Wednesday, November 12, 2014

We hit the Speed limit of python (Peer closed the socket connection)

Python is a beautiful language but we know the limitations of the python compared with compiled language. So here I'm explaining a scenario where python was not the ideal choice. We are using python for network programming, specifically a TCP client/server program which read packets from another C based server. We picked python because on the requirements the traffic volume wasn't huge enough to consider compiled languages like C/C++ or Java, and we are already using python so more comfortable with it. 

The Network Topology 

The data flowing from Server A to Server B via TCP. The Server B is implemented in python for now. This setup was running for more than a year without any problems or crashing. Then it started showing problems. The Server B crashing sometime because of the socket connection between the Server A and Server B getting closed by the Server A, We don't know why. Bellow listed are the errors that we are getting, one interesting thing is; 
these two errors won't happen together, some time Errno 104, some time Errno 107. 

  1. Errno: 104 - Peer closed the socket connection. 
  2. Errno: 107 - Transport endpoint closed. 

I initially thought this was happening because of python's long running process exiting due to memory overflow or some other reasons. We thought this because python's reputation as a long running daemon is not great. Then we figure out that long running daemon wasn't the reason for this issue, since the main python process wasn't consuming more RAM and it's very stable in memory usage. Our python server using very simple data structures, so python VM very efficiently handling the Memory. Then I started to look into other areas of program, by changing the settings and tuning parameters to see any of it fixes the issue. All of the options seems to work for a while but it throws the same error after few days. I was stuck and no idea about the issue and spend around a week to go through the program's in and out to see any possible causes. But no clue from it. Finally Stack Overflow came into help, IRC wasn't helpful for these type of very specific issues. Programmers can't live without Google and Stack Overflow. I put the question on the SO, people helped me to make the question better by adding the TCP traces picked up using tcpdump. With in hours I got the answer I was looking for. It's mainly due the python socket reading related. In the Python side we are not clearing the TCP packets from the receive buffer of TCP socket completely, only reading few bytes (300B to 10KB) at a time. So which causing the Python server to send receive window size flag of TCP protocol as ZERO for multiple times In the heavy traffic period. This might triggering this issue. That was the idea I got from the stack Overflow. The problem on the python side is that, it's a single threaded server which reading the data from the socket, one thing we are right now not doing is reading until socket is empty due to some implementation restrictions. From the SO answer I got some quick solutions to try, Remove unwanted codes from the main loop, so the total time taken to process one socket read would be as small as possible. Tweak with the Kernel parameters for the TCP sending and receiving buffer size. Improve the Code base or re-implement it in C completely or do the main part as C extension. The method 1 and 2 was easy go ahead with, So I did it right away. It gives considerable difference and the crashes reduced considerably. But still it happened once or twice. So I know that it's not enough and need reimplementation of the parts which causing this problem. But we are holding on the reimplementation since it will take lot of time to change the existing stuff. Any way the current changes give enough time to try more things to improve the existing code and investigate the problem more. Mean while I did some more investigation locally to understand the issue more. From that what I understood was, when the network is heavily loaded the Python server side not clearing the traffic quick enough and which forcing the remote server to close the TCP socket (Not yet found why it's closing it). Right after the socket is closed on the remote end, Python side of the code might be trying socket.send or socket.recv and it will fail since no active socket. So in the beginning I mentioned that I'm getting two errors, this is because some time right after the remote server closed the socket, the next socket interaction in the python side may be socket.send which was triggering the error Errno 104. If it was socket.recv it would have been Errno 107. One other thing I learned after this was that, the TCP server usually handles the slow remote servers by blocking the new massage write into the sending buffer. In my environment I understood the remote server A is not blocking the write(Non-blocking socket) instead closing the socket( This could be due to timeout).If the TCP server has option to cache the extra packets into disk then it could have handled this peak traffic period. 

A TCP Server has to provide atleast these features 

Way to handle the buffer overflow if the client is very slow. Write the buffer overflowed data into durable queue or file system. When the Client comes back, the server has to automatically deliver the pending messages in FIFO order. 

Final Thoughts 

If these options are there with the TCP server that I was handling, this problem couldn't have happened. So finally now I can say that Python could have handled this situation if the traffic spikes some times only - In our case that was the situation. Python couldn't have done it if the rate of server is more than the rate at which Python can clear the socket backlog. 
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