well, we can hardcode 300 second as minimum refresh time.

anyway, I think there is to be update on the engine.
what bigger the board grows, that more using todays active topics as base change.log looks good.

what comes to bandwidth.
if the server is on 10M net, it would be better not to give this to it's users.
then only 150 users running on 20sec refresh would eat up 90-95% of the bandwidth.

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