Detecting unexpectedly closed sockets

[Alex Tweedly]

At 20:25 17/10/2004 +0200, Tomas Franzén 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.