Re: big responses to slow clients: Rack vs PSGI

[James Tucker <jftucker@gmail.com>]

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I agree with you, with a caveat. I think that the model is better for some
specific use cases.

_ry and I had this debate at reasonable length back before node, when he
was writing flow and I prepped the first thin async patch/rack hack.

The fundamental problem here is a competition of use cases and ideals. It
would be ideal for server authors if Ruby had lightweight coroutines that
could be scheduled across threads and relinquish control on IO. You can
emulate some of this with Fibers, and Neverblock went really far down this
path, as did Goliath. There's really no substitute for real user
schedulable goroutines and a decent IO subsystem though - we can build the
latter, but the prior is squarely in MRI's control - and they're now headed
further down the path of thread isolation.

Skipping a very large background and diatribe of these choices in MRI, I
will say that the chosen path is really not suitable for very high scale
processes. To be clear, I'm not saying "ruby doesn't scale" in the sense
that we can't use it for services that handle Mqps and up. You can - it's
not cheap, but it's entirely doable, and the engineering cost is on par
with other choices, assuming reasonable problem solving. What I am saying
is that Ruby, as designed and where it is headed, will not be ideal for
tens of thousands of open sockets, or more. It does not have enough
lightweight datastructures. It doesn't have the right IO primitives, and it
doesn't have the right concurrency primitives. The newer concurrency
primitives that are being discussed also make solving the general case of
this problem harder. The general case assumes that task latencies are
unpredictable, and in that space, you need a scheduler that can perform
memory and cpu offloading. MRI will not deliver either of those
capabilities.

The reason I describe the above, is that if we accept that Ruby is only
good for limited scale and cost per process, then we can view the problem a
little differently. Instead of trying to force it, or constantly work
around those limitations in library level systems, we can work around the
problem in flow control and traffic management systems. Indeed this is what
we do today, though as you note, often by accident/side effect. The kinds
of catastrophic load balancing challenges such as the rap genius saga will
remain, and slow client challenges will also. We can deal with them. At
scale you never really get to escape these anyway, except by engineering
systems to specifically combat those challenges. Either you're so fast and
efficient that attacks are impractical, or you start needing to put
appropriate limitations in place in upstream defenses. Ultimately the prior
is brittle and as such the latter eventually gets deployed by
SRE/sysadmins/ops folks.

I also don't mean to say that it would be bad to explore some more of these
ideas. I think it would, particularly if it could lead to more examples of
what servers need in order to be efficient, as strong cases for MRI/Ruby
designers to consider. I also think there are good cases to be made for
alternative servers for specific use cases - I know you specifically Eric
have done great work in this area. I would encourage you, absolutely, to
freely depart from Rack for those use cases. I'd also be really happy to
eat my words if you find some way of taking on elixir style scalability
with MRI, though on a personal level I don't know if it's worth the time.
As always, thank you for everything you've done, and for the discussion.

On Nov 15, 2016 3:10 PM, "Eric Wong" <e@80x24.org> wrote:

> I've been poking around in Plack/PSGI for Perl5 some months,
> and am liking it in some ways more than Rack.
>
> This only covers server-agnostic web applications; IMHO exposing
> applications to server-specific stuff defeats the purpose of
> these common specs.
>
> In Rack, one major problem I have is streaming large responses
> requires calling body.each synchronously.
>
> For handling writing large responses to slow clients, this means
> a Rack web server has 2 choices:
>
>
> 1) Block the calling Thread, Fiber, or process until the
>    slow client can consume the input.  This hurts if you have
>    many slow clients blocking all your threads.
>
>       body.each { |buf| client.write(buf) }
>       body.close
>
>    Simple, but your app is at the mercy of how fast the client
>    chooses to read the response.
>
>
> 2) Detect :wait_writable/:wait_readable (EAGAIN) when writing to
>    the slow client and start buffering the response to memory or
>    filesystem.
>
>    This may lead to out-of-memory or out-of-storage conditions.
>
>    nginx does this by default when proxying, so Rubyists are
>    often unaware of this as it's common to use nginx in front
>    of Rack servers for this purpose.
>
>    Something like the following should handle slow clients
>    without relying on nginx for buffering:
>
>       tmp = nil
>       body.each do |buf|
>         if tmp
>           tmp.write(buf)
>         else
>           # the optimistic case:
>           case ret = client.write_nonblock(buf, exception: false)
>           when :wait_writable, :wait_writable # EAGAIN :<
>             tmp = Tempfile.new(ret.to_s)
>             tmp.write(buf)
>           when Integer
>             exp = buf.bytesize
>             if exp > ret # partial write :<
>                tmp = Tempfile.new('partial')
>                tmp.write(buf.byteslice(ret, exp - ret))
>             end
>           end
>         end
>       end
>
>       if tmp
>         server_specific_finish(client, tmp, body)
>       else
>         body.close if body.respond_to?(:close)
>       end
>
>    Gross; but smaller responses never get buffered this way.
>    Any server-specific logic is still contained within the
>    server itself, the Rack app may remain completely unaware
>    of how a server handles slow clients.
>
>
>
> PSGI allows at least two methods for streaming large responses.
> I will only cover the "pull" method of getline+close below.
>
> Naively, getline+close is usable like the Rack method 1) for
> body.each:
>
>       # Note: "getline" in Plack/PSGI is not required to return
>       # a "line", so it can behave like "readpartial" in Ruby.
>       while (defined(my $buf = $body->getline)) {
>           $client->write($buf);
>       }
>       $body->close;
>
> ...With all the problems of blocking on the $client->write call.
>
> On the surface, the difference between Rack and PSGI here is
> minor.
>
>
> However, "getline" yielding control to the server entirely has a
> significant advantage over the Rack app calling a Proc provided
> by the server: The server can stop calling $body->getline once
> it detects a client is slow.
>
>       # For the non-Perl-literate, it's pretty similar to Ruby.
>       # Scalar variables are prefixed with $, and method.
>       # calls are "$foo->METHOD" instead of "foo.METHOD" in Ruby
>       # if/else/elsif/while all work the same as in Ruby
>       # I will over-comment here assuming readers here are not
>       # familiar with Perl.
>
>       # Make client socking non-blocking, equivalent to
>       # "IO#nonblock = true" in Ruby; normal servers would only
>       # call this once after accept()-ing a connection.
>       $client->blocking(0);
>
>       my $blocked; # "my" declares a locally-scoped variable
>
>       # "undef" in Perl are the equivalent of "nil" in Ruby,
>       # so "defined" checks here are equivalent to Ruby nil checks
>       while (defined(my $buf = $body->getline)) {
>           # length($buf) is roughly buf.bytesize in Ruby;
>           # I'll assume all data is binary since Perl's Unicode
>           # handling confuses me no matter how many times I RTFM.
>           my $exp = length($buf);
>
>           # Behaves like Ruby IO#write_nonblock after the
>           # $client->blocking(0) call above:
>           my $ret = $client->syswrite($buf);
>
>           # $ret is the number of bytes written on success:
>           if (defined $ret) {
>               if ($exp > $ret) { # partial write :<
>
>                   # similar to String#byteslice in Ruby:
>                   $blocked = substr($buf, $ret, $exp - $ret);
>
>                   last; # break out of the while loop
>               } # else { continue looping on while }
>
>           # $! is the system errno from syswrite (see perlvar manpage
>           # for details), $!{E****} just checks for $! matching the
>           # particular error number.
>           } elsif ($!{EAGAIN} || $!{EWOULDBLOCK}) {
>               # A minor detail in this example:
>               # this assignment is a copy, so equivalent to
>               # "blocked = buf.dup" in Ruby, NOT merely
>               # "blocked = buf".
>               $blocked = $buf;
>
>               last; # break out of the while loop
>           } else {
>               # Perl does not raise exceptions by default on
>               # syscall errors, "die" is the standard exception
>               # throwing mechanism:
>               die "syswrite failed: $!\n";
>           }
>       }
>       if (defined $blocked) {
>           server_specific_finish($client, $blocked, $body);
>       } else {
>           $body->close;
>       }
>
> In both my Rack and PSGI examples, I have a reference to a
> server_specific_finish call.  In the Rack example, this method
> will stream the entire contents of tmp (a Tempfile) to the
> client.
>
> The problem is tmp in the Rack example may be as large as
> the entire response.  This sucks for big responses.
>
> In the PSGI example, the server_specific_finish call will only
> have the contents of one buffer from $body->getline in memory at
> a time.  The server will make further calls to $body->getline
> when (and only when) the previous buffer is fully-written to the
> client socket.  There is only one (app-provided) buffer in
> server memory at once, not entire response.
>
> Both server_specific_finish calls will call the "close" method
> on the body when the entire response is written to the client
> socket.  Delaying the "close" call may make sense for logging
> purposes in Rack, even if body.each is long done running, and is
> obviously required in the PSGI case since further "getline"
> calls need to be made before "close".
>
>
> The key difference is that in Rack, the data is "pushed" to the
> server by the Rack app.  In PSGI, the app may instead ask the
> server to "pull" that data.
>
>
> Anyways, thanks for reading this far.  I just felt like writing
> something down for future Rack/Ruby-related projects.  I'm not
> sure if Rack can change without breaking all existing apps
> and middlewares.
>
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>
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