Serving Flask Applications with uWSGI
In my older post, I have shared how to run the built-in development server to serve Flask web service. For production, we need to use more powerful web servers. Flask complies to the WSGI 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 serve a simple flask application using uWSGI, we can run the following command:
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 add more options via the command line, it becomes less convenient.
We can also use a config file (in
ini format) to run uWSGI:
[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 worker-reload-mercy=5
Save this file as
uwsgi.ini (or whatever you like), and then run the application using
uwsgi --ini uwsgi.ini.
If the extension of config file is
ini, we can also directly run uWSGI:
Note that the first line (
[uwsgi]) is necessary and must not be omitted.
Below are the meanings of some of these options:
chdir option specifies the directory to run the application.
wsgi-file and callable
wsgi-file specify the source file where your application resides.
callable sets the name of your application.
For example, if you define your application like this:
app = Flask(__name__), then you should set
callable = app.
With master mode on, the master process will restart a worker process if it dies.
Generally, it is a good idea to turn master mode on with
master = true.
See also: https://stackoverflow.com/q/35072176/6064933
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.
die-on-term = true, you can not kill uwsgi process via
Instead, uWSGI will reload a process if it were killed.
This may not be what you want.
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.
true is a sane choice.
lazy-apps=true, each process will initialize applications independently of each other.
If you do not set this option, you may run into troubles.
For example, someone reported that PyTorch model does not run properly if
lazy-apps option is not set.
See also official discussion here.
It will add a custom prefix to uwsgi processes so that you can find the processes for a specific application easily. For example, with the following config:
proc-name-spaced = demo
the output will be:
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=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.
pidfile will create a file under specified location with the pid of uWSGI master process.
To kill the uWSGI process safely, you can run the following command:
uwsgi --stop demo.pid
When I try to kill the uwsgi process via
I found that uwsgi takes long time to stop and prints the following message when it finally stops:
worker 1 buried after 30 seconds goodbye to uWSGI. VACUUM: pidfile removed.
To close the uwsgi processes 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 sets the number of seconds uWSGI will wait for the worker to exit before killing it forcefully.
Then when you start uwsgi, you will see the following log message:
your mercy for graceful operations on workers is 5 seconds
- NGINX - Python - UWSGI kill issue
- Configuring uWSGI for Production: The defaults are all wrong: https://www.youtube.com/watch?v=p6R1h2Nn468
License CC BY-NC-ND 4.0