Rate Limiting

Date: Wed 2023-03-29

Permalink: https://www.dominic-ricottone.com/posts/2023/03/rate-limiting/

In October, my public MoinMoin wiki started getting hammered with rapid requests. It wasn’t a major concern of mine at first.

  1. It was nowhere near the scale of a DOS attack.
  2. MoinMoin has built-in surge protection that always allows logged-in users access.
  3. As a principle, I’m in favor of public documents remaining truly public. Even to potentially bad actors.

But then it started impacting my uptime percentage. Oh boy do I love seeing that perfect, bright-green 100%. And it turns out, that’s all the motivation I need to start walking back a principle.

I’ve been using upptime for quite a while as my uptime monitor/tracker. It’s an exceedingly simple system that can be hosted freely on GitHub Pages. Well, as long as you are willing and able to tweak with the CI/CD limits. It ships with some defaults that are a touch too far for an unpaid account.

The major advantage to upptime is that my monitor is 100% isolated from my true production servers. The CI/CD does all maintenance automatically. Certificate expirations, host uptime, and end user error are all neatly folded out of the equation.

The extra advantage is GitHub Issues as a web admin ticketing system. For the most part, I host services by and for myself. It would never be worthwhile to spin up a ticketing system just for server administration. But there are still lessons learned and personal notes that I would ideally staple to incident reports, even just for myself. GitHub Issues is a clunky beast, but it’s more than enough for this use case.

This is all to say, you can find a timeline of events and a conversation I had with myself at #52.

I did briefly wonder if upptime was in error, and reporting a false positive. I certainly could not replicate the issues. And it wouldn’t be the first time I needed to tweak the CI/CD due to GitHub’s race to the bottom on unpaid account limits. As I now know, my inability to replicate was a result of MoinMoin’s clever surge protection.

I did a trial run with Uptime Robot but quickly realized that it reported the same issues. And if an uglier, paywalled monitor (that has no ticketing system!) was going to give me the same outcomes, why would I give it another thought? Haven’t logged in since.

I then did some exploration into the realm of building my own monitor on another cloud platform (GCP?). That didn’t make it even as far as a trial run.

While this all began in October, it took me several months to take this seriously as a threat (to my uptime streak).

My first approach was to try to understand the spam. I added more and more thorough connection logging, and even some HTTP headers. I monitored logs in the hope that I could predict incidents–even intervene. I tried to parse metrics in the aftermath of incidents to better predict the next one.

This didn’t really amount to anything. As far as I can tell, I’ve randomly become the latest client (victim?) of a particularly zealous spider.

My second approach was sticky rate limiting. I have always sat my services behind an instance of HAProxy, because I’m a huge fan of premature optimization and building for scale that never actualizes. This made the introduction of rate limiting (and the aforementioned logging) incredibly simple.

I started with limits that mirrored MoinMoin’s own surge protection, figuring that someone more intelligent that I probably came up with those defaults. Those turned out to be a bit too loose, so I spent the next few months playing a back-and-forth tuning game.

In the end, I’ve landed on a configuration that has staved off spam and maintained uptime since mid-January. (Leaving aside a brief certificate expiration… oops.) I’m extremely pleased with this result, and my uptime streak is so beautiful.

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