Re: [boost] [http] Formal Review

Subject: Re: [boost] [http] Formal Review
From: Vinícius dos Santos Oliveira (vini.ipsmaker_at_[hidden])
Date: 2015-08-15 15:22:27

2015-08-14 2:06 GMT-03:00 Vinícius dos Santos Oliveira <
vini.ipsmaker_at_[hidden]>:

> 2015-08-13 22:29 GMT-03:00 Lee Clagett <forum_at_[hidden]>:
>
>> On Thu, Aug 13, 2015 at 11:43 AM, Vinícius dos Santos Oliveira <
>> vini.ipsmaker_at_[hidden]> wrote:
>>
>> > 2015-08-12 19:27 GMT-03:00 Lee Clagett <forum_at_[hidden]>:
>> >
>> > > Anyway - I was thinking along the same lines at various points.
>> Having a
>> > > function that pre-generates HTTP messages is very useful IMO, and
>> should
>> > > likely be included. Designing a good parsing concept would be a bit
>> more
>> > > work I think, but probably worth it too. I'm not sure how the author
>> > > intends to swap out parsers in the current design. Having a fixed
>> parser
>> > > seems acceptable, but the author almost seemed to suggest that it
>> could
>> > be
>> > > selectable somehow.
>> > >
>> >
>> > A parser doesn't make sense for all communication channels.
>> >
>> > Do you have an example of a communication channel where a parser concept
>> wouldn't work? They wouldn't necessarily always provide the same output or
>> behave the same way, but a communication channel has a defined format. Any
>> implementation reading that format generally has _some_ output, which is
>> pretty much a parser IMO. Sorry for the bikeshedding on this, its not
>> really necessary, but this stuck out for some reason.
>>
>
> Well, in my previous answer, I think I ended focusing on the wrong part of
> the proposal. Let me fix this issue in this email. And thank you for
> helping me figuring out my mistake (or "keep insisting" or "not losing hope
> on me", what you prefer).
>
> To handle a HTTP request, you read the metadata, progressively download
> the body and then the trailers. It's wise to avoid reading partial metadata
> because the request can only be handled after the whole metadata has been
> read. However, the body can be handled as it is received.
>
> You're arguing that you always (1) fill a buffer and then (2) parse it.
> CGI uses environment variables, not a contiguous chunk of memory or stream
> of bytes. It still could work, though. The headers would be serialized into
> the buffer (not nice).
>
> Using the buffer/view approach, messages are always in serialized format.
> Unless you store/cache the parsing result (doing allocation or some fixed
> size, as you do not know amounts ahead-of-time), this will consume more
> time to handle, as you'll need to reparse every time an information is
> asked ("give me header host", "give me header cookie"). If you do store
> parsing result, you're just storing the message using a
> masked-as-not-message-based-when-it's-not API. And unless your buffer is
> used to store more info than the real network traffic, the view needs to be
> get information from the socket too, not just the buffer. It's not a pure
> parser, there is state not found on the buffer (imagine how you'd handle
> progressive download where lots of the traffic was already discarded to
> handle the rest of the message), so the view needs the socket, which is
> storing information not present in the buffer. It's like wasting much more
> CPU usage to avoid some more memory usage. These are just the basic changes
> of impact.
>
> On a high-level side of thinking, buffer/view approach makes all requests
> immutable by default. You cannot fake or inject data in the headers while
> you pass the headers along a chain of handlers. There are workarounds.
> Also, if you cannot forge the HTTP message, you cannot create it and send
> to the socket. You always need to use the generator. The problem with the
> proposed generator is the lack of consideration for other HTTP backends.
> More care need to be given to capabilities like happens in Boost.Http (is
> chunking available? 100-continue? can I upgrade? ...?). Also, like I've
> stated in one of the previous emails, it's tricky to get interoperability
> right with these not explicit generators (is chunking going to be
> implicitly used?).
>
> I need to think more, maybe I'll send another email with more comments.
> You can have these meanwhile.
>

Okay, I though enough about the proposal.

I was thinking how I could write an implementation for such proposal.

The problem is that you'd have to keep the headers (the whole metadata
actually) in the buffer as long as the current request-response pair isn't
finished. If you use a fixed-size buffer, you may never be able to finish
the request, even if you progressively download the body. And if the
view/parser only reads the buffer, progressive download is already kind of
hard and may very well involve a complex API. If the view also takes the
socket (not only the buffer) as argument, this may be partially addressed.
With this approach, the message will have to be reparsed so many times that
you may as well just expose an HTTP parser to the user and don't try to
provide anything else.

The parser approach is too low level. And this approach to turn it into a
high level API kills a lot of HTTP features/use cases like progressively
downloading the body, a least with this low amount of investment.

Also, you mixed two proposals (actually a little more) into one:

   - A design closer to tcp::socket.
   - The socket+buffer+parser/view approach.

And I may want to criticize a point that is a consequence of only one
proposal, but with the mixed solution, I end up criticizing both. I'll
trust your judgement to correctly separate which critique goes to which
proposal.

-- 
Vinícius dos Santos Oliveira
https://about.me/vinipsmaker