[Tilesathome] Distributing tiles at home server chores?

[Frederik Ramm]

Hi,

    prepare for a solid round of backseat driving ;-)

The dev server cannot cope with running the tiles at home server. I don't 
know why exactly that is. Let's summarize what the tiles at home server does:

A. Managing the re-render queue

A1. accepting re-render requests
A2. handing them out to clients
A3. watching whether they really get completed

B. Accepting tile uploads

B1. unzip files being delivered
B2. sort them into the filesystem, update metadata MySQL

C. Serving tiles to web browsers

C1. serving through Apache from file system
C2. serving through 404 handler script (if Apache doesn't find the file, 
do a database lookup)

D. Statistics and cleanup jobs

Tasks "A" and "D" are negligible; "A" could conceivably be run on a 
totally different server with only little overhead, but it probably 
isn't even worth the effort to outsource it as the request queue only 
receives and dispatches one request every five seconds (on average).

"B" is very i/o intensive. There is considerable potential for 
optimising the code, but still "B" will have to create thousands of 
files on the hard disk for every tileset uploaded, and in addition write 
stuff to the database. The load doesn't become smaller if we consider 
that numerous processes do "B" at the same time, needlessly vying for 
resources.

"C" has massively improved since tiles are on the hard disk as opposed 
to in a BLOB in MySQL. I assume, but cannot prove, that "C" is only slow 
because there's so much "B" going on.

So in my eyes, "B" is the main culprit.

Unless we manage to drastically optimise the software at this point, we 
should think about distributing "B" to more than one server. How could 
this be done?

I could imagine that tiles could be distributed in a "striped" fashion - 
one server takes all level-12 tiles where x mod 3=0, one those with x 
mod 3=1, and one those with x mod 3=2 and so on. But how would a slippy 
map then load tiles from these servers? It could be done via a proxy, 
but it will probably be difficult to configure an Apache or Squid with 
the rules required to find the right server of origin for a tile:

* if z==12 compute x mod 3 and select server 0,1,or 2
* if z>12 compute relevant level-12 tile x coordinate (x/2**(z-12)) mod 
3 and select server 0,1,or 2
* z<12 - special case, probably only have one server with all these tiles

Any thoughts?

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00.09' E008°23.33'