Google App Engine: how to parallelize downloads using TaskQueue or Async Urlfetch?

My Gae application retrieves JSON data from a third party site; given an ID representing the item to download , the item's data on this site is organized in multipl开发者_如何学JAVAe pages so my code has to download chunks of data, page after page, until the data of the last available page is retrieved.

My simplified code looks like this:

class FetchData(webapp.RequestHandler):
  def get(self):
    ...
    data_list = []
    page = 1
    while True:
      fetched_data= urlfetch.fetch('http://www.foo.com/getdata?id=xxx&result=JSON&page=%s' % page)
      data_chunk = fetched_data["data"] 
      data_list = data_list + data_chunk
      if len(data_list) == int(fetched_data["total_pages"]):
         break
      else:
         page = page +1 
    ...  
    doRender('dataview.htm',{'data_list':data_list} )

The data_list results is an ordered list where the first item has data of page number 1 and the last item has data of the latest page; this data_list, once retrieved, is rendered in a view.

This approach works 99% of times but sometimes, due to the 30 seconds limit imposed by Google App Engine, on items with many pages i get the dreaded DeadlineExceededError. I would like to know if using TaskQueue|Deferred|AsyncUrlfetch I could improve this algorithm parallelizing in some way the N urlfetch calls.

Use this: http://code.google.com/appengine/docs/python/urlfetch/asynchronousrequests.html

Which is simple like so:

def handle_result(rpc):
    result = rpc.get_result()
    # ... Do something with result...

# Use a helper function to define the scope of the callback.
def create_callback(rpc):
    return lambda: handle_result(rpc)

rpcs = []
for url in urls:
    rpc = urlfetch.create_rpc()
    rpc.callback = create_callback(rpc)
    urlfetch.make_fetch_call(rpc, url)
    rpcs.append(rpc)

# ...

# Finish all RPCs, and let callbacks process the results.
for rpc in rpcs:
    rpc.wait()

I have resolved with this:

chunks_dict = {}

def handle_result(rpc, page):
    result = rpc.get_result()
    chunks_dict[page] = result["data"]

def create_callback(rpc, page):
    return lambda: handle_result(rpc, page)

rpcs = []
while True:
    rpc = urlfetch.create_rpc(deadline = 10)
    rpc.callback = create_callback(rpc, page)
    urlfetch.make_fetch_call(rpc, 'http://www.foo.com/getdata?id=xxx&result=JSON&page=%s' % page)
    rpcs.append(rpc)
    if page > total_pages:
       break
    else:
       page = page +1   
for rpc in rpcs:
    rpc.wait()

page_keys = chunks_dict.keys()
page_keys.sort()
for key in page_keys:
    data_list= data_list + chunks_dict[key]