[Geoprisma-dev] Variable substitution in GeoPrisma configuration

[Etienne Dube]

Hi all,

I've been thinking of a useful feature to add to GeoPrisma: variable 
substitution for configuration parameters.


Rationale:

Suppose you have a very long and complex XML configuration file. This 
file will be deployed in different environment, all with different 
characteristics: development, test and production. Each environment can 
have its own file paths and different URLs to access the services. It is 
somewhat of a pain to maintain three separate XML configuration files 
for these environments. Wouldn't it be awesome if instead, we could just 
use placeholder variables for the environment-specific parts of the 
configuration, and have GeoPrisma load the values for these variables 
from an external source (be it a properties file or a database table) 
and substitute them dynamically at runtime?

For example:

In gp.xml:

<services>
   <wms>
     <name>S_MyWMS</name>
     <srs>*</srs>
     <format>*</format>
     <source>http://${s_mywms_host}/mywms</source>
   </wms>
</services>

In gp.properties for development env.:

s_mywms_host=devserver.bovine-university.edu

In gp.properties for production env.:

s_mywms_host=prodserver.bovine-university.edu

... and so on.


Implementation considerations:

Now from what I've gathered, the logical place to add such a feature 
would be in Config.php. However the different implementations of the 
Config abstract class have their own way of getting their configuration 
attributes from their respective source (XML file or DB), so changes to 
these subclasses would be required as well (probably in 
XMLConfig.simpleXMLToArray() and PGSQLMapContextConfig.getRecords() ).

A few questions as well:

  - Do we want to allow variable substitution for all configuration 
parameters? Or should we restrict it to only specific elements such as 
services?
  - Should we provide a way to escape the variable placeholders (${...} 
-- another syntax could be used as well), to allow the uncommon 
occurrences of passing a string containing ${...} as is?
  - What would be the best way to reference the external properties file 
/ table? Should it be a parameter in the configuration, or do we want to 
have it hardcoded (e.g. always look for a gp.properties file in the same 
location as gp.xml).

As always, any input is appreciated.

Thanks!
Etienne

-- 
Etienne Dubé
Developer

Boreal - Information Strategies
202A-101 rue du Moulin
Magog, Québec, J1X 4A1, Canada
http://www.boreal-is.com

Tel. :  +1 514 313 5951 ext:1131
Email: etienne.dube at boreal-is.com