How is DeadlineExceededException implemented in Google App Engine for Java?

Question

Applications on Google App Engine must have web requests that return response data within 30 seconds. When this time is exceeded, a DeadlineExceededException exception is thrown:

/time.jsp
java.lang.ClassCastException: com.google.apphosting.api.DeadlineExceededException cannot be cast to javax.servlet.ServletException
    at org.apache.jasper.runtime.PageContextImpl.handlePageException(PageContextImpl.java:754)
    at org.apache.jsp.time_jsp._jspService(time_jsp.java:66)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:806)
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1093)
    at com.google.apphosting.runtime.jetty.SaveSessionFilter.doFilter(SaveSessionFilter.java:35)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
    at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:360)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:712)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405)
    at com.google.apphosting.runtime.jetty.AppVersionHandlerMap.handle(AppVersionHandlerMap.java:237)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:139)
    at org.mortbay.jetty.Server.handle(Server.java:313)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:506)
    at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:830)
    at com.google.apphosting.runtime.jetty.RpcRequestParser.parseAvailable(RpcRequestParser.java:76)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:381)
    at com.google.apphosting.runtime.jetty.JettyServletEngineAdapter.serviceRequest(JettyServletEngineAdapter.java:125)
    at com.google.apphosting.runtime.JavaRuntime.handleRequest(JavaRuntime.java:235)
    at com.google.apphosting.base.RuntimePb$EvaluationRuntime$6.handleBlockingRequest(RuntimePb.java:4755)
    at com.google.apphosting.base.RuntimePb$EvaluationRuntime$6.handleBlockingRequest(RuntimePb.java:4753)
    at com.google.net.rpc.impl.BlockingApplicationHandler.handleRequest(BlockingApplicationHandler.java:24)
    at com.google.net.rpc.impl.RpcUtil.runRpcInApplication(RpcUtil.java:359)
    at com.google.net.rpc.impl.Server$2.run(Server.java:800)
    at com.google.tracing.LocalTraceSpanRunnable.run(LocalTraceSpanRunnable.java:56)
    at com.google.tracing.LocalTraceSpanBuilder.internalContinueSpan(LocalTraceSpanBuilder.java:510)
    at com.google.net.rpc.impl.Server.startRpc(Server.java:756)
    at com.google.net.rpc.impl.Server.processRequest(Server.java:348)
    at com.google.net.rpc.impl.ServerConnection.messageReceived(ServerConnection.java:459)
    at com.google.net.rpc.impl.RpcConnection.parseMessages(RpcConnection.java:319)
    at com.google.net.rpc.impl.RpcConnection.dataReceived(RpcConnection.java:290)
    at com.google.net.async.Connection.handleReadEvent(Connection.java:419)
    at com.google.net.async.EventDispatcher.processNetworkEvents(EventDispatcher.java:762)
    at com.google.net.async.EventDispatcher.internalLoop(EventDispatcher.java:207)
    at com.google.net.async.EventDispatcher.loop(EventDispatcher.java:101)
    at com.google.net.rpc.RpcService.runUntilServerShutdown(RpcService.java:251)
    at com.google.apphosting.runtime.JavaRuntime$RpcRunnable.run(JavaRuntime.java:373)
    at java.lang.Thread.run(Unknown Source)

How is it done? Do you have any open source code?

Answer 1

The Request Timer

A request handler has a limited amount of time to generate and return a response to a request, typically around 30 seconds. Once the deadline has been reached, the request handler is interrupted.

The Java runtime environment interrupts the servlet by throwing a com.google.apphosting.api.DeadlineExceededException. If the request handler does not catch this exception, as with all uncaught exceptions, the runtime environment will return an HTTP 500 server error to the client.

The request handler can catch this error to customize the response. The runtime environment gives the request handler a little bit more time (less than a second) after raising the exception to prepare a custom response.

While a request can take as long as 30 seconds to respond, App Engine is optimized for applications with short-lived requests, typically those that take a few hundred milliseconds. An efficient app responds quickly for the majority of requests. An app that doesn't will not scale well with App Engine's infrastructure.

Part of the runtime == Not using threads. This feature is a modification of the VM for sure, and I wouldn't hold my breath waiting for the source code.