True, and in the case of the HN page that was up during the outage it was actually the intended content page at the time due to some backend problems, it was a customized static page not an error page.

Not every webpage is created equally.

When we say API, we usually think of it as Twitter API that sort of thing, which we can send a JSON or XML and get back something easily parasable.

Sites like HN doesn't have that kind of service. Instead, it returns HTML. That's fine. No one said API can't return HTML.

When some part of the Twitter API becomes unavailable, the API server should return 503 when we asks those APIs to return response. If these bad APIs are also used to power some part of the frontend, then the frontend will not work properly. For example, viewing thumbnail from dashboard may be down. But the rest of the page is functioning. In that case, you can't send 503 from the frontend. It doesn't make sense. So if any crawler reading Twitter.com it shouldn't see 503 when it hit the home page.

When HN is down, it should return 503. There are two reasons. First, HN is not functional anymore. The page you see may just be a maintenance page configured in Nginx, much like 404 error page. Secondly, HN is itself an API service. When it is broken, it is broken. It just doesn't work anymore. When you try to access it through a Python script, it doesn't return in the format you expect it to return. When you debug you realize it is not returning anything like you were expecting because most of the HTML structure is gone. This is not an API format change. It's simply the backend is not functional anymore.

And semantically, when your site is under a maintenance mode, 503 makes sense. 200 is not that evil. It just doesn't give anyone a better clue.

That is ridiculous. You think the status intended to be conveyed for all HN URLs was "it is up"? It was intended that any automated tools trying to figure out if HN was up or not would decide "Yes, it's up."?

If that was intended, then it was a poor, mis-guided, un-helpful intention.

But HN worker has already said it was not intended, there was not much intention involved at all, they just had other things to worry about (getting the site back up) and weren't thinking about it, due to lack of a pre-planning for how to handle an outage. https://news.ycombinator.com/item?id=7016141

At any rate, your ideology of HTTP status codes does not seem to match that of the actual HTTP designers, or of anyone trying to actually use HTTP status codes for anything. If you aren't going to use HTTP status codes for anything, then it hardly matters what they are, so there's no point in arguing about it. But as soon as you try writing software that uses HTTP status codes for anything, you will start hating sites that give you error messages with 200 OK response codes.

Precisely, I think people try to put too much semantic meaning into HTTP status codes. If you read the spec, most of the exotic semantics are actually part of the WebDAV spec and not actually the HTTP spec.

For something like a pure REST API, basic status codes are fine. It's when you start breaking the REST abstraction (which a human-readable landing page surely does) that it gets tempting to start misusing status codes.

For any kind of non-RESTful or procedural API, HTTP status codes are simply not adequate and application-specific error handling is necessary (HTTP response 200, app-specific-error 999, etc.).

If you don't think any meaning should be taken from http status codes, then why set/send them at all, why do they exist?

If any meaning is going to be taken from them at all, then the difference between "OK" and "Server Error" seems pretty basic and fundamental.

Well, to illustrate my point just look at the WebDAV spec vs the HTTP spec. WebDAV shows how you can add more semantics to HTTP response codes.

But when you think about it, for a pure REST API you really only need classic HTTP response codes.

Response codes only make sense in the context of a resource. Once you introduce query strings or start to stretch the meaning of HTTP verbs the abstraction starts to leak.

So when designing an API it's smart to just handle errors at the application level rather than at the protocol level.

Protcol level errors are for things that are outside of the application. An error response with data validation errors should return a 200, for example.