Suppose you want to fetch some data from a remote service. The remote service has two instances, both of which are slightly flaky. What you want to do is send off requests to both servers simultaneous (this is the easy part) and then wait for the first one to give you a result. The latter uses the new API that I'm about to describe here.
from google.appengine.api import urlfetch, apiproxy_stub_map
urls = ['http://service1.com', 'http://service2.com'] # Etc.
rpcs = []
for url in urls:
rpc = urlfetch.create_rpc(deadline=1.0)
urlfetch.make_fetch_call(rpc, url)
rpcs.append(rpc)
rpc = apiproxy_stub_map.UserRPC.wait_any(rpcs)
# Now rpc is the first rpc that returned a result. Have at it!
That's all! If you're interested in learning more about this handy class method, just check out its docstring in the App Engine SDK. Note that technically you should loop until it doesn't return None.
You can also repeatedly call wait_any() to get subsequent result. Make sure to remove the rpc it returns (if any) from the list, since otherwise it will return the same rpc over and over again: the specification of wait_any() says it returns the first rpc in the given list that completes, regardless of whether you have seen it before.
Also note that there currently is no way to cancel the other RPCs, which is why I passed a low deadline to the create_rpc() call. The problem is that even if you completely ignore the other RPCs, the App Engine runtime still waits for them to finish or timeout.
Finally, there is also a similar class method UserRPC.wait_all(), which waits until all RPCs in the list you pass it are complete. (It doesn't return anything.)
PS. Don't look too closely at the implementation of these methods. It may change as we think of a better way to do it. But we're committed to the API.
