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MapCache has the ability to forward any incoming request that it cannot natively respond to (either by returning a tile directly, by merging multiple tiles, etc...).
This setup allows mapcache to be placed transparently in front of an existing OGC-service supplying server to accelerate tiled or getmap requests for a selected number of grids, while maintaining service compatibility for, e.g., unsupported grids, WFS requests, ...
The proxying of requests is configured inside the WMS MapCache service, which is semantically awkward.
The configuration for this behavior is activated by a succession of entries inside the <forwarding_rule> element of the wms <service>. Rules are tested for in the order in which they appear in the mapcache.xml configuration file, and the first one that matches is used. If no rules are defined, or if no rule matches the incoming request, an error is returned to the user.
<service type="wms" enabled="true"> <forwarding_rule name="first rule"> <!-- rule tests --> <!-- proxy destination --> </forwarding_rule> <forwarding_rule name="second rule"> <!-- rule tests --> <!-- proxy destination --> </forwarding_rule> </service>
A <forwarding_rule> consists of a set of matching rules and an <http> block defining where the request should be forwarded to.
The rules apply to the KVP parameters that were passed in the incoming request, and are added with the <param> keyword:
<forwarding_rule name="first rule"> <param name="SERVICE" type="values">WFS,WCS</param> <!-- ... !> <forwarding_rule>
The “type” attribute is the same that what is allowed for dimensions, i.e. allowed values are “values”, “regex”, and “intervals”. In the previous example, the rule would match any incoming request having ...&SERVICE=WFS&... or ...&SERVICE=WCS&... in its request parameters.
<forwarding_rule name="first rule"> <param name="SERVICE" type="values">WFS,WCS</param> <param name="LAYERS" type="values">somelayername</param> <!-- ... !> <forwarding_rule>
Multiple rules can be used if the filtering has to be done on mutiple parameters. In the previous example, the rule would match a WFS or WCS request that concerns the “somelayername” layer only.
A <forwarding_rule> that has no <param> child will match any incoming request that could not be serviced by MapCache directly from its cache, and can be used to forward all unsupported request to a full OGC compliant server so that an un-cached response can be returned to the client.