[judofyr]

I’ve been doing some research for this earlier, and my conclusion was: This is very hard, if not impossible, to implement automatically. The main problem is that it’s impossible to handle exceptions correctly without making the whole stack aware of it.

Currently, when an exception occurs, the system can simply change the response (since the response hasn’t been sent to the client yet, but is only buffered inside the system). With this approach, a response can be in x different states: before flushing, after the 1st flushing, … and after the xth flushing. And after the 1st flushing, the status, headers and some content has been sent to the client.

Imagine that something raises an exception after the 1st flushing. Then a 200 status has already been sent, togeher with some headers and some content. First of all, the system has to make sure the HTML is valid and at least give the user some feedback. It’s not impossible, but still a quite hard problem (because ERB doesn’t give us any hint of where tags are open/closed). The system also need to take care of all the x different state and return correct HTML in all of them.

Another issue is that we’re actually sending an error page with a 200 status. This means that the response is cacheable with whatever caching rules you decied earlier in the controller (before you knew that an error will occur). Suddenly you have your 500.html cached all over the placed, at the client-side, in your reverse proxy and everywhere.

Let’s not forget that exceptions don’t always render the error page, but do other things as well. For instance, sometimes an exception is raised to tell the system that the user needs to be authenticated or doesn’t have permission to do something. These are often implemented as Rack middlewares, but with automatic flushing they also need to take care of each x states. And if it for instance needs to redirect the user, it can’t change the status/headers to a 302/Location if it’s already in the 1st state, and therefore needs to inject a <script>window.location=’foo’</script> in a cacheable 200 response.

Of course, the views shouldn’t really raise any exceptions because it should be dumb. However, in Rails it’s very usual in Rails to defer the expensive method calls to the view. The controllers sets everything up, but it’s not until it needs to be rendered that it’s actually called. This increases the possibilty that an exception is raised in the rendering phrase.

Maybe I’m just not smart enough, but I just can’t come up with a way to tackle all of these problems (completely automated) without requiring any changes in the app.

[runeb]

Good points, and I agree. Sending 200 OK before you know if the response truly is OK is problematic, at best. I'm not sure I agree with you about authentication / authorization. These should do their job before a controller gets a chance to render / first flush a view and would thus get a chance to send other headers. In the extreme the first middleware would flush a 200 header right away and then call down the middleware stack, but thats not how I read this proposal.

Anything that might happen while actually rendering a view is a concern here (and as your get more lazy that could be quite a lot), but you'd normally sort out auth before actually rendering/flushing anything.

[judofyr]

The authentication / authorization was merely an example, and I agree that most of these solutions are done before rendering occurs.

However, my point is that in order to take advantage of flushing, you want to start sending the HTML as soon as possible and this forces you to decide the status and the headers. If there's something in your stack which requires different status/header, you either need to evaluate it earlier in the request or hack around it by appending different HTML.

The more you decide to evaluate earlier, the less efficient becomes the flushing. So for every piece in the stack you have to make a choice: What is the chance that this requires different status/headers/content? How much do we gain by deferring? How can we hack around it if we've already started sending a response?

This is something you can't do automatically, and as far as I can see, this isn't mentioned in Yehuda's post at all.

[raggi]

It does require changes in the app, but app authors who need this kind of performance benefit will be willing to accept that hit. The solution is far better than the alternatives:

- Allowing users to flush manually (people screw this up real bad) - Changing the rack spec (allowing for #each on the body to be lazily yielded, and terminating on nil or the like) - Moving to an always async stack (totally kills most users)

Yes, there are plenty of issues with this, and I agree with your concern, but it is also something which can have a marked effect on performance for users. It's also worth noting that a well componentised partial can render an error in-place of the partial itself, for example, rendering a page that contains the whole layout, and a single red box of errors (say a render of the _new partial can be added to the buffer after a _create fails, instead of rendering the success box). Yes, that requires some refactoring of the application (rather than using for example, the standard 302 approach).

It's also worth noting that a larger class of applications that would find this actually useful should generally have reasonable test coverage and code maturity. Whilst this isn't always the case, we also don't protect users from eval, and other evil tools, in ruby or rails.

[judofyr]

From the post:

For Rails 3.1, we wanted a mostly-compatible solution with the same programmer benefits as the existing model, but with all the benefits of automatic flushing

And from there he goes on with very specific implmentation details and the only caveat is some API change. This gives the impression that this is something you can easily enable for any app.

I just want to point out that 100% automatic flushing is pretty much impossible with the current state of Rack/Rails, and there's still plenty of work before there's anything near flushing support in Rails.

In addition, everyone should be aware of the trade-off you're making with flushing (potentially sending 500 responses as 200 Ok etc.)

[raggi]

Yeah, I had already told Yehuda about the potential concurrency issues for apps switching over to this, as fibers are stacks and stack switching can lead to concurrency issues, especially dropping in a re-ordering of the render chain like this. He said this is another of the reasons it should be optional.

I'm surprised he didn't raise this in the article, but I guess the article was more about how it could work, than how it will in the final build, or how it should in all cases.

I've got to disagree partially, or at least present a different point of view about the 200/500 argument. It could be considered acceptable for the response to be a 200, as the server has not completely errored out, it is only a portion of the response that has errored, and at the application level. It seems that some apps would return a 500 in this case, and then render a page, suggesting that the server and app are broken. This is really something that could very quickly turn into a bikeshed discussion, but you can probably see my point even if you don't agree (I'm not sure I agree in all cases, but it is food for thought).

As you probably well know, I've been doing the async on rails and other frameworks game in ruby for about as long as anyone else that's publicly producing code in this arena (in ruby), and I have to say that this solution that they came up with is actually far better than most of the other hacks. It would be really nice if ruby had performant generators for yielding, but without them, we're left with less options. This fiber approach is getting us pretty close, and I think it's worth exploring, even if it turns out to be a bad idea for most people.

The Rack API won't really ever change significantly for the better in this arena. People don't seem to want to lose the simple #call returning a tuple protocol, and without making that either: a) asynchronous (in as much as not relying on a return value, but a call to a response object), or b) some significant changes to the contract for body, we can not really optimise many ways to provide both simplicity and reasonable levels of granular control of IO. ryah of node.js fame was writing an IO driven server years ago when I was first working on the async rack hack that is in thin, zbatery, rainbows and flow. We had a lot of discussions back then about how best to fit this stuff into the Rack API. We both desired to use something similar to Enumerator#next, but this would never fit much better into the existing setups, and as already noted, is not very performant by default.

As you say, this is not trivial, but I would argue that this means we need to experiment with more approaches, as neither of the currently presented solutions (my async api, and this fiber api) are ideal, nor is buffering large volumes of response data in memory, or taking a bigpipe / highly ajax style approach.

[judofyr]

I see your point and I agree that it's possible to make your app gracefully handle exceptions. However, there's no way Rails can automate it for you: you still have to carefully design your app to handle it.

If Rails can help you make it easier, that's great, I just don't see how it's a mostly-compatible solution as Yehuda wrote in the post.

I fully agree that we should explore this option, but at the same time we should make people aware of the trade-offs and not present it as some setting you can simply enable by config.automatic_flushing = true (which makes Rails do all the hard work).

Yehuda didn't even mention the word exception in the blog post, so I wasn't sure if was aware of the issues or not.

[runeb]

That breaks caching, as pointed out by judofyr. I can imagine having to remove caching will eat up the performance gains pretty fast.