Blog Redirects, Azure Style

Last year, I set up an AppEngine app to redirect the old blog’s URLs to the WordPress install. It’s a relatively simple Flask web application, although it turned out to be around 700 lines of code (quite a bit to just serve redirects). While it ran fine for over a year on Google Cloud without me touching anything, and fitting into the free tier, I had to move it, as part of my divestment from GSuite (which is only vaguely linked to me leaving Google).

I could have just migrated the app on a new consumer account for AppEngine, but I decided to try something different, to avoid the bubble, and to compare other offerings. I decided to try Azure, which is Microsoft’s cloud offering. The first impressions were mixed.

The good thing of the Flask app I used for redirection being that simple is that nothing ties it to any one provider: the only things you need are a Python environment, and the ability to install the requests module. For the same codebase to work on AppEngine and Azure, though, there seems to be a need for a simple change. Both providers appear to rely on Gunicorn, but AppEngine appears to be looking for an object called app in the main module, while Azure is looking for it in the application module. This is trivially solved by defining the whole Flask app inside application.py and having the following content in main.py (the command line support is for my own convenience):

#!/usr/bin/env python3

import argparse

from application import app


if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument(
        '--listen_host', action='store', type=str, default='localhost',
        help='Host to listen on.')
    parser.add_argument(
        '--port', action='store', type=int, default=8080,
        help='Port to listen on.')

    args = parser.parse_args()

    app.run(host=args.listen_host, port=args.port, debug=True)

The next problem I encountered was with the deployment. While there’s plenty of guides out there to use different builders to set up the deployment on Azure, I was lazy and went straight for the most clicky one, which used GitHub Actions to deploy from a (private) GitHub repository straight into Azure, without having to install any command line tools (sweet!) Unfortunately, I hit a snag in the form of what I think is a bug in the Azure GitHub Action template.

You see, the generated workflow for the deployment to Azure is pretty much zipping up the content of the repository, after creating a virtualenv directory to install the requirements defined for it. But while the workflow creates the virtualenv in a directory called env, the default startup script for Azure is looking for it in a directory called antenv. So for me it was failing to start until I changed the workflow to use the latter:

    - name: Install Python dependencies
      run: |
        python3 -m venv antenv
        source antenv/bin/activate
        pip install -r requirements.txt
    - name: Zip the application files
      run: zip -r myapp.zip .

Once that problem was solved, the next issue was to figure out how to set up the app on its original domain and have it serve TLS connections as well. This turned out to be a bit more complicated than expected because I had set up CAA records in my DNS configuration to only allow Let’s Encrypt, but Microsoft uses DigiCert to provide the (short lived) certificates, so until I removed that it wouldn’t be able to issue (oops.)

After everything is set up, here’s a few more of the differences between the two services, that I noticed.

First of all, Azure does not provide IPv6, although since they use CNAME records this can change at any time in the future. This is not a big deal for me, not only because the IPv6 is still dreamland, but also because the redirection would point to WordPress, that does not support IPv6. Nonetheless, it’s an interesting point to make, that despite Microsoft having spent years preparing for IPv6 support, and having even run Teredo tunnels, they also appear to not be ready to provide modern service entrypoints.

Second, and related, it looks like on Azure there’s a DNAT in front of the requests sent to Gunicorn — all the logs show the requests coming from 172.16.0.1 (a private IP address). This is opposite to AppEngine that shows the actual request IP in the log. It’s not a huge deal, but it does make it a bit annoying to figure out if there’s someone trying to attack your hostname. It also makes it funny that it’s not supporting IPv6, given it does not appear to need for the application itself to support the new addresses.

Speaking of logs, GCP exposes structured request logs. This is a pet peeve of mine, which GCP appears to at least make easier to deal with. In general, it allows you to filter logs much more easily to find out instances of requests being terminated with an error status, which is something that I paid close attention to in the weeks after deploying the original AppEngine redirector: I wanted to make sure my rewriting code didn’t miss some corner cases that users were actually hitting.

I couldn’t figure out how to get a similar level of detail in Azure, but honestly I have not tried too hard right now, because I don’t need that level of control for the moment. Also, while there does seem to be an entry in the portal’s menu to query logs, when I try it out I get a message «Register resource provider ‘Microsoft.Insights’ for this subscription to enable this query» which suggests to me it might be a paid extra.

Speaking of paid, the question of costs is something that clearly needs to be kept in clear sight, particularly given recent news cycles. Azure seems to provide a 12 months free trial, but it also gives you £150 of credit for 14 days, which don’t seem to match up properly to me. I’ll update the blog post (or write a new one) with more details after I have some more experience with the system.

I know that someone will comment complaining that I shouldn’t even consider Cloud Computing as a valid option. But honestly, from what I can see, I will be likely running a couple more Cloud applications out there, rather than keep hosting my own websites, and running my own servers. It’s just more practical, and it’s a different trade-off between costs and time spent maintaining thing, so I’m okay with it going this way. But I also want to make sure I don’t end up locking myself into a single provider, with no chance of migrating.

One thought on “Blog Redirects, Azure Style

  1. If the requests are all coming front the same address, then that’s SNAT (not DNAT). Are there any http headers with the original IP address?

    Like

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