Output Specification

Koop output plugins are used to transform data coming from a Koop provider to any specific output format (e.g GeoServices response, vector-tiles, WMS, RSS, CSV etc). Generally, Output plugin class defines service routes and handlers that accept incoming requests, handle errors and filters or transforms provider data before sending it back to the client. Koop-core ships with a default output plugin koop-output-geoservices.

An output plugin directory must contain index.js file that exports a class with following properties:

  • version – Version for your output plugin
  • type – Value of this property must be 'output'
  • routes - An array of objects with path, methods and handler. path defines the output plugin routes, methods contain valid HTTP methods and handler is the class method name for handling the request.

A basic example is below:

const version = require('../package.json');

class OutputPlugin {
    static type = 'output';

    static version = version;

    static routes = [{
        path: '/define/your/route/path',
        methods: ['get'], // HTTP methods recoginized by Express.js
        handler: 'serve', // Class method that will handle the request
    }, ];

    async serve(req, res, next) {
        try {
            const geojson = await this.model.pull(req); // pull geojson from provider
            const outputData = this.#filterTransform(geojson); // filter/transform
            res.status(200).json(outputData); // send response
        } catch (err) {

    #filterTransform(geojson) {
        // filter/transform geojson here
        return transformedGeojson

module.exports = Output

Inside the request handler serve, we have access to Express request and response object. request is passed to Koop provider using any of pull methods specified below to retrieve geoJSON data from the provider:

  • pull (request) - Mostly used
  • pullLayer (request)
  • pullCatalog (request)
  • pullStream (request) - Only used if provider implements Node.js streams to send response

Depending upon what the provider you are using supports for sending geoJSON, call the appropriate pull method by this.model._pullMethod_ . Once data is received, we can pass it through our custom transform or filter function. In the example above, we have a class method called #filterTransform which takes geojson from the provider then filters the geoJSON which could implement any sort of custom data manipulation. Then, we finally return the transformed output data to the client. We also wrap all the operations of data retrieval and transformation in try...catch block to handle any errors.

See the koop-output-flat source files for an example of a simple output-plugin.

Improve this page