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I just ran into these "prefixes" and can't figure them out. At first I saw a tile URL beginning with c. and was assuming it is a cached version. Then I noticed prefixes of a. and b.*. All of the prefixed URLs seem to be some sort of cached or older versions of the tiles in question but what?

I couldn't find information in the Slippy map wiki article ( and the related questions here don't seem to help either ( &

asked 02 Jun '11, 16:04

jaakkoh's gravatar image

accept rate: 0%

Round-robin load balancers?

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answered 03 Jun '11, 06:37

ivanatora's gravatar image

accept rate: 7%

Normaly the web browser would not simultaniously download more then a few urls from a server. This leads to slower download when browsing the map. To trick the browser to download more tiles faster OpenLayers have support for several servers. Therefore the tile server have several domains for the same server. This trick makes the browser open up more connections to the server witch leads to faster download.

The tile server have two cache servers in front that are load balanced by theese urls. This might cause some tiles to be older on one server then the other.

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answered 02 Jun '11, 19:19

Gnonthgol's gravatar image

Gnonthgol ♦
accept rate: 16%

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question asked: 02 Jun '11, 16:04

question was seen: 6,046 times

last updated: 03 Jun '11, 06:37

NOTICE: is no longer in use from 1st March 2024. Please use the OpenStreetMap Community Forum