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Comment by TobTobXX | Hacker News Reader

There are two hard problems in IT: cache invalidation, naming things and off-by-one errors.

Github raw seems like the simplest system for solving cache invalidation: invalidate the cache of a changed file when it’s pushed.

They have access to both GitHub and the raw service. I know there are usually all sorts of layers between that make interconnectivity logistically complicated, but am I wrong that at the top-level it’s that simple?

That is too simple for the feature they are using. The client itself has its own cache and the only way to fully prevent traffic from a client is to tell it content it caches will remain valid for some amount of time into the future.

For URLs that return the latest entry there is no valid amount of time known in advance by GitHub unless they want to introduce mandatory publication delays. For URLs of specific change sets, they should never be corrected again and an infinite cache is pretty much valid unless a user overrides good git practices.

I think GitHub frequently misidentifies which scenario they are in and when they return 1 day for a current state URL users notice, while when they return 5 minutes for a permanent change set that gets a lot if traffic they lost network capacity.

Isn’t this where stuff like ETAGs are supposed to help? Not completely solve, but at least help reduce the problem a bit more?

Yes, but cache invalidation is a hard problem, so most services side-step etags (+ if-none-match), only to hit caches elsewhere.

> The client itself has its own cache

Let’s leave this aside since it both has known client-side mitigations, and is not the cause of the issue that was posted.

Once you are using one style of caching its usually a mistake to introduce another even if the style is marginal. They very clearly have an issue with max age with clients/CDNs in the second half of the thread and probably have similar problems on internal transparent proxies, etc.

Tag it with etag = hash, done. The client side isn't hard part.

The server side would require pushing any invalidation to (I imagine) whole tree of caches, which isn't exactly that hard if you plan for it from the start and have some way of upstream telling downstream file changes, but, well, they probably don't as I'd imagine they didn't expected people to pin their infrastructure to some binary blob on github that mutates

Etag doesn't let you "fully prevent traffic from a client" (GP's exact words). They'll still send a request to which you need to reply with a 304 after checking the resource.

They wouldn’t even need to check the resource. If the hash (which you get for free since it’s part of the git commit) was the etag then they can quickly reply to that request from an edge.

You get it "for free" if you load up the repo and check some files. That's not free at all.

In fact, loading a file by name from a Git repo is rather expensive, and is definitely not the way their CDN should be keeping things in cache (gotta load ref, uncompress and parse commit object, uncompress and read tree object(s), just to get the blob's hash. Every one of those objects is both deflated and delta-encoded.)

Nono: don't do it at lookup, set the etag when the repo is pushed.

I'm open to the idea that it's less computation to simply hash the files instead of deflate and decide, but my point was the hash is already calculated on the client when the changed file is added to the repo.

You could also just commit a file with the intended commit hash, make that the indicator for changes and use the commit in other requests. Has the added benefit that clients only need to fetch a tiny file if nothing changed.

GitHub responded that it was simply a bug in that Cache-Control was set to a day instead of 5 minutes. It's already been fixed.

There are actually only two hard problems in computer science:

0) Cache invalidation

1) Naming things

5) Asynchronous callbacks

2) Off-by-one errors

3) Scope creep

6) Bounds checking

You forgot the 'Segmentation Fault' at the last line

It is on the list, but was never written to stdout because of a NULL pointer exception.

Haven't seen that variant before! Thanks.

asynchronous callbacks are more of "wrong solution" than hard problem...

Bounds checking as separate from off-by-one just means you stop using C arrays. That's not hard. And why point at callbacks specifically? And scope creep is not a computer science problem; it's easy to avoid if people decide to avoid it.

So this list is too bloated for the joke to work well, I think. Even before we talk about how off-by-one gets ruined this way.

7) Yoda logic

And the oxford comma as well, apparently

This is not actually a situation where the Oxford comma disambiguates, though I also had the gut feeling one should be there

That's clearly a type of off by one error.

You multithreading have forgotten.

Cache invalidation is a large class of multi threading problems, that’s just as much an issue between threads as between servers.

You can eliminate the cache invalidation problem by not reusing old names for new things.

newester_new_final_v4_new.txt

Doesn't that just move the problems to more "naming things" though?

Yes, naming things is intimately tied to cache invalidation. That's why the two things are together in that maxim. Not sure why people think it has to so with naming variables or functions...

> That's why the two things are together in that maxim.

I think you missed the joke.

Did they?

I _think_ it became a joke, accidentally.

> There are two hard problems in IT: cache invalidation, naming things and off-by-one errors.

That's three things with an off-by-one error.

> Not sure why people think it has to so with naming variables or functions...

I think it’s open to interpretation. And I think it is valid to apply the saying to naming of variables, functions, and many other things.

See

https://skeptics.stackexchange.com/questions/19836/has-phil-...

Links to

https://www.karlton.org/2017/12/naming-things-hard/

Links to a bunch of places.

None of the ones I looked at seem to confidently say what was originally meant by the saying.

Either way, regardless of what the guy meant when he originally used to say it, it’s allowed to apply a saying to new situations.

That doesn't solve cache invalidation; that just means you're always invalidating the cache even in cases where you don't actually want to.

You just solve that by using another level of indirection. Duh.

Content addressing causes the name to change only when the content changes, which also means the name doesn't change if the content doesn't change, this by definition you don't have spurious cache invalidations

And how do you check what the new name of the content you haven't seen is?

Serious question? There will be a root object that has a stable name through updates. When you write that, you embed current content hashes in it.

Someone could get the cached of it, of course. But since it content addresses what it links to, it should avoid incoherent groups of cached data.

I agree this would provide for increased coherency, but the additional indirection is hardly ideal. And really it's just kicking the caching problem up to a super-object that will probably be thrashed much harder than any individual file, and may be larger than each individual file.

All in all it's a "certified hard problem", there's a bunch of domain specific nuance an HN thread couldn't hope to capture.

The extra indirection may or may not be a problem, of course. For a lot of places this is used, it is "index.html," after all.

That said, I do find folks bend way too many things into fitting this pattern; such that I do not mean to be dismissive of the criticism.

But where client gets the new name it should download for?

As long as name client asks for (even if it is reference) is constant, it can have cache problems. Of course it makes it simpler as youc an opt to cache it so short that invalidation is less of an issue, but that's again working around cache invalidation

There are many problems with that though. For example, if your CSS changes you change the filename... but now you need to change the HTML file that references it. You can't change that easily.

Or what if your CSS change just deletes some unused classes... it'd be fine for users to keep the old version until it expires. If you rename the resource you'll be causing a lot of users to wait unnecessarily. Not a huge problem, unless you're Meta or Google.

And so on.

When people say cache invalidation is hard it's best to believe them, because it is.

Sounds a little bit like the standard recipe of any functional programming enthusiast: Just use pure functions!

Except that they‘re colliding all the time with that tiny problem that the world isn‘t pure... People expect the same page at the same URL, with different content, tomorrow, so good luck choosing a different one than the one they have bookmarked and that ranks on Google.

Caching doesn‘t get any less hard by trying to define the problem away.

Not using old names for new things in this case means abandoning the concept of Git branch tags.

The Oxford comma would be useful here

You pretty much never have a list where "things" is somewhere other than the end.

The edit adding ", concurrency," and updating "two" to "three" arrived before the message, so it wasn't applied.

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