Overview

Currently the map layer data utilized by the Standard Search UI 2D and 3D maps rely on external tile services.  In cases where the DDF deployment is on a private network and potentially has no or sporadic connectivity to a tile server, the DDF Search UI functionality will be limited to textual search and result visualization.  Even in cases where a tile server is available, map performance map may be poor due to network latency or bandwidth limitations.  An ideal solution to this problem would include

• local tile caching
  • automatic cache seeding based on user interaction with the map 
  • administrative seeding
• tile service aggregation (1 DDF endpoint aggregates multiple external WMS services)
• runs in the DDF container without extensive modification
• includes administration services DDF can utilize
• open source licensing

GeoWebCache

http://geowebcache.org/ - licensed under LGPL.  

GWC provides seeding/caching of tiles from WMS providers and a proxy capability to get around cross domain policy issues when accessing external WMS services. GWC also provides aggregation of tile services, and multiple endpoints for accessing tile data, including WMS 1.1.1, WMTS 1.0.0, and TMS 1.0.0.

Endpoints

• geowebcache/rest
  • rest API which includes seeding tiles from remote WMS providers and reloading configurations
• geowebcache/demo
  • web pages for showing configured WMS providers and viewing map tiles - disabled for DDF
• geowebcache/home
  • web page providing a service listing (WMTS/WMS/TMS) and runtime statistics - disabled for DDF
• geowebcache/service
  • endpoint for WMS/WMTS/TMS services

Configuration 

• WMS services are configured via an XML file - geowebcache.xml, but DDF will utilize the GWC REST services with an admin UI plugin
• the configuration can be reloaded at runtime using the geowebcache//rest/reload endpoint 
• Disk quotas can be configured via an xml configuration file.  See (http://geowebcache.org/docs/current/configuration/diskquotas.html)

Security

GWC utilizes spring security for its endpoints.  Luckily, security can be disabled by modifying the spring security config (WEB-INF/acegi-config.xml).  DDF security works with the GWC endpoints as with any of the other DDF endpoints.

GeoWebCache REST Service examples

Retrieve a list of configured layers

curl -v -k "https://localhost:8993/geowebcache/rest/layers" 

<layers>
  <layer>
    <name>img states</name>
    <atom:link xmlns:atom="http://www.w3.org/2005/Atom" rel="alternate" href="https://localhost:8993/geowebcache/rest/layers/img+states.xml" type="text/xml"/>
  </layer>
  <layer>
    <name>raster</name>
    <atom:link xmlns:atom="http://www.w3.org/2005/Atom" rel="alternate" href="https://localhost:8993/geowebcache/rest/layers/raster.xml" type="text/xml"/>
  </layer>
</layers>

Retrieve a specific layer

curl -v -k "https://localhost:8993/geowebcache/rest/layers/img+states.xml"

<wmsLayer>
  <name>img states</name>
  <mimeFormats>
    <string>image/gif</string>
    <string>image/jpeg</string>
    <string>image/png</string>
    <string>image/png8</string>
  </mimeFormats>
  <wmsUrl>
    <string>http://demo.opengeo.org/geoserver/wms?</string>
  </wmsUrl>
  <wmsLayers>nurc:Img_Sample,topp:states</wmsLayers>
</wmsLayer> 

Delete a layer

curl -v -k -XDELETE "https://localhost:8993/geowebcache/rest/layers/raster.xml"

Add a layer

curl -v -k -XPUT -H "Content-type: text/xml" -d @states.xml "https://localhost:8993/geowebcache/rest/layers/states.xml"

<wmsLayer>
  <name>states</name>
  <mimeFormats>
    <string>image/gif</string>
    <string>image/jpeg</string>
    <string>image/png</string>
    <string>image/png8</string>
  </mimeFormats>
  <wmsUrl>
    <string>http://demo.opengeo.org/geoserver/wms</string>
  </wmsUrl>
  <wmsLayers>nurc:Img_Sample,topp:states</wmsLayers>
</wmsLayer>

 Modify a layer

curl -v -k -XPOST -H "Content-type: text/xml" -d @states2.xml "https://localhost:8993/geowebcache/rest/layers/states.xml"

<wmsLayer>
  <name>states</name>
  <mimeFormats>
    <string>image/gif</string>
    <string>image/jpeg</string>
    <string>image/png</string>
  </mimeFormats>
  <wmsUrl>
    <string>http://demo.opengeo.org/geoserver/wms</string>
  </wmsUrl>
  <wmsLayers>nurc:Img_Sample,topp:states</wmsLayers>
</wmsLayer>

GeoWebCache Layer Schema

The layer schema for GWC can be found here: http://geowebcache.org/schema/1.5.0/geowebcache.xsd.  Key wmsLayer elements:

Prototype in DDF

• See https://github.com/jlcsmith/ddf/tree/geowebcache/catalog/geowebcache
  
  

Remaining work

• documentation
• fix logging to use PAX Web logging (currently logging goes to std out)
• create admin UI plugin for
  • service configuration 
    • ability to add/modify/delete tile services
  • seeding
  • diskquota control (see http://geowebcache.org/docs/current/configuration/diskquotas.html)
• verify HTTPS to external tile services - how do certs need to be configured?
• consider removal the use of the HTTP proxy in DDF for tile services (GWC is its own proxy)