Socket open and close on 1 sec or to hold open

Question

I need to have periodically communication with plc ( on every 1 sec ), I send message and I receive message. I use Socket class for this communication. Do I need to every 1 sec to open connection ( socket=new Socket(ipaddress, port) ), send messagethen socket.close() and so on , or to hold socket opet all time ?

Answer 1

I'll assume you're talking about TCP sockets here...

Apart from the obvious inefficiencies involved in setting up a TCP connection every second you're also likely to end up accumulating sockets in TIME_WAIT (hopefully on your client).

I've written about TIME_WAIT and the problems it causes with regards to server scalability and stability here on my blog: http://www.serverframework.com/asynchronousevents/2011/01/time-wait-and-its-design-implications-for-protocols-and-scalable-servers.html

Given the rate that you are opening and closing sockets (once a second would result in 240 (60*4) sockets sitting in TIME_WAIT in the normal (4 minute) 2MSL TIME_WAIT period) this shouldn't prove too much of a problem assuming the TIME_WAIT sockets ARE ending up on the client and not on the server and assuming you're not connecting to lots of servers every second, but... If you have many clients connecting to your server every second and you are not making sure that your server doesn't accumulate sockets in TIME_WAIT state then you may limit your server's scalability.

The alternative is to hold the socket connection open and only reopen it if and when it gets disrupted. This may prove slightly more complex for you to initially program but pooling the connection in this way is likely to be considerably more efficient (when you DO need to send data you just send the data and don't need to go through the TCP handshake to set the connection up) and much more resource efficient on the client; you're not perpetually holding 240 sockets in TIME_WAIT...

Answer 2

Keeping the socket always connected will reduce network traffic and computation time of the client. However, if the server uses blocking I/O it may run out of connection threads if many clients are remaining connected. You will also have to deal with dropped connections due to timeout, network issues and server downtime.