I need some assistance on a project I am working on. It's a library itself using Jersey 1.x (1.19.1) aiming at HTTP posting a JSON document and getting the corresponding JSON response from a server.
I am facing a problem when the response from the server is "big". The JSON document that is posted by my client application contains several jobs that must be executed by the server, and the JSON document sent back by the server is made of the outputs of these jobs. The jobs can be considered independent from each other. The server works in streaming mode, which means it starts to process the jobs before it receives the entire JSON document posted by the client. And it starts to send the outputs of the jobs as soon as they are finished. So the server starts to reply to my client application while it is still posting the request. Here is my problem. When the request gets big so gets the response (more jobs to do), my application freezes and at some point terminates.
I spent a lot of time trying to figure out what's happening and here is what is found and what I infered.
Jersey, for handling HTTP communication is using a class from the JDK (in rt.jar) I forgot the exact name and don't have access to my work right now but let's call it HttpConnection. In this class there is a method checkError() that is invoked and throws a IOException with only a message saying it was impossible to write to server. Debugging I was able to understand that an attribute of this class named trouble was set to true because a write() method caught an IOException before. checkError() throws a IOException based on that trouble boolean flag. It's not possible to easily see the cause IOException because the classes of the JRE are compiled without the debugging symbols but I managed to see that this IOExeption was a "connection reset by peer" problem.
Then I tried to understand why the server resets the connection. I used a HTTP proxy that captures the HTTP traffic between my client application and the server but this gave me no more clues, it even seems that the proxy is unable to handle properly the connection with the server as well!
So I tried to use Wireshark to capture the traffic and see what's wrong. Here is what I found.
On client side, packets corresponding to the post of the request JSON document are sent and the server starts to reply shortly after, as explained above. The server side sends more and more packets and I noticed that the buffer of the TCP layer (called TCP window in Wireshark) on client side has a size that decreases more and more as the server sends packets. Until it beomes full (size: 0 byte). So the TCP layer on server side cannot send data to the TCP layer on client side anymore and thus becomes full too. The conversation, in the end is only about retrying to send data, on both sides, failing again and again. Ultimately the server decides to send a reset packet. This corresponds to the cause IOExcpetion I mentioned above I believe.
My understanding is: as long as the server does not start to stream the response everything is fine. When the server starts to send the response, the TCP buffer on client side starts to get filled. But as the client application does not read the response yet, the content of this buffer is not consumed. When the server has sent enough data to fill this buffer it cannot send anymore data and the buffer of its TCP layer gets full too because the server continues to push data. As a result, the client application cannot finish to send the request JSON document. The communication is blocked on both sides and the server decides to reset the connection.
My conclusion is: the code, as currently written, does not support such full duplex communication, because the response from the server is not consumed as it is received. Indeed, walking through the Jersey code that is executed by my library, by debugging, it is clear that the pattern is:
- first: connection.getOutputStream().write()
- and then: response.getInputStream().read()
In my opinion, the root cause of the problem is that the library I am working on uses Jersey in this synchronous manner which does not fit well the way the server works (streaming the response while the request is still being sent to it).
I searched a lot on the Internet a solution keeping Jersey 1.19.1 for me to improve the library with as few impacts as possible but I failed. This is the reason why I am asking help here now ;)
So basicaly my question is: is it possible to do what I need to do keeping Jersey client library 1.19.1 and if yes how? If not, what HTTP client library should I use for my library (to write a post request and read the corresponding response at the same time) and if you could give me a basic example so I can be on track quickly it would be much appreciated.
One last thing: curl works just fine, I can fully post the exact same JSON document and get the response using it, so there is no problem on server side as I suspected at the very beginning of my investigation. And it scales fine (I tried to send huge JSON documents). Of course I made sure the HTTP header of the post is the same in the case of my library and in the curl case.
Thanks a lot for reading me and for your answers.
Best regards,
Loïc
