Develop Reference

VirtualBox Flask Server / Ubuntu 16.04 32-bit / Server stops responding periodically

I'm not sure where to post this since I don't really know if this is an issue with Flask, VirtualBox (6.0) or Ubuntu (16.04 LTS).
Intro
My company installs Ubuntu VMs for our clients that run a local Flask web server on their system. I know Flask is not meant to be used for production, but each individual server handles about 15 users max (and often just 1 or 2) and usually only one or two users at a time. So far it's been working well for dozens of clients over several months.
Problem
We have a couple of clients where the application becomes inaccessible in users' browsers after a certain period of time. I can SSH to the VM and see that the Flask service still seems to be running without issue. But the users are not able to connect to the IP address and port of the VM via their browser like they could before (eg. http://<vm_ip>:<port_number>).
Restarting the service doesn't resolve the issue, but restarting the entire VM does resolve it.
The flask application runs as a service, but all the service is doing is triggering a bash script that runs python run.py.
The VM network adapter is set to "NAT" and it serves the Flask app on the same IP address as the host machine. When the app becomes inaccessible, I'm not able to connect to the application on the host machine using either http://<vm_ip>:<port_number> or http://localhost:<port_number>. We have this same configuration on many client systems and it works without issue for all other clients.
Nothing should change in the application configuration when the VM restarts. All networking and port settings are static. So something is clearly getting hung up. Since the application output doesn't show anything off and the Flask service seems to restart fine internally, I'm guessing the issue is with Ubuntu or VirtualBox (or something on the client machine/network).
Question
I'm wondering what would cause a Flask server that runs very reliably in most cases, to periodically stop responding after a certain amount of time, even though the VM is accessible and Flask appears to be running fine internally.
I apologize for lack of details and logs - our access to the client machine is limited. I will try to get my hands on these.

strange behavior with flask on windows

I'm maintaining a web application built with Flask with python 2.7 coupled with Jinja and angularjs .In a Linux environment, everything is working fine.
On windows when I run the application on cmd or git bash (python app.py), I only see that the server is running and in which port (and everything else is working fine in the browser), but the problem is that the logs in console aren't shown like in a Linux terminal.
For example, I can't see the requests like: POST /login..or an exceptional mission or even a simple print "test" Dosen"to show (still everything is working in a browser).
Even worst, when I terminate the server with "ctr+c" all the previous messages and logs are printed in the terminal, all together in one single dump!
--- Update ---
when i use the command python -u app.py
it's even worst , the application dose'nt run in the browser anymore , no log in console and when i termiante it shows this :
screenshot of terminal

I would suggest that you creat a proper uWSGI gateway.
An example setup could be:
Linux 18.04 ->
Nginx / Apache reverse web proxy mode ->
Gunicorn (which has a debugger you can attach, this will output to the regular log files like other applications. Systemd logs I think) ->
Flask web framework.
Hope this helps.

Google app engine endpoints api not working in localhost

I am trying to start a python app-engine project on localhost. I am in a Windows 8.1 machine (Python version 2.7). The project code is copied from here link. The server seems to start but the page wouldn't properly load due to some problem. Probably it can't reach the api server.
Here is a shot from firebug showing the failed request
What seems off to me is that in command prompt (I started the dev server from there) the API server is started at some random port other than the module port. But the webpage is trying to access the http://127.0.0.1:8000/_ah/api/static/proxy.html... page at the module port. I tried accessing it at the API server port it does open some page there. (I mean I tried opening http://127.0.0.1:1532/_ah/api/static/proxy.html... seperately) It returns some json I don't understand...
{app_id: dev~udatut-bs, rtok: '0'}
Command prompt log:
Here's the difference between the outputs in localhost and *.appspot.com
Should get this:
But stuck here:(check buttons are not loaded)

I finally found a solution here.
Steps for windows:
Goto your google appengine installation folder and then ./google/appengine/toots. (C:\Program Files (x86)\Google\google_appengine\google\appengine\tools) if you followed default installation instructions.
Open the file appengine_rpc.py and look for the line (towards the end of the file)
opener.add_handler(fancy_urllib.FancyProxyHandler())
and comment it out like
# opener.add_handler(fancy_urllib.FancyProxyHandler())
Close the app and restart it again.

How to setup WSGI server to run similarly to Apache?

I'm coming from PHP/Apache world where running an application is super easy. Whenever PHP application crashes Apache process running that request will stop but server will be still ruining happily and respond to other clients. Is there a way to have Python application work in a smilar way. How would I setup wsgi server like Tornado or CherryPy so it will work similarly? also, how would I run several applications from one server with different domains?

What you are after would possibly happen anyway for WSGI severs. This is because any Python exception only affects the current request and the framework or WSGI server would catch the exception, log it and translate it to a HTTP 500 status page. The application would still be in memory and would continue to handle future requests.
What we get down to is what exactly you mean by 'crashes Apache process'.
It would be rare for your code to crash, as in cause the process to completely exit due to a core dump, the whole process. So are you being confused in your terminology in equating an application language level error to a full process crash.
Even if you did find a way to crash a process, Apache/mod_wsgi handles that okay and the process will be replaced. The Gunicorn WSGI server will also do that. CherryPy will not unless you have a process manager running which monitors it and the process monitor restarts it. Tornado in its single process mode will have the same problem. Using Tornado as the worker in Gunicorn is one way around that plus I believe Tornado itself may have some process manager in it now for running multiple process which allow it to restart processes if they die.
Do note that if your application bug which caused the Python exception is bad enough and it corrupts state within the process, subsequent requests may possibly have issues. This is the one difference with PHP. With PHP, after any request, whether successful or not, the application is effectively thrown away and doesn't persist. So buggy code cannot affect subsequent requests. In Python, because the process with loaded code and retained state is kept between requests, then technically you could get things in a state where you would have to restart the process to fix it. I don't know of any WSGI server though that has a mechanism to automatically restart a process if one request returned an error response.

If you're in an UNIX-like environment, you can run mod_wsgi under Apache in Daemon Mode. This means there will be a separate process for the Python code, and even if it crashes the server will continue running normally (and hopefully the WSGI process will restart itself). A WSGI application can run under multiple processes and multiple threads per process.
As for running multiple domains in the same server, check Name-Based Virtual Hosts.

How to get Intellij-idea close Flask dev server properly?

I use IDEA 10.5 for my Flask experimentation. Flask has en embedded test server (like Django does)
When I launch my test class, the dev server launches as well on port 5000. All good.
* Running on http://127.0.0.1:5000/
When I click on the "Stop process" button (red square), I get the message saying the process is finished :
Process finished with exit code 143
However the server is still alive (responds to requests) and I can see I still have a python process running.
Obviously this prevents me from relaunching the test straight away, I have to kill the server process first.
How do you manage to get both your program and the server ending at the same time ?

I guess what happens is that you start your flask app which then is forking the development server as a new process. If you stop the app the forked process is still running.
This looks like a problem, that cannot easily be solved within the means of your IDE. You could add something to your main to kill the already running server process, before starting the app again, but that seems ugly.
But why don't you just start your app with app.run(debug=True) as described in flask doc? The server will reload automatically everytime you changed your app so you don't have to stop and restart it manually.
EDIT:
Something a bit quirky just came to my mind: if you just need a comfortable way to kill the server from within the IDE all you have to do is to introduce a syntactical error in one of the places the reloader monitors, save the file and the server will choke on it and die :)

This doesn't happen anymore with newer versions (tested with PyCharm 2.0)