NAME
Plack::App::CGIBin::Streaming - allow old style CGI applications to use
the plack streaming protocol
SYNOPSIS
in your app.psgi:
use Plack::App::CGIBin::Streaming;
Plack::App::CGIBin::Streaming->new(root=>...)->to_app;
DESCRIPTION
With Plack already comes Plack::App::CGIBin.
"Plack::App::CGIBin::Streaming" serves a very similar purpose.
So, why do I need another module? The reason is that Plack::App::CGIBin
first collects all the output from your CGI scripts before it prints the
first byte to the client. This renders the following simple clock script
useless:
use strict;
use warnings;
$|=0;
my $boundary='The Final Frontier';
print <<"EOF";
Status: 200
Content-Type: multipart/x-mixed-replace;boundary="$boundary";
EOF
$boundary="--$boundary\n";
my $mpheader=<<'HEADER';
Content-type: text/html; charset=UTF-8;
HEADER
for(1..100) {
print ($boundary, $mpheader,
'<html><body><h1>'.localtime()."</h1></body></html>\n");
$|=1; $|=0;
sleep 1;
}
print ($boundary);
Although multipart HTTP messages are quite exotic, there are situations
where you rather want to prevent this buffering. If your document is
very large for example, each instance of your plack server allocates the
RAM to buffer it. Also, you might perhaps send out the "<head>" section
of your HTTP document as fast as possible to enable the browser load JS
and CSS while the plack server is still busy with producing the actual
document.
"Plack::App::CGIBin::Streaming" compiles the CGI scripts using
CGI::Compile and provides a runtime environment similar to
"Plack::App::CGIBin". Compiled scripts are cached. For production
environments, it is possible to precompile and cache scripts at server
start time, see the "preload" option below.
Every single request is represented as an object that inherits from
Plack::App::CGIBin::Streaming::Request. This class mainly provides means
for handling response headers and body.
Options
The plack app is built as usual:
$app=Plack::App::CGIBin::Streaming->new(@options)->to_app;
@options is a list of key/value pairs configuring the app. The
"Plack::App::CGIBin::Streaming" class inherits from Plack::App::File.
So, everything recognized by this class is accepted. In particular, the
"root" parameter is used to specify the directory where your CGI
programs reside.
Additionally, these parameters are accepted:
request_class
specifies the class of the request object to construct for every
request. This class should implement the interface described in
Plack::App::CGIBin::Streaming::Request. Best if your request class
inherits from Plack::App::CGIBin::Streaming::Request.
This parameter is optional. By default
"Plack::App::CGIBin::Streaming::Request" is used.
request_params
specifies a list of additional parameters to be passed to the
request constructor.
By default the request constructor is passed 2 parameters. This list
is appended to the parameter list like:
$R = $class->new(
env => $env,
responder => $responder,
@{$self->request_params//[]},
);
preload
In a production environment, you probably want to use a (pre)forking
server to run the application. In this case is is sensible to
compile as much perl code as possible at server startup time by the
parent process because then all the children share the RAM pages
where the code resides (by copy-on-write) and you utilize your
server resources much better.
One way to achieve that is to keep your CGI applications very slim
and put all the actual work into modules. These modules are then
"use"d or "require"d in your app.psgi file.
As a simpler alternative you can specify a list of "glob" patterns
as "preload" value. "Plack::App::CGIBin::Streaming" will then load
and compile all the scripts matching all the patterns when the app
object is created.
This technique has benefits and drawbacks:
pro: more concurrent worker children in less RAM
see above
con: no way to reload the application on the fly
when your scripts change you have to restart the server. Without
preloading anything you could just kill all the worker children
(or signal them to do so after the next request).
pro/con: increased privileges while preloading
the HTTP standard port is 80 and, thus, requires root privileges
to bind to. scripts are preloaded before the server opens the
port. So, even if it later drops privilges, at preload time you
still are root.
Runtime environment
Additional to the environment provided by CGI::Compile, this module
provides:
the global variable $Plack::App::CGIBin::Streaming::R
For the request lifetime it contains the actual request object. This
variable is "local"ized. There is also a way to access this variable
as class method.
If you use a Coro based plack server, make sure to replace the guts
of this variable when switching threads, see "swap_sv()" in
Coro::State.
"Plack::App::CGIBin::Streaming->request" or
"Plack::App::CGIBin::Streaming::request"
This function/method returns the current request object or "undef"
if called outside the request loop.
%ENV is populated
everything from the plack environment except keys starting with
"plack" or "psgi." is copied to %ENV.
"STDIN" and "STDOUT"
Both, "STDIN" and "STDOUT" are configured to use the
Plack::App::CGIBin::Streaming::IO PerlIO layer. On output, the layer
captures the data and sends it to the request object. Flushing via
$| is also supported. On input, the layer simply converts calls like
"readline STDIN" into a method call on the underlying object.
You can use PerlIO layers to turn the handles into UTF8 mode.
However, refrain from using a simple "binmode" to reverse the
effects of a prior "binmode STDOUT, ':utf8'". This won't pop the
Plack::App::CGIBin::Streaming::IO layer but neither will it turn off
UTF8 mode. This is considered a bug that I don't know how to fix.
(See also below)
Reading from "STDIN" using UTF8 mode is also supported.
Pitfalls and workarounds
SIGCHLD vs. SIGCLD
During the implementation I found a wierd bug. At least on Linux, perl
supports "CHLD" and "CLD" as name of the signal that is sent when a
child process exits. Also, when Perl calls a signal handler, it passes
the signal name as the first parameter. Now the question arises, which
name is passed when a child exits. As it happens the first assignment to
%SIG{CHLD} or $SIG{CLD} determines that name for the rest of the
lifetime of the process. Now, several plack server implementations, e.g.
Starman, rely on that name to be "CHLD".
As a workaround, "Plack::App::CGIBin::Streaming" contains this code:
BEGIN {
local $SIG{CHLD}=$SIG{CHLD};
}
If your server dies when it receives a SIGCHLD, perhaps the module is
loaded too late.
binmode
Sometimes one needs to switch STDOUT into UTF8 mode and back. Especially
the *back* is problematic because the way it is done is often simply
"binmode STDOUT". Currently, this won't revert the effect of a previous
"binmode STDOUT, ':utf8'".
Instead use:
binmode STDOUT, ':bytes';
EXAMPLE
This distribution contains a complete example in the eg/ directory.
After building the module by
perl Build.PL
./Build
you can try it out:
(cd eg && starman -l :5091 --workers=2 --preload-app app.psgi) &
Then you should be able to access
* <http://localhost:5091/clock.cgi?30>
* <http://localhost:5091/flush.cgi>
The clock example is basically the script displayed above. It works in
Firefox. Other browsers don't support multipart HTTP messages.
The flush example demonstrates filtering. It has been tested wich
Chromium 35 on Linux. The script first prints a part of the page that
contains the HTML comment "<!-- FlushHead -->". The filter recognizes
this token and pushes the page out. You should see a red background and
the string "loading -- please wait". After 2 seconds the page should
turn green and the string should change to "loaded".
All of this very much depends on browser behavior. The intent is not to
provide an example that works for all of them. Instead, the capabilities
of this module are shown. You can also test these links with "curl"
instead.
The example PSGI file also configures an access_log and an error_log.
AUTHOR
Torsten Förtsch <torsten.foertsch@gmx.net>
COPYRIGHT
Copyright 2014 Binary.com
LICENSE
This program is free software; you can redistribute it and/or modify it
under the terms of the the Artistic License (2.0). A copy of the full
license is provided by the LICENSE file in this distribution and can be
obtained at
<http://www.perlfoundation.org/artistic_license_2_0>
SEE ALSO
* Plack::App::CGIBin
* CGI::Compile
* Plack::App::CGIBin::Streaming::Request
* Plack::App::CGIBin::Streaming::IO