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Lately I've been toying a lot with [Plack](http://plackperl.org/) and two Perl web framework: [Tatsumaki](http://search.cpan.org/perldoc?Tatsumaki) and [Dancer](http://search.cpan.org/perldoc?Dancer). I use both of them for different purposes, as their features complete each other.
Plack
-----
If you don't already know what Plack is, you would want to take a look at the following Plack resources:
- [Plack (redesigned) website](http://plackperl.org)
- [Plack documentation](http://search.cpan.org/perldoc?Plack)
- [Miyagawa's screencast](http://bulknews.typepad.com/blog/2009/11/plack-and-psgi-screencast-and-feedbacks.html)
- [Plack advent calendar](http://advent.plackperl.org/)
> As [sukria](http://www.sukria.net/) is planning to talk about [Dancer](http://perldancer.org) during the [FPW 2010](http://journeesperl.fr/fpw2010/index.html), I will probably do a talk about Plack.
After reading some code, I've started to write two middleware: the first one add ETag header to the HTTP response, and the second one provides a way to limit access to your application.
**\*** Plack::Middleware::ETag
This middleware is really simple: for each request, an [ETag](http://en.wikipedia.org/wiki/HTTP_ETag) header is added to the response. The ETag value is a sha1 of the response's content. In case the content is a file, it works like apache, using various information from the file: inode, modified time and size. This middleware can be used with [Plack::Middleware::ConditionalGET](http://search.cpan.org/perldoc?Plack::Middleware::ConditionalGET), so the client will have the ETag information for the page, and when he will do a request next time, it will send an "if-modified" header. If the ETag is the same, a 304 response will be send, meaning the content have not been modified. This module is [available on CPAN](http://search.cpan.org/perldoc?Plack::Middleware::ETag).
Let's see how it works. First, we create a really simple application (we call it app.psgi):
``` perl
#!/usr/bin/env perl
use strict;
use warnings;
use Plack::Builder;
builder {
enable "Plack::Middleware::ConditionalGET";
enable "Plack::Middleware::ETag";
sub {
['200', ['Content-Type' => 'text/html'], ['Hello world']];
};
};
```
Now we can test it:
``` example
% plackup app.psgi&
% curl -D - http://localhost:5000
HTTP/1.0 200 OK
Date: Sat, 03 Apr 2010 09:31:43 GMT
Server: HTTP::Server::PSGI
Content-Type: text/html
ETag: 7b502c3a1f48c8609ae212cdfb639dee39673f5e
Content-Length: 11
% curl -H "If-None-Match: 7b502c3a1f48c8609ae212cdfb639dee39673f5e" -D - http://localhost:5000
HTTP/1.0 304 Not Modified
Date: Sat, 03 Apr 2010 09:31:45 GMT
Server: HTTP::Server::PSGI
ETag: 7b502c3a1f48c8609ae212cdfb639dee39673f5e
```
**\*** Plack::Middleware::Throttle
[With this middleware](http://git.lumberjaph.net/p5-plack-middleware-throttle.git/), you can control how many times you want to provide an access to your application. This module is not yet on CPAN, has I want to add some features, but you can get the code from git. There is four methods to control access:
Plack
Middleware::Throttle::Hourly: how many times in one hour someone can access the application
P
M::T::Daily: the same, but for a day
P
M::T::Interval: which interval the client must wait between two query
- by combining the three previous methods
To store sessions informations, you can use any cache backend that provides `get`, `set` and `incr` methods. By default, if no backend is provided, it will store informations in a hash. You can easily modify the defaults throttling strategies by subclassing all the classes.
Let's write another application to test it:
``` perl
#!/usr/bin/env perl
use strict;
use warnings;
use Plack::Builder;
builder {
enable "Plack::Middleware::Throttle::Hourly", max => 2;
sub {
['200', ['Content-Type' => 'text/html'], ['Hello world']];
};
};
```
then test
``` example
% curl -D - http://localhost:5000/
HTTP/1.0 200 OK
Date: Sat, 03 Apr 2010 09:57:40 GMT
Server: HTTP::Server::PSGI
Content-Type: text/html
X-RateLimit-Limit: 2
X-RateLimit-Remaining: 1
X-RateLimit-Reset: 140
Content-Length: 11
Hello world
% curl -D - http://localhost:5000/
HTTP/1.0 200 OK
Date: Sat, 03 Apr 2010 09:57:40 GMT
Server: HTTP::Server::PSGI
Content-Type: text/html
X-RateLimit-Limit: 2
X-RateLimit-Remaining: 0
X-RateLimit-Reset: 140
Content-Length: 11
Hello world
% curl -D - http://localhost:5000/
HTTP/1.0 503 Service Unavailable
Date: Sat, 03 Apr 2010 09:57:41 GMT
Server: HTTP::Server::PSGI
Content-Type: text/plain
X-RateLimit-Reset: 139
Content-Length: 15
Over rate limit
```
Some HTTP headers are added to the response :
- **X-RateLimit-Limit**: how many request can be done
- **X-RateLimit-Remaining**: how many requests are available
- **X-RateLimit-Reset**: when will the counter be reseted (in seconds)
This middleware could be a very good companion to the [Dancer REST stuff](http://www.sukria.net/fr/archives/2010/03/19/let-the-dancer-rest/) [added recently](file:///easily-create-rest-interface-with-the-dancer-1.170/).
another Tatsumaki application with Plack middlewares
----------------------------------------------------
To demonstrate the use of this two middleware, [I wrote a small application](http://git.lumberjaph.net/p5-feeddiscovery.git/) with Tatsumaki. This application fetch a page, parse it to find all the feeds declared, and return a JSON with the result.
``` example
% GET http://feeddiscover.tirnan0g.org/?url=http://lumberjaph.net/blog/
```
will return
``` example
% [{"href":"http://lumberjaph.net/blog/index.php/feed/","type":"application/rss+xml","title":"i'm a lumberjaph RSS Feed"}]
```
This application is composed of one handler, that handle only **GET** request. The request will fetch the url given in the **url** parameter, scrap the content to find the links to feeds, and cache the result with Redis. The response is a JSON string with the informations.
The interesting part is the app.psgi file:
``` perl
my $app = Tatsumaki::Application->new(['/' => 'FeedDiscovery::Handler'],);
builder {
enable "Plack::Middleware::ConditionalGET";
enable "Plack::Middleware::ETag";
enable "Plack::Middleware::Throttle::Hourly",
backend => Redis->new(server => '127.0.0.1:6379',),
max => 100;
$app;
};
```
The application itself is really simple: for a given url, the Tatsumaki::HTTPClient fetch an url, I use [Web::Scraper](http://search.cpan.org/perldoc?Web::Scraper) to find the \*link rel="alternate"\* from the page, if something is found, it's stored in Redis, then a JSON string is returned to the client.
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