== Retro == Featherweight Web Development -- Author: Sebastien Pierre [email protected] -- Revision: 0.9.5 (12-Fev-2010)

Retro is a lightweight Python Web WSGI-based toolkit that is designed for easy prototyping and development of Web services and Web apps.

Some facts:

Retro is notably one of the very few web toolkit able to do HTTP streaming (aka. Comet) out of the box.
Retro comes with batteries included and features a simple, fast, reactor-based WSGI server.
Retro makes use of Python decorators for declarative web development.

Core concept: web services are "just" message-passing over HTTP. Regular Python classes (and their methods) methods can be turned into web services using only decorators.

Seeing is Believing

Retro is for hackers, so here is the code.

The Telephone: A Basic Web Service

<<< 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"

The Watch: A less basic Web Service

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 "<html><body><pre>%s</pre></body></html>" % (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 Chat: A less basic Web Service

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.

Reference

Features

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

Architecture

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

Dispatcher

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 Request object

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 'POST'ed by the client, by a form like this one:

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.

Note _____________________________________________________________________________
It is important to note that *you must return the result* of the
'request.[respond, returns, redirect, bounce, ...]' methods. These methods
actually return a generator that will be used by Retro to produce the
content of the response.

Sessions

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

Deployment

Standalone

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.

With FastCGI

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.

With modwsgi

Using [modwsgi] for 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.

References

[FLUP]: FLUP, random WSGI stuff, Alan Saddi, http://trac.saddi.com/flup [BEAKER]: BEAKER [MODWSGI]: modwsgi, http://code.google.com/p/modwsgi/ [POUND]: Pound proxy, http://www.apsis.ch/pound/

EOF - vim: ts=2 sw=2 syn=kiwi et

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Metadata

Seeing is Believing

The Telephone: A Basic Web Service

The Watch: A less basic Web Service

The Chat: A less basic Web Service

Reference

Features

Architecture

Dispatcher

The Request object

Sessions

Deployment

Standalone

With FastCGI

EOF

With modwsgi

EOF

References

EOF - vim: ts=2 sw=2 syn=kiwi et

← Metadata

Owner

Metadata

retro retro copied to clipboard

Metadata

Seeing is Believing

The Telephone: A Basic Web Service

The Watch: A less basic Web Service

The Chat: A less basic Web Service

Reference

Features

Architecture

Dispatcher

The Request object

Sessions

Deployment

Standalone

With FastCGI

EOF

With modwsgi

EOF

References

EOF - vim: ts=2 sw=2 syn=kiwi et

← Metadata

Owner

Metadata

retro
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