I have a Flask based app that uses flask-assets to bundle, compile and minify css and Javascript. A timeout occurs when I start the app with gunicorn using
gunicorn --bind=0.0.0.0:8000 --workers=3 --log-level=INFO manage:app
The timeout message states that:
[2018-04-01 11:15:13 +0000] [10] [INFO] Starting gunicorn 19.6.0
[2018-04-01 11:15:13 +0000] [10] [INFO] Listening at: http://0.0.0.0:8000 (10)
[2018-04-01 11:15:13 +0000] [10] [INFO] Using worker: sync
[2018-04-01 11:15:13 +0000] [16] [INFO] Booting worker with pid: 16
[2018-04-01 11:15:13 +0000] [17] [INFO] Booting worker with pid: 17
[2018-04-01 11:15:13 +0000] [18] [INFO] Booting worker with pid: 18
[2018-04-01 11:15:46 +0000] [10] [CRITICAL] WORKER TIMEOUT (pid:17)
[2018-04-01 11:15:47 +0000] [17] [INFO] Worker exiting (pid: 17)
[2018-04-01 11:15:47 +0000] [19] [INFO] Booting worker with pid: 19
I have defined the assets in an assets.py file with the content
from flask_assets import Bundle
app_css = Bundle('app.scss', filters='libsass', output='styles/app.css')
app_js = Bundle('app.js', filters='jsmin', output='scripts/app.js')
vendor_css = Bundle(
'vendor/semantic.css',
filters='cssmin',
output='styles/vendor.css')
vendor_js = Bundle(
'vendor/jquery.min.js',
'vendor/semantic.min.js',
'vendor/tablesort.min.js',
'vendor/zxcvbn.js',
filters='jsmin',
output='scripts/vendor.js')
The relevant part in the main app __init__.py is
from flask_assets import Environment
from .assets import app_css, app_js, vendor_css, vendor_js
def create_app(config_name):
...
# Set up asset pipeline
assets_env = Environment(app)
dirs = ['assets/styles', 'assets/scripts']
for path in dirs:
assets_env.append_path(os.path.join(basedir, path))
assets_env.url_expire = True
assets_env.register('app_css', app_css)
assets_env.register('app_js', app_js)
assets_env.register('vendor_css', vendor_css)
assets_env.register('vendor_js', vendor_js)
The question is, how can the timeout be avoided? Is it possible to precompile the assets before gunicorn starts waiting?
The solution requires two changes to work, enabling preload for gunicorn and run the asset pipeline in the factory. To enable preloading, add --preload to the gunicorn command:
gunicorn --bind=0.0.0.0:8000 --workers=3 --preload --log-level=INFO manage:app
If not otherwise specified, the asset pipeline will be run in a lazy manner, i.e., when its assets are first required by a request. This means that if build() is not explicitly called, the first request will block and possibly timeout. Therefore, for each Bundle object call bundle in the create_app function:
app_css.build()
app_js.build()
vendor_css.build()
vendor_js.build()
If only the preloading is enabled in gunicorn or the Bundle not explicitly built before, the build command is called with the first request and then times out.
Bonus: To speed up your Travis CI build, add app/static/.webassets-cache to the cache in .travis.yml with:
cache:
pip: true
directories:
- core/app/static/.webassets-cache