In my old post, I have shared how to run the builtin development server to serve the web API. For production, we need to use more powerful web servers. Flask complies to the WSGI (Web Server Gateway Interface) specification and can work with any web server that conforms to WSGI.

In this post, I want share how to run Flask applications using uWSGI — a popular WSGI-compliant server.

# Install and simple running

To install uWSGI, we can use pip:

pip install uwsgi


To run a simple flask application using uWSGI, we can use the following commands:

uwsgi --http 0.0.0.0:8081 --module flask-app:app --master --processes 2 --threads 2
# or run following command
# uwsgi --socket 0.0.0.0:8081 --protocol=http --module flask-app:app


# Run uWSGI via config

When we use more options via the command line, it becomes less convenient. We can also use an ini file to config uWSGI like this:

[uwsgi]
chdir=/path/to/project
wsgi-file=flask-demo.py
callable=app
http=0.0.0.0:1234
need-app=true
lazy-apps=true
master=true
processes=2
threads=2
single-interpreter=true
die-on-term=true
procname-prefix-spaced=flask demo
vacuum=true
pidfile=demo.pid


Save this file as uwsgi.ini (or whatever you like), and then run the application using uwsgi --ini uwsgi.ini. If the extention of config file is ini, we also directly run uWSGI as this: uwsgi uwsgi.ini.

Below are meanings of the some of these options:

## chdir

The chdir option specifies the directory to run the application.

## wsgi-file and callable

wsgi-file specify the source file where your application resides. callable are the name of your application. For example, if you define your application like this: app = Flask(__name__), then you should set callable = app.

## master

With master mode on, the master process will restart a worker process if it dies. Generally, it is good idea to turn master mode on with master = true.

## single-interpreter

By default, uwsgi will enable multiple interpreter mode, i.e., running multiple apps in a single process using different interpreter. If we are running a single app, it is not necessary to run in this mode.

Ref:

## die-on-term

without die-on-term = true, you can not kill uwsgi process via kill xxx. Instead, uWSGI will reload a process if it were killed. This may not be what you want.

## need-app

if need-app = true, uWSGI will not run if it can not find the application to run. Otherwise, it will load even without an application. When you request the service, you will get HTTP 500 responses.

## lazy-apps

With lazy-apps=true, each process will initialize applications indepently of each other. If you do not set this option, you may run into troubles. For example, someone report that PyTorch model are not runnning properly using uWSGI and flask if lazy-apps option is not set.

See also official discussion here.

## proc-name-spaced

It will add a custom prefix to uwsgi process so that you can find the process for a specific application easily.

proc-name-spaced = demo


will produce the following output:

jdhao 29516  1.0  0.0 110104 14852 pts/31   S+   21:16   0:00 demo uWSGI master
jdhao 29520  5.5  0.0 186184 28080 pts/31   S+   21:16   0:00 demo uWSGI worker 1
jdhao 29521  0.0  0.0 110104  7644 pts/31   S+   21:16   0:00 demo uWSGI http 1


## py-autoreload

py-autoreload=2 will check if file is modified every 2 seconds and restart application if modification detected. This should only be used for debugging, and not for production.

Ref:

## pidfile

The option pidfile will create a file under specified location with the pid of uWSGI master process. To kill the uWSGI process safely, you can use the following command:

uwsgi --stop demo.pid


Ref:

# worker-reload-mercy

When I try to kill the uwsgi process via Ctrl-C, I found that uwsgi takes long time to stop and print the following message when it finally stops:

worker 1 buried after 30 seconds
goodbye to uWSGI.
VACUUM: pidfile removed.


To close the uwsgi process quicker, we can add the worker-realod-mercy option and set it to a small value:

# set it to 5 seconds
worker-reload-mercy = 5


It set the number of seconds uWSGI will wait for the worker to exit before killing it brutally.

Then when you start uwsgi, you will see the following log message:

your mercy for graceful operations on workers is 5 seconds


Ref: