process with the same arguments and sends TERM signal to the process that was started previously so it can shutdown gracefuly.

If you paid attentions 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 address

• your web server needs to perform a graceful shutdown when it receives TERM signal. Gunicorn already does that. uWSGI is not supported yet.

anything else?

See the usage with hupwatch --help for more information on possible configuration options.

There is also some details important detail of handling failures and what to do when hupwatch receives 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 of init. 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-exit flag.