I created an App Engine backend to serve http requests for a long running process. The backend process works as expected when the query references an input of small size, but times out when the input size is large. The query parameter is the url of an App Engine BlobStore blob, which is the input data for the backend process. I thought the whole point of using App Engine backends was to avoid the timeout restricts that App Engine frontends possess. How can I avoid getting a timeout?
I call the backend like this, setting the connection timeout length to infinite:
HttpURLConnection connection = (HttpURLConnection)(new URL(url + "?" + query).openConnection());
connection.setRequestProperty("Accept-Charset", charset);
connection.setRequestMethod("GET");
connection.setConnectTimeout(0);
connection.connect();
InputStream in = connection.getInputStream();
int ch;
while ((ch = in.read()) != -1)
json = json + String.valueOf((char) ch);
System.out.println("Response Message is: " + json);
connection.disconnect();
The traceback (edited for anonymity) is:
Uncaught exception from servlet
java.net.SocketTimeoutException: Timeout while fetching URL: http://my-backend.myapp.appspot.com/somemethod?someparameter=AMIfv97IBE43y1pFaLNSKO1hAH1U4cpB45dc756FzVAyifPner8_TCJbg1pPMwMulsGnObJTgiC2I6G6CdWpSrH8TrRBO9x8BG_No26AM9LmGSkcbQZiilhC_-KGLx17mrS6QOLsUm3JFY88h8TnFNer5N6-cl0iKA
at com.google.appengine.api.urlfetch.URLFetchServiceImpl.convertApplicationException(URLFetchServiceImpl.java:142)
at com.google.appengine.api.urlfetch.URLFetchServiceImpl.fetch(URLFetchServiceImpl.java:43)
at com.google.apphosting.utils.security.urlfetch.URLFetchServiceStreamHandler$Connection.fetchResponse(URLFetchServiceStreamHandler.java:417)
at com.google.apphosting.utils.security.urlfetch.URLFetchServiceStreamHandler$Connection.getInputStream(URLFetchServiceStreamHandler.java:296)
at org.someorg.server.HUDXML3UploadService.doPost(SomeService.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
As you can see, I’m not getting the DeadlineExceededException, so I think something other than Google’s limits is causing the timeout, and also making this a different issue from similar stackoverflow posts on the topic.
I humbly thank you for any insights.
Update 2/19/2012: I see what’s going on, I think. I should be able to have the client wait indefinitely using GWT [or any other type of client-side async framework] async handler for an any client request to complete, so I don’t think that is the problem. The problem is that the file upload is calling the _ah/upload App Engine system endpoint which then, once the blob is stored in the Blobstore) calls the upload service’s doPost backend to process the blob. The client request to _ah/upload is what is timing out, because the backend doesn’t return in a timely fashion. To make this timeout problem go away, I attempted to make the _ah_upload service itself a public backend accessible via http://backend_name.project_name.appspot.com/_ah/upload, but I don’t think that google allows a system service (like _ah/upload) to be run as a backend. Now my next approach is to just have ah_upload immediately return after triggering the backend processing, and then call another service to get the original response I wanted, after processing is finished.
The solution was to start a backend process as a tasks and add that to the task queue, then returning a response to client before it waits to process the backend task (which can take a long time). If I could have assigned ah_upload to a backend, this would have also solved the problem, since the clien’t async handler could wait forever for the backend to finish, but I do not think Google permits assigning System Servlets to backends. The client will now have to poll persisted backend process response data, as Paul C mentioned, since tasks can not respond like a normal servlet.