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Updated Postgis async (markdown)

Bonnasseau 2013-09-24 09:01:02 -07:00
parent 2d7b25561c
commit db6d4dda4d
1 changed files with 14 additions and 2 deletions
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Postgis-async.md

@ -9,7 +9,7 @@ Mapnik uses the painter's algorithm to render maps. It means that layers are dra
In this case, the renderer spends a lot of time waiting for PostGis to perform the query that will feed with features.
The asynchronous_request parameter in PostGIS pulgin aims to parallelize querries on the remote server and rendering : while a layer is rendering, queries are sent ahead.
The asynchronous_request parameter in PostGIS pulgin aims to parallelize queries on the remote server and rendering : while a layer is rendering, queries are sent ahead.
## When to use it
@ -39,6 +39,18 @@ Set a value to *max_async_connection* and *asynchronous_request* to *true* in yo
<Parameter name="password"></Parameter>
<Parameter name="table">world_worldborders</Parameter>
<Parameter name="asynchronous_request">true</Parameter>
<Parameter name="max_async_connection">3</Parameter>
<Parameter name="max_async_connection">4</Parameter>
</Datasource>
</Layer>
### How to set max_async_connection
*max_async_connection* sets the size of the number of databases connections that can run in parallel for the rendering of one map. Concretely, it means how many layers to load features ahead.
Let's consider an example, with the number of layers geographical features for 7 layers :
1. countries 500
1. urban areas 550
1. parcs 620
1. commercial areas 580
1. lakes 570
1. roads 2500
1. cities 300
Let's we assume database query time and drawing time are equal and proportionnal to the number of features in the layer.
The largest layer is *roads* ; it is 4 time larger thant the others. Hence we should launch the query to get the features for roads before the drawing of layer *urban areas*, so that the query is finished when the drawing of roads is about to start.