Using CherryPy/Cherryd to launch multiple Flask instances

Question

Per suggestions on SO/SF and other sites, I am using CherryPy as the WSGI server to launch multiple instances of a Python web server I built with Flask. Each instance runs on its own port and sits behind Nginx. I should note that the below does work for me, but I'm troubled that I have gone about things the wrong way and it works "by accident".

Here is my current cherrypy.conf file:

[global]
server.socket_host = '0.0.0.0'
server.socket_port = 8891
request.dispatch: cherrypy.dispatch.MethodDispatcher()
tree.mount = {'/':my_flask_server.app}

Without diving too far into my Flask server, here's how it starts:

import flask
app = flask.Flask(__name__)

@app.route('/')
def hello_world():
    return "hello"

And here is the command I issue on the command line to launch with Cherryd:

cherryd -c cherrypy.conf -i my_flask_server

Questions are:

1. Is wrapping Flask inside CherryPy still the preferred method of using Flask in production? https://stackoverflow.com/questions/4884541/cherrypy-vs-flask-werkzeug

2. Is this the proper way to use a .conf file to launch CherryPy and import the Flask app? I have scoured the CherryPy documentation, but I cannot find any use cases that match what I am trying to do here specifically.

3. Is the proper way to launch multiple CherryPy/Flask instances on a single machine to execute multiple cherryd commands (daemonizing with -d, etc) with unique .conf files for each port to be used (8891, 8892, etc)? Or is there a better "CherryPy" way to accomplish this?

Thanks for any help and insight.

Answer 1

I can't speak for Flask, but I can for CherryPy. That looks like the "proper way"...mostly. That line about a MethodDispatcher is a no-op since it only affects CherryPy Applications, and you don't appear to have mounted any (just a single Flask app instead).

Regarding point 3, you have it right. CherryPy allows you to run multiple Server objects in the same process in order to listen on multiple ports (or protocols), but it doesn't have any sugar for starting up multiple processes. As you say, multiple cherryd commands with varying config files is how to do it (unless you want to use a more integrated cluster/config management tool like eggmonster).

Answer 2

Terminology: Mounting vs Grafting

In principle this is a proper way to serve a flask app through cherrypy, just a quick note on your naming:

It is worth noting here that tree.mount is not a configuration key by itself - tree will lead to cherrypy._cpconfig._tree_config_handler(k, v) being called with the arguments 'mount', {'/': my_flask_server.app}.

The key parameter is not used at all by the _tree_config_handler so in your config "mount" is just an arbitrary label for that specific dict of path mappings. It also does not "mount" the application (it's not a CherryPy app after all). By that I mean, it does not cherrypy.tree.mount(…) it but rather cherrypy.tree.grafts an arbitrary WSGI handler onto your "script-name" (paths, but in CherryPy terminology) namespace.

Cherrypy's log message somewhat misleadingly says "Mounted <app as string> on /"]

This is a somewhat important point since with graft, unlike mount, you cannot specify further options such as static file service for your app or streaming responses on that path.

So I would recommend changing the tree.mount config key to something descriptive that does not invite reading too much semantics about what happens within CherryPy (since there is the cherrypy.tree.mount method) due to that config. E.g., tree.flask_app_name if you're just mapping that one app in that dict (there can be many tree directives, all of them just getting merged into the paths namespace) or tree.wsgi_delegates if you map many apps in that dict.

Using CherryPy to serve additional content without making an app of it

Another side note, if you want cherrypy to e.g. provide static file service for your app, you don't have to create a boilerplate cherrypy app to hold that configuration. You just have to mount None with the appropriate additional config. The following files would suffice to have CherryPy to serve static content from the subdirectory 'static' if they are put into the directory where you launch cherryd to serve static content (invoke cherryd as cherryd -c cherrypy.conf -i my_flask_server -i static:

static.py

import cherrypy
# next line could also have config as an inline dict, but
# file config is often easier to handle
cherrypy.tree.mount(None, '/static-path', 'static.conf')

static.conf

# static.conf
[/]
tools.staticdir.on = True
tools.staticdir.root = os.getcwd()
tools.staticdir.dir = 'static'
tools.staticdir.index = 'index.html'