[Tilesathome] Is it me?

[Rob Reid]

Ed Loach wrote the following on 07/09/2008 08:24:
> I'm having problems downloading data this evening. I'm on my third attempt at the tile it locked up at and have:
>
> This is version 10416 (Saurimo) of tilesgen running on MSWin32, ID: 38271
> [#1   0% Download] Tileset (12,2140,1367) around 51.21,8.13
> [#1   0% Download] Downloading: Map data for captionless,tile,maplint...
> [snip]
> Aha! On the 5th attempt it gained an extra few lines:
>
> [#1   0% Download] Trying ROMA
> [#1   0% Download] Trying smaller slices
> [#1   0% Download] Downloading: Map data (slice 1 of 10)...
>
> And now seems to have stopped there instead.
>
> The 6th attempt seemed to work normally, and is now transforming, etc.
>   
Ed, I can't say if its just you or not as I'm have not been running a 
client for the last few hours but what you are seeing is normal client 
behaviour under certain circumstances.

- The pause after at the point 'Downloading: Map data.....' is when it  
trying to download the map data for the tiles you have requested.  This 
data is downloaded from a main OSM api server which is separate to the 
T at H servers and has to balance the load between requests from T at H 
clients and other sources. If this server is too busy your client will 
see a long pause (can be a few minutes) while it waits for data and then 
it will time out and move on to trying other sources of map data such as 
ROMA or XAPI.
- ROMA and XAPI are other sources of the map data maintained separately 
and up to about 5 min behind the OSM api server in terms of updating the 
map data. It's possible for these map sources to be busy or unavailable 
as well and again the client will pause until it times out.
- If the client still can't get data it trys requesting the data in 
smaller chunks in the hope that a request for a smaller amount of data 
might succeed. I think these requests just go to the main OSM api server.
- If the client has tried all available map sources and still got no 
data it will stop trying to process the current tile. If you are in loop 
mode it will pass the tile back to the T at H and inform it that it could 
not get any data.

There is a few useful sources of info to tell if the problems are just 
you are affecting everyone.
- The T at H server <http://tah.openstreetmap.org/> page shows some log 
info so is useful to see if other people are handing back tiles due to 
be unable to get data, look for messages like 'no data here!'. This 
server is also the one that hands out tilesets and receives uploaded 
tilesets so if you can't access this page then your client will probably 
be having problems too.
- This page 
<http://munin.openstreetmap.org/openstreetmap/puff.openstreetmap-lighttpd_queues.html> 
shows the queue on the main OSM api server, look for the 'tah 80' line.
- This page 
<http://munin.openstreetmap.org/openstreetmap/tah.openstreetmap.html#T@H> 
shows various charts for the T at H server like how full the upload queue 
is etc.
With the munin charts its hard to say what values are ok and which imply 
problems, all I can really say is that if you watch them occasionally 
you will build up a picture of normal behaviour and be able to spot 
unusual behaviour.

cheers

rcr