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== Retro
== Featherweight Web Development
-- Author: Sebastien Pierre <sebastien@ivy.fr>
-- 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:
> <form action="/api/postThis" method="POST">
> <input type="text" name="name" value="File name" />
> <!-- NOTE: You need the 'enctype=multipart/form-data' for this to
> work properly with the following Retro snippet -->
> <input type="file" enctype='multipart/form-data' 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.
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](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](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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