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Updated Postgis async (markdown)

Bonnasseau 2013-09-24 06:43:20 -07:00
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Postgis-async.md

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The PostGIS pluging can be used asynchrnonously since Mapnik 2.3 which can reduce map rendering time if you have several PostGIS layers.
Since Mapnik 2.3 the PostGIS pluging can be used asynchrnonously to reduce the overall map rendering time.
## How it works
Mapnik uses the painter's algorithm to render maps. It means that layers are drawn sequentially. The inner algorithmer is :
@ -16,6 +16,7 @@ The asynchronous_request parameter in PostGIS pulgin aims to parallelize querrie
Must have :
* You have already applyed all the [rendering optimizations with PoistGIS](/wiki:OptimizeRenderingWithPostGIS/)
* you have a lot of PostGIS layers
Nice to have :
* rendering time for layers are quite homogeneous
* your PostGIS database is on another server
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## When not to use it
* you use cache-features=false ; if you want to reduce map rendering time, you should not send the same data base query twice if you have enough RAM to store the query restults
* you have less thant 3 PostGIS layers
* you have very heterogenous layers : for example, a huge road layers that takes times longer to render than the other layers
* you have very heterogenous layers : for example, a huge road layers that takes 8 times longer to render than the other layers
## How to use it
### Usage from XML
Set a value to *max_async_connection* and *asynchronous_request* to *true* in your existing PostGIS datasources :
<Layer name="countries" status="on" srs="+proj=latlong +datum=WGS84">
<StyleName>countries_style_label</StyleName>
<Datasource>
<Parameter name="type">postgis</Parameter>
<Parameter name="host">dbhost</Parameter>
<Parameter name="dbname">admin</Parameter>
<Parameter name="user">postgres</Parameter>
<Parameter name="password"></Parameter>
<Parameter name="table">world_worldborders</Parameter>
<Parameter name="asynchronous_request">true</Parameter>
<Parameter name="max_async_connection">3</Parameter>
</Datasource>
</Layer>