[Tilesathome] Rate limiting on upload page

[OJW]

We're testing a system that only accepts uploads if the server is more than a 
certain amount idle.  The reason being that map viewing is a realtime thing 
that people rely on for a certain response rate, so some CPU/disk-access time 
needs to be reserved for that no matter how many uploads are coming in.

The message is currently "501, server is quite busy", and the limit is 
currently "must be > 20% idle".

Unfortunately (because it's in PHP), this calculation is only done _after_ the 
file is transferred, which isn't ideal for any contributors with limited 
bandwidth.

If you'd like to know in advance whether your upload is likely to succeed, 
I'll create a page that says "GO" or "STOP" depending on what the current 
status is. Don't forget that status may change rapidly!

http://dev.openstreetmap.org/~ojw/Upload/go_nogo.php

Any suggestions for limiting mechanisms that work earlier in the upload 
process (e.g. apache config) would be appreciated.  Remember that the system 
mustn't oscillate - anything which checks once per minute and 
allows/disallows all uploads within the subsequent minute will give you a 
cycle of one minute overloaded, next minute totally idle.

Why not rate-limit requests instead?  Well there's a long and variable 
time-delay between requests and uploads, plus many people don't take any 
notice of the requests queue and run their own one.  Limiting at the upload 
stage is likely to be more reliable at the moment.  But if you have an idea 
for rate-limiting in requests queue, or in the client, or at any other point 
in the application, please post your code to SVN.

If the tiles at home client would like to implement it, we could have a priority 
for uploads, so that if the request you took was priority-1 (means that 
someone is waiting for it) then you could say so while uploading and get 
better access to the upload page.

Regards,

OJW