Τα αντικείμενα Deferreds (“καθυστερούμενα”;) αποτελούν ένα κομβικό σημείο στη λειτουργία του Twisted. Επειδή η εξυπηρέτηση των αιτήσεων γίνεται διαδοχικά σε non-blocking mode, ο κώδικας εξυπηρέτησης για ένα event δεν πρέπει να καθυστερεί. Τι θα συμβεί π.χ. όταν κατά την εξυπηρέτηση μιας αίτησης πρέπει αν διαβάσουμε έναν δεύτερο απομακρυσμένο διαδικτυακό πόρο; Εδώ μπαίνει η χρήση των Deferreds.
Στο παράδειγμα που ακολουθεί, η εξυπηρέτηση της αίτησης web βασίζεται στην ανάγνωση μιας δεύτερης ιστοσελίδας. Η ανάγνωση αυτή γίνεται μέσω της μεθόδου getPage(), η οποία επιστρέφει αμέσως ένα αντικείμενο Deferred. Στο αντικείμενο αυτό προσθέτουμε ένα callback κκαι ένα errback, τα οποία θα κληθούν μετά την επιτυχή ανάγνωση (ή το σφάλμα ανάγνωσης, αντίστοιχα). Στο μεταξύ, το framework είναι σε θέση να εξυπηρετήσει άλλες αιτήσεις.
from twisted.application import internet,service
from twisted.web import server,resource
from twisted.web.client import getPage
# definitions of resources
class Root(resource.Resource): # the root resource class
isLeaf = False
class Page(resource.Resource): # the sample page resource class
isLeaf = True
def render_GET(self,request):
""" called to handle a GET request """
# request next web page - a deferred is returned.
# When callback fires, page content will be
# the 1st param in callback invocation.
# If errback fires, 1st param will be a failobj (a description of error)
dfr = getPage("http://sample.page.url")
# add a callback and errback to deferred object
# we pass also the request object as additional param
dfr.addCallback(self.handlePageOK,request)
dfr.addErrback(self.handlePageError,request)
# notify the framework that we are not done yet
return server.NOT_DONE_YET # more of response to follow
def handlePageOK(self,content,request):
""" callback for getPage(). 'content' is the page text """
# forward reply to client as is
request.write(content)
# signal end of reply
request.finish()
def handlePageError(self,failobj,request):
""" callback for getPage(). 'failobj' holds the error description """
# change reply to text/plain, error message may contain invalid HTML chars
request.setHeader('Content-type','text/plain')
# reply with description of error
request.write(str(failobj))
# signal end of reply
request.finish()
# main program part
# build the resources tree
root = Root() # the "/" resource
root.putChild("page",Page()) # the "/data" resource
# here we connect a "Site" (subclass of "twistewd.web.http.HTTPFactory") to a "resource"
site = server.Site(root)
# create an object that represents our application
# object must be called exactly 'application' to use the twistd launcher!
application = service.Application("simple-web-server")
# create the echo service that our application will offer
web_echo_service = internet.TCPServer(50007,site)
# add the web echo service to our application object
web_echo_service.setServiceParent(application)
Παρατηρήστε ότι:
Χρησιμοποιούμε την τεχνική των Deferreds και την εξαγωγή πληροφορίας με τη βοήθεια των regular expressions:
import re
from twisted.application import internet,service
from twisted.web import server,resource
from twisted.web.client import getPage
# definitions of resources
class Root(resource.Resource): # the root resource class
isLeaf = False
class Page(resource.Resource): # the sample page resource class
isLeaf = True
# rexps for data extraction - common for all objects
rexp = re.compile(r'<tr class="TRstyle[01]">(.+?)</tr>',re.DOTALL)
rexp2 = re.compile(r'<td[^>]+>(.+?)</td>',re.DOTALL)
rexp3 = re.compile(r'<span[^>]+>(.+?)</span>',re.DOTALL)
def render_GET(self,request):
""" called to handle a GET request """
# request next web page - a deferred is returned.
dfr = getPage("http://weather.forecast.url")
# add a callback and errback to deferred object
# we pass also the request object as additional param
dfr.addCallback(self.handlePageOK,request)
dfr.addErrback(self.handlePageError,request)
# notify the framework that we are not done yet
return server.NOT_DONE_YET # more of response to follow
def handlePageOK(self,content,request):
""" callback for getPage(). 'content' is the page text """
# change reply to text/plain
request.setHeader('Content-type','text/plain')
m = self.rexp.findall(content.decode("iso8859_7"))
for item in m:
m2 = self.rexp2.findall(item)
if len(m2)<10: continue # skip irregular entries
day,date = self.rexp3.findall(m2[0])
time = m2[1]
grad = m2[3].replace(u"°",unichr(176))
humid = m2[4]
wind = m2[5]
sky = m2[8]
outstr = "%s\t%s\t%s\t%s\t%s\t%s\t%s\n" % (day,date,time,grad,humid,wind,sky)
request.write(outstr.encode("utf_8"))
# signal end of reply
request.finish()
def handlePageError(self,failobj,request):
""" callback for getPage(). 'failobj' holds the error description """
# change reply to text/plain, error message may contain invalid HTML chars
request.setHeader('Content-type','text/plain')
# reply with description of error
request.write(str(failobj))
# signal end of reply
request.finish()
# main program part
# build the resources tree
root = Root() # the "/" resource
root.putChild("page",Page()) # the "/data" resource
# here we connect a "Site" (subclass of "twistewd.web.http.HTTPFactory") to a "resource"
site = server.Site(root)
# create an object that represents our application
# object must be called exactly 'application' to use the twistd launcher!
application = service.Application("simple-web-server")
# create the echo service that our application will offer
web_echo_service = internet.TCPServer(50007,site)
# add the web echo service to our application object
web_echo_service.setServiceParent(application)