The WPS Hook¶

In GeoScript JS, the geoscript/process module provides a Process constructor. A process object wraps a function with a title, description, and additional metadata about the inputs and outputs. With the GeoServer scripting extension, when a script exports a process, it is exposed in GeoServer via the WPS interface.

To better understand how to construct a well described process, we’ll examine the parts of the previously provided buffer.js script:

var Process = require("geoscript/process").Process;

exports.process = new Process({
  title: "JavaScript Buffer Process",
  description: "Process that buffers a geometry.",
  inputs: {
    geom: {
      type: "Geometry",
      title: "Input Geometry",
      description: "The target geometry."
    },
    distance: {
      type: "Double",
      title: "Buffer Distance",
      description: "The distance by which to buffer the geometry."
    }
  },
  outputs: {
    result: {
      type: "Geometry",
      title: "Result",
      description: "The buffered geometry."
    }
  },
  run: function(inputs) {
    return {result: inputs.geom.buffer(inputs.distance)};
  }
});

When this script is saved in the $GEOSERVER_DATA_DIR/scripts/wps directory, it will be available to WPS clients with the identifier js:buffer. In general, the process identifier is the name of the script prefixed by the language extension.

First, the require function is used to pull in the Process constructor from the geoscript/process module:

var Process = require("geoscript/process").Process;

Next, a process is constructed and assigned to the process property of the exports object. This makes it available to other JavaScript modules that may want to import this process with the require function in addition to exposing the process to GeoServer’s WPS. The title and description provide WPS clients with human readable information about what the process does.

exports.process = new Process({
  title: "JavaScript Buffer Process",
  description: "Process that buffers a geometry.",

All the work of a process is handled by the run method. Before clients can execute a process, they need to know some detail about what to provide as input and what to expect as output. In general, processes accept multiple inputs and may return multiple outputs. These are described by the process’ inputs and outputs properties.

inputs: {
  geom: {
    type: "Geometry",
    title: "Input Geometry",
    description: "The target geometry."
  },
  distance: {
    type: "Double",
    title: "Buffer Distance",
    description: "The distance by which to buffer the geometry."
  }
},

The buffer process expects two inputs, named geom and distance. As with the process itself, each of these inputs has a human readable title and description that will be provided to WPS clients. The type property is a shorthand string identifying the data type of the input. See the Process API docs for more detail on supported input and output types.

outputs: {
  result: {
    type: "Geometry",
    title: "Result",
    description: "The buffered geometry."
  }
},

The buffer process provides a single output identified as result. As with each of the inputs, this output is described with type, title, and description properties.

To see what this process metadata looks like to a WPS client, call the WPS DescribeProcess method:

http://localhost:8080/geoserver/wps
  ?service=WPS
  &version=1.0.0
  &request=DescribeProcess
  &identifier=js:buffer

Finally, the run method is provided.

  run: function(inputs) {
    return {result: inputs.geom.buffer(inputs.distance)};
  }
});

The run method takes a single inputs argument. This object will have named properties corresponding the the client provided inputs. In this case, the geom property is a Geometry object from the geoscript/geom module. This geometry has a buffer method that is called with the provided distance. See the Geometry API docs for more detail on available geometry properties and methods.

The run method returns an object with properties corresponding to the above described outputs - in this case, just a single result property.

To see the results of this processs in action, call the WPS Execute method:

http://localhost:8080/geoserver/wps
  ?service=WPS
  &version=1.0.0
  &request=Execute
  &identifier=js:buffer
  &datainputs=geom=POINT(0 0)@mimetype=application/wkt;distance=10