Persistent Connection on a web server HTTP1.1

Question

I'm trying to write a web server in C under Linux using protocol HTTP1.1 . I've used select for multiple requests and I'd like to implement persistent connections but it didn't work so far 'cause I can't set a timeout properly. How can I do it? I think about setsockopt function:

setsockopt(connsd, SOL_SOCKET, SO_RCVTIMEO, (char *)&tv, sizeof(tv))

where tv is a struct timeval. This isn't working either.

Any suggestions?

Answer 1

SO_RCVTIMEO will only work when you are actually reading data. select() won't honor it. select() takes a timeout parameter in its last argument. If you have a timer data structure to organize which connections should timeout in what order, then you can pass the soonest to timeout time to select(). If the return value is 0, then a timeout has occurred, and you should expire all timed out connections. After processing live connections (and re-setting their idle timeout in your timer data structure), you should again check to see if any connections should be timed out before calling select() again.

There are various data structures you can use, but popular ones include the timing wheel and timer heap.

A timing wheel is basically an array organized as a circular buffer, where each buffer position represents a time unit. If the wheel units is in seconds, you could construct a 300 element array to represent 5 minutes of time. There is a sticky index which represents the last time any timers were expired, and the current position would be the current time modulo the size of the array. To add a timeout, calculate the absolute time it needs to be timed out, modulo that by the size of the array, and add it to the list at that array position. All buckets between the last index and the current position whose time out has been reached need to be expired. After expiring the entries, the last index is updated to the current position. To calculate the time until the next expiration, the buckets are scanned starting from the current position to find a bucket with an entry that will expire.

A timer heap is basically a priority queue, where entries that expire sooner have higher priority than entries that expire later. The top of a non-empty heap determines the time to next expiration.

If your application is inserting a lots and lots of timers all the time, and then cancelling them all the time, then a wheel may be more appropriate, as inserting into the wheel and removing from the wheel is more efficient than inserting and removing from a priority queue.

Answer 2

The simplest solution is probably to keep a last-time-request-received for each connection, then regularly check that time and if it's too long ago then close the connection.