<<< from retro import ajax, on, run, Component

class Telephone(Component):

def _init( self ): Component.init_(self) self.tube = []

@ajax(GET="/listen") def listen( self ): if self.tube: m = self.tube[0] ; del self.tube[0] return m

@ajax(GET="/say/{something:rest}") def say( self, something ): self.tube.append(something)

run(components=Telephone()) >>>

run this script with python:

<<< $ python telephone.py Dispatcher: @on GET /listen Dispatcher: @on POST /channels:burst Dispatcher: @on GET /say/{something:rest} Retro embedded server listening on 0.0.0.0:8000 >>>

and then interact with your new web service using curl

curl http://localhost:8000/say/hello
null
curl http://localhost:8000/say/world
null
curl http://localhost:8000/listen
"hello"
curl http://localhost:8000/listen
"world"

Now "The Telephone" was rather trivial. Let's do something more impressive : a web page that displays the time.

from retro import ajax, on, run, Component import time

<<< class Watch(Component):

@on(GET="/time") def getTime( self, request ): def stream(): while True: yield "

%s

" % (time.ctime()) time.sleep(1) return request.respondMultiple(stream())

run(components=Watch()) >>>

Now start firefox and go to http://localhost:8000/time (you won't be able to open two tabs with it, if you want to test concurrency, use another browser or another machine).

The Watch was pretty interesting, but the time.sleep(1) in the middle of the generator doesn't really help performance. Let's do some more fancy stuff.

• Based on WSGI
• Embedded reactor-based WSGI server
• Flup for FCGI/SCGI/AJP connection
• Plain old CGI mode
• "Comet"/HTTP streaming support
• Decorators for declarative web services
• Lightweight

               APPLICATION  ----------------- COMPONENT
               -----------           -------- COMPONENT

To be able to server pages and content in your application, you have to express a mapping between URL and actual methods of your components.

In Retro, you define a mapping by exposing a method of your component using the @on decorator:

class Main(Component):
	...
	@on(GET="/index")
	def index( self, request ):
		return request.respond("Hello, world !")

Here, we've defined the 'index' method (the name is not important), and exposed to react to a GET HTTP method sent to the /index URL. Restarting your server and going to http://localhost:8080/index will give you this text:

Hello, world !

The @on decorator parameters are made of the following elements:

• a parameter name (like GET, POST, GET_POST), where the HTTP methods are uppercase and joined by underscores

• a parameter value, which is an expression that defines matching URLs.

• an optional priority, which allows one mapping with higher priority to be used in preference when more than one mapping matches the URL.

The parameter value expression can contain specific parts that will be matched and given as arguments to the decorated method:

	@on(GET="/int/{i:integer}")
	def getInteger(self, request, i):
		return request.respond("Here is number %d" % (i))

Now, if you go to http://localhost:8080/int/0, or http://localhost:8080/int/1, http://localhost:8080/int/2, you will see these numbers printed out.

Generally speaking, anything between { and } in parameter expressions will be interpreted as a matching argument. The format is like that:

{ NAME : EXPRESSION }

where NAME must match an argument of the decorated method, and where EXPRESSION is either a regular expression (as of Python re module), or one of the following values:

• word, alpha: any sequence of alphabetical chars
• string: everything that is not a /
• digits: a sequence of digits, cast to an int
• number: a floating point or an int, negative or positive
• int, integer: an int, negative or positive
• float: an float, negative or positive
• file: two alphanumeric words joined by a dot
• chunk: everything which is neither a / nor a .
• rest, any : the rest of the URL
• range: two integers joined by : (Python-style)

Once you have your mapping right, you may want to do more complicated things with your exposed methods… and we'll see in the next section what we can do with the mysterious 'request' parameter.

The retro.core.Request class defines a class that represents an HTTP request. The request parameter we've seen in the previous section is an instance of this class, that represents the request sent by the client browser and received by the Retro server.

The request object offers different kind of functionalities:

• Accessing the request parameters, cookies and data: whether POST or GET, whether url-encoded or form-encoded, parameters and attachments are retrievable using the param, cookies and data methods.

• Accessing request method, headers and various information: TODO

• Creating a response: the request object contains methods to create specific responses, whether it is serving a local file, returning JSON data, redirecting, returning an error, or simply returning specific content. These methods are mainly respond, returns, redirect, bounce, notFound, localFile.

In practice, you need only to know a few things. First, how to get access to parameters.

Say you have a request to that URL:

http://localhost:8080/api/doThis?a=1&b=2

and that you have a handler bound to /api/doThis, here is how you would get access to a and b:

@on(GET_POST='/api/doThis')
def doThis( self, request ):
	a = request.get('a')
	b = request.get('b')
	if a is None or b is None:
		return request.respond(
			"You must give proper 'a' and 'b' parameters",
			status=400
		)
	else:
		return request.respond(
			"Here a:%s and here is b:%s" % (a,b)
		)

This is the simplest, and most common case. Now if you want to receive a file that was POSTed by the client, by a form like this one:

<form action="/api/postThis" method="POST">
	<input type="text" name="name" value="File name" />
	<input type="file" name="file" value="Upload file" />
</form>

you would do the following:

@on(POST='/api/postThis')
def postThis( self, request ):
	file_name = request.get('name')
	file_data = request.get('file')
	file_info = request.file('file')
	return request.respond(
		"Received file %s, of %s bytes, with content type:%s" %
		(file_info.get("filename"),len(file_data),file_info.get("contentType"))
	)

we see here that the file data is available as any other parameter, but that we can use the request.file method to get more information about the file, like its content type and original filename.

Session management is another important aspect of web applications. Retro session manage is provided by Alan Saddi's FLUP session middleware or by the Beaker library.

Retro comes with its own WSGI server, which is the more well-tested solution for deploying applications that make use of advanced WSGI features (such as request streaming using yield.

Usually, a standalone Retro application ends with code similiar to this one (here we have an application composed of two FileServer and PageServer custom components).

  def createApp():
    return Application(components=(
      FileServer(),
      PageServer(),
    ))

  def start( app=None, options=None ):
    if app is None: app = createApp()
    name = "My application"
    run(
      app=app, name=name, method=STANDALONE, port=options.get("port") or 8888, sessions=False,
    )

  if __name__ == "__main__":
    options = {}
    for a in sys.argv[1:]:
        a=a.split("=",1)
        if len(a) == 1: v=True
        else: v=a[1];a=a[0]
        options[a.lower()] = v
    start(options=options)

If your main server script is server.py, you can simply start it like this:

python server.py PORT=8888
Dispatcher: @on  GET /crossdomain.xml
Dispatcher: @on  GET /lib/css/{css:[\w\-_]+\.css}
Dispatcher: @on  GET /lib/swf/{script:\w+\.swf}
Dispatcher: @on  GET /lib/images/{image:[\w\-_]+\.(png|gif|jpg)}
...
Retro embedded server listening on 0.0.0.0:8888

The next thing is to make this application available to your website, meaning you have to map it to port 80. You can bind it directly to port 80, but it's a better idea to use proxying.

Here is how to do it with Apache:

ProxyPass        /myapp http://localhost:8888/
ProxyPassReverse /myapp http://localhost:8888/
ProxyPreserveHost  On

You can also use something like Pound, where a configuration would look like this:

  ListenHTTP
      Address localhost
      Port    80
      
      # Your Retro application
      Service
          URL "/myapp/.*"
          BackEnd
              Address 0.0.0.0
              Port    8888 
          End
      End
  End

Pound is definitely cooler, as the configuration is easy and it's a very fast reverse proxy. What you can do is use Apache, Lighttpd or Nginx to serve static files (like '/lib/.*'), overriding your Retro application default server for static files.

Retro can be connected to an FCGI (FastCGI) enabled server thanks to the Python FLUP module. Here is how to do it using Lighttpd:

First create a myapp.fcgi script:

#!/usr/bin/env python
from retro import *
from retro.contrib.localfiles import LocalFiles
from myapp import server
print "Retro: Starting FCGI"
run(
    app        = server.createApp()
    name       = os.path.splitext(os.path.basename(__file__))[1],
    method     = FCGI,
    sessions   = False
)
# EOF

The above script expects you to have a myapp.server.createApp() function that returns the retro application.

Second, create a myapp.lighttpd.conf script:

server.modules += ( "mod_fastcgi" )
server.document-root = "."
server.port     = 8888
fastcgi.debug   = 1
fastcgi.server  = (
    ".fcgi" => ( 
        "localhost" => (
            "min-procs" => 2,
            "socket"    => "/tmp/retro-fastcgi.socket",
            "bin-path"  => "/full/path/to/myapp.fcgi",
        )
    )
)

Notice that you have to give the full absolute path to myapp.fcgi in your bin-path field of the lighttpd configuration file.

Now type the following command:

$ lighttpd -D myapp.lighttpd.conf
2009-04-02 14:29:02: (log.c.75) server started 
2009-04-02 14:29:02: (mod_fastcgi.c.1325) --- fastcgi spawning local 
  proc: /home/sebastien/Projects/Public/Retro/Examples/FCGI/myapp.fcgi 
  port:0
  socket /tmp/retro-fastcgi.socket 
  min-procs: 4 
  max-procs: 4 
2009-04-02 14:29:02: (mod_fastcgi.c.1350) --- fastcgi spawning 
  port: 0 
  socket /tmp/retro-fastcgi.socket 
  current: 0 / 4 
  ...

And now open your browser and go to http://localhost:8888/myapp.fcgi/. You can now play with Ligghttpd configuration options to change the root URL for your application.

Note that Lighttpd FCGI configuration is quite painful, as it's very hard to know what doesn't work when things don't work. I'd recommend to use Apache's modwsgi or a combination of Retro standalone and 'pound' load balancer.

Using modwsgifor Apache is by far the easiest solution to get a Retro application up and running:

WSGIScriptAlias /altitude /home/sebastien/Servers/Datalicious.ca/altitude.wsgi

#!/usr/bin/python
import sys, os
sys.stdout = sys.stderr

import myapplication.server
application = myapplication.server.createApp()
# EOF

And some tips:

• In your startup script extend and manipulate sys.path to make sure you control the environment properly.

• In your startup script, change the current working directory to make sure you run at the proper location

• WSGI is by default very similiar to CGI (one process per request), but with more work in configuration you can get it work like FCGI.