You have arrived at the mid-week hump. Have a $container of $beverage and enjoy some reading before you speed along with the rest of your life.

New articles

Tagged data in Haskell (SICP 2.4.2)

Structure and Interpretation of Computer Programs, the well-known wizard book, teaches us to (maybe not, after all?) tag data to distinguish multiple representations of the same value, and to be able to create generic functions compatible with all representations. But SICP is written with Lisp in mind. What happens if we try to translate it to Haskell?

Full article (5–13 minute read): Tagged data in Haskell (SICP 2.4.2)

Flashcard of the week

Some wisdom around system robustness.

Why should you not test external services your system depends on in a liveness probe?

A liveness probe, a type of health check, is something that pokes your system to see if it's responding at all. If it's not, it should be restarted.

If an important external service is down, the system may be unable to generate a sensible response to some requests, so it might seem like a good idea to include that important external service in the liveness probe, i.e. fail the liveness probe if the external service does not respond timely.

However, that would be incorrect, and there's a simple reason why:

Restarting our system cannot fix problems with external services.

We probably want to include external services in some of our health checks, but a health check tied to system restarts – a liveness probe – is not the right place.

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