Cold Paths

Faced with yet another crisis caused by a bug hidden in a cold path, I found myself Googling for a quick link to Slack out to the engineering team about cold paths. Unfortunately, I can’t find a focused write-up; and so here I am writing this.

A cold path is a path through the code or situation that rarely happens. By contrast, hot paths happen frequently. You don’t find bugs in hot paths. By nature, bugs are found in places that you didn’t think to look. Bugs are always in cold paths — every bug is found in a path colder than all the paths you tested.

Here are some real world “cold paths” with big consequences:

Rare events are hard to predict. That’s just the nature of them. As engineers, I belive it’s our responsibility to do our best to try harder and get better at planning for these rare bugs. Is that it? Try harder?

Better: Don’t have cold paths

Smaller programs

I watched one of Gil Tene’s many amazing talks on Azul’s C4 garbage collector (not this talk, but similar) where he claimed that normally it takes 10 years to harden a garbage collector. Azul didn’t have 10 years to produce a viable business, so they avoided almost all cold paths in the collector and they were able to harden it in 4 years (I never tried verifying this claim).

For a garbage collector, this means things like offering fewer options, or having a simpler model to avoid cold paths around promoting objects between generations. For your app it will mean something different.

You can test less to achieve high quality by reducing the size of your application. Less edge cases is equivalent to less testing surface area, which implies less testing work and fewer missed test cases. There’s something to be said for avoiding config options and making solutions less generic.

Avoid fallbacks

While I worked at AWS I had this beaten into my skull, but thankfully they’ve published guidence an excellent piece titled “avoiding fallback in distributed systems”. The hope is that, when system 1 fails you would like to automatically fallback to system 2.

For example, let’s say we have a process that sends logs to another service. For the hot path, we send logs directly via an HTTP request. But if the log service fails (e.g. overloaded, maintenence, etc.) we fallback by writing to a file and have a secondary process send those logs to the service when it comes back.

  • System 1: directly send logs to server
  • System 2: send asynchronously via file append

If system 2 is more reliable than system 1, then why don’t we always choose system 2? Always write to the file and ship logs asynchronously rather than send directly to the server. This is surprisingly strong logic that isn’t considered often enough. More often, by asking the question you end up finding a way to make system 1 more robust.

In cases where fallback can’t be avoided they suggest always exercising the fallback. For example, on every request, randomly decide to use either system 1 or system 2, thereby ensuring that the cold path isn’t cold because both are exercised on the hot path, at least sometimes.

Know your capacity for testing

In “files are fraught with problems”, Dan Luu demonstrates that it’s unexpectedly difficult to write a file to disk correctly. Juggling issues like handling random power loss or strange ext4 behavior becomes a full-time job. It’s a lot to keep in your head, just to write a file.

Is it better to:

  1. Ignore the cold paths and hope for the best
  2. Correctly implement & test each file write event and ship late
  3. Use a system that does it correctly for you, like MySQL or SQLite

Choice #3 delegates the testing of all those pesky cold paths to a 3rd party. Therefore, #3 is always the best choice, unless your company is in the file writing business (e.g. you’re AWS and working on DynamoDB or S3).

Alternnate take on the same idea: Choose boring technology


The practice of avoiding cold paths is often presented as “simple code”. Unfortunately, “simple” has such wildly varying meanings that it’s often antagonistic to use it outside a mathematical setting. I’ve found that centering conversations around “avoiding cold paths” gives more clarity on how to proceed.

In system design, the conversation about what is “simple” is even tougher due to the amorphous nature of it. The principle of “avoiding cold paths” can be extended to mean, “delegating cold paths” to a trusted third party, like an open source project or a cloud provider. An earnest discussion about your capacity for testing might be appropriate. It lets you disengage from “building cool stuff” and instead view it as “testing burden I’d rather not have”.