At 20:25 17/10/2004 +0200, Tomas Franz�n wrote:
What would be a good value for N? That is, how often should I send a keep-alive?
Tricky question - I wouldn't make any guesses without more info - see questions below.
The server should be able to handle many (preferably MANY) clients at once. Still, the teachers need to get feedback about the disconnected state of the student as soon as possible.
In wireless environments, where students carry around their laptops and run out of power, the connections may be unexpectedly closed quite often.
Can you define "many" and "MANY" ? (i.e. 50 v 500 v 5,000 v 50,000 ....)
Does anyone have experience in handling tons of clients at once with Rev? Got any tips?
[ I don't have that experience with Rev - any tips I give are general-purpose, network protocol or distributed algorithm tips. ]
Questions (to help answer how many seconds should N (keepalive timer) be) (sorry if these are repeating info you gave earlier in the thread ...)
1. are you using TCP (open socket) or UDP (open datagram socket) ?
2. if TCP - do you open a connection per transaction (cf most HTTP / www) or open one and keep it open for multiple transactions (e.g persistent connections, as added in HTTP 1.1) ?
2a. (if persistent connection) does the server close inactive TCP connections to preserve its TCP connections (most servers will only be able to have a limited number of TCP connections open simultaneously)?
3. how many transactions per second (or per minute or per hour - whichever is appropriate) would a typical teacher and a typical student be involved in ?
4. probably most important - how quickly do you need to be able to respond ?
5. what kind of failures are you trying to detect ? - loss of power, network, etc. - student disconnecting deliberately (playing hooky :-)
6. what kind of response will be made when a loss is detected ?
what order of magnitude time-frame is involved ?
(i.e. machine speed, human speed, school administration speed ?)if you were doing UDP transactions with a need for machine-speed response and "many" <= 50 - then you might make N as small as 1 second (but more likely 5)
if you were doing TCP with a new TCP connection per transaction (and hence per keepalive), and many <= 5000 and a typical response would be the teacher walking somewhere to react, I'd make N be 60 seconds.
if it's hard to answer the above questions ahead of deployment, then make N somewhere between those two (5 and 60) and easy to change, and see what happens .....
-- Alex.
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