Dear Tomcat users/dev team, We are understanding the impact of HTTP/2 in our application as HTTP/2 provides better throughput and performance. Before directly tuning HTTP/2 in application, we thought of analyzing certain use cases which our application demands in standalone environment.
Our use case is very simple. Java based standalone client is making simple POST request to the server and server read the file name from the request and push the requested file to client. Here, client can request multiple files same time by sending multiple requests concurrently, so server should be able to send multiple files concurrently depending on configured concurrency level. Currently, in this test only single client is making requests to the server with concurrency 5. Server is not overloaded and not performing any other tasks. Machine has more than 500GB empty space and not running any heavy applications. Test: We used different set of files for this test. Files with sizes between 1GB - 5GB and concurrency > 5. We are using traditional connector protocol HTTP11NIOProtocol with HTTP/2 is turned on. Observations: HTTP/1.1 - With HTTP/1.1 given sample code works fine. Only drawback here is it opens multiple TCP connections to satisfy HTTP/1.1 HTTP/2 - With HTTP/2, it is expected to be only one TCP connection and multiple streams to handle the traffic. Tomcat HTTP/2 debug logs suggest that only one connection being used and multiple streams are spawned as expected. So far everything is fine. But sample code does not work consistently with higher concurrency (> 3). We captured the stack trace of tomcat process which is attached here. Couple of tomcat threads are waiting to acquire semaphore for socket write operation. When write operation is stuck servlet is not able to push any data to client and client is also stuck waiting for more data. I don't see any error/exception at the client/server. streamReadTimeout and streamWriteTimeout are configured as -1 so they are infinitely waiting for the write semaphore. Outcome of this is client is able to receive only partial data from server and at some point server stuck to send any more data. We also tried IOUtils file transfer related APIs still it didn't help. I have also tried with Async non-blocking IO but the observations are same. Our actual requirement is very similar where java based http client would request bulk data concurrently from server and server should push that without any trouble. But, it is not limited to files only. Server can push serialized java bulk objects over the stream concurrently. Note that sample code works fine most of the time if I enable HTTP/2 logs either in client or tomcat. So I would suggest not to turn on HTTP/2 debug logs to conclude anything. Following components are used in sample code for the test 1. Client - Java 11.0.10 httpclient - (client\Client.java) 2. Server - Tomcat 9.0.46 3. Servlet - AsyncServlet - (server\Server.java) 4. Operating system - Windows 10 5. Machine specifications - 32GB RAM and 500GB open space. 5. Latency - None, client and server are running on same machine 6. Set of files - You can use any random files whose sizes are between 1GB-5GB to reproduce the issue. Refer attachment for 1. Client side code 2. Server side servlet 3. server.xml 4. Tomcat Stacktrace 5. Tomcat server logs Could you please go through sample code along with server.xml. Here are my few questions 1. Why HTTP/2 is failing for such use case where large files are concurrently pushing to the client. I believe this is a very common use case must have be assessed earlier. 2. Connector configuration in server.xml is correct for HTTP/2 ? 3. Does AsyncServlet is written properly for such type of use case ? If not, then please suggest correct way to manage it. 4. If you are already aware of similar problem, can you point out any alternatives or recommendations. Thanks in advance for your support. Regards, Kedar Deshmukh
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