Talking to OGC Web Services in JSON Or How I Learned to Stop - - PowerPoint PPT Presentation

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Talking to OGC Web Services in JSON Or How I Learned to Stop - - PowerPoint PPT Presentation

Apr 15, 2023 357 likes •786 views

Talking to OGC Web Services in JSON Or How I Learned to Stop Worrying and Love XML Processing in JavaScript 1 Hi, my name is Alexey Valikov http://xing.to/va https://github.com/highsource @orless 2 and Im (also) into GIS and

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Talking to OGC Web Services in JSON

Or How I Learned to Stop Worrying and Love XML Processing in JavaScript

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Hi, my name is …

Alexey Valikov http://xing.to/va https://github.com/highsource @orless

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… and I’m (also) into GIS and Web Mapping

GIS 2go Internal apps for the Deutsche Bahn

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Web Mapping means JavaScript

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JavaScript speaks JSON

{ "type": "Feature", "geometry": { "type": "Point", "coordinates": [125.6, 10.1] }, "properties": { "name": "Dinagat Islands" } }

[ˈdʒeɪsən]

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GIS-Services are based on OGC Standards

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… specified by XML Schemas

More than 50 Schemas More than 100 individual versions Over 8MB and 800 individual XSD files

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OGC Web Services speak XML

GetCapabilities <wfs:WFS_Capabilities … /> GetFeature <wfs:FeatureCollection … /> Client WFS Server

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JS Apps have a communication problem

JS App WFS Server {”JS”:”ON”} ??? XSD

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<x:ml/>

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Jsonix provides a solution

Mapping

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WFS Server {”JS”:”ON”}

Jsonix

XSD <x:ml/> JS App

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What is Jsonix?

Jsonix is a powerful Open-Source JavaScript library for XML↔JS conversion https://github.com/highsource/jsonix

Bidirectional, type-safe, strongly-structured Driven by declarative XML↔JS mappings… … which can be generated from XML schemas automatically

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Further Jsonix features

Works in browsers Works in Node.js Works with AMD, CommonJS and without (globals) Namespace-aware Supports almost all of the XML Schema simple types (including binary types and QNames) Supports xsi:type And much more

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Jsonix Example

Parse WMS Capabilities JSFiddle: http://bit.do/jsonix-001

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Parsing with Jsonix - Code Walk-Through

var getCapabilitiesUrl = …; // First we create a context for XLink and WMS mappings var context = new Jsonix.Context([XLink_1_0, WMS_1_3_0], …); // Then we create an unmarshaller (parser) var unmarshaller = context.createUnmarshaller(); // And finally use this unmarshaller // to parse an XML document from the given URL unmarshaller.unmarshalURL(getCapabilitiesUrl, function(result) { // We’ll get results in a callback function $('#json').html(JSON.stringify(result, null, 2)); });

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Jsonix creates pretty JSON

<WMS_Capabilities … version="1.3.0" updateSequence="163"> <Service> … </Service> <Capability> <Request> … </Request> <Exception> … </Exception> <Layer> … <Layer queryable="1">…</Layer> <Layer queryable="1" opaque="0">…</Layer> … </Layer> </Capability> </WMS_Capabilities> { "WMS_Capabilities": { "version": "1.3.0", "updateSequence": "163", "service": { … }, "capability": { "request": { … }, "exception": { … }, "layer": { … , "layer": [ … ] } } } }

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… and can serialize this JSON back to XML

// Create (or reuse) the Jsonix Context var context = new Jsonix.Context([XLink_1_0, WMS_1_3_0], …); // Create a marshaller (serializer) var marshaller = context.createMarshaller(); // Serialize JSON as XML string or DOM node $('#xml').text(marshaller.marshalString(result));

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The same thing without Jsonix?

OpenLayers 3: ol.format.

WMSCapabilities

JSFiddle: http://bit.do/jsonix-002

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Let’s take a closer look into the OL3 code

  • l.format.WMSCapabilities
  • ca. 28 kB, ca 0.8 KLoC

Type-safe Strongly-structured Only parsing Written manually Super exciting code

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Why yet another XML-JS tool?

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Jsonix is strongly-structured and type-safe And that’s unparalleled

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What does “strongly-structured” mean?

<WMS_Capabilities … version="1.3.0" updateSequence="163"> <Service> … </Service> <Capability> <Request> … </Request> <Exception> … </Exception> <Layer> … <!-- Just one Layer element --> <Layer queryable="1"> … </Layer> </Layer> </Capability> </WMS_Capabilities>

How should the Layer element be represented in JSON? As a single element or as an array? capability.layer.layer? capability.layer.layer[0]? You can’t decide it just based on the XML instance You have to know the schema

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Jsonix is strongly-structured

Jsonix knows the structure of the XML from the mapping … and respects the defined cardinalities, element orders, properties, naming and so on Jsonix always produces JSON and XML-structures which respect the definitions from the mapping

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Jsonix produces reliable structures

<WMS_Capabilities … version="1.3.0" updateSequence="163"> <Service> … </Service> <Capability> <Request> … </Request> <Exception> … </Exception> <Layer> … <Layer queryable="1"> … </Layer> </Layer> </Capability> </WMS_Capabilities>

layer is an array: capability.layer.layer[0] Because the mapping says so:

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Strong, reliable structures === simpler code

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What does “type-safe” mean?

<WMS_Capabilities … version="1.3.0" updateSequence="163"> <Service> … </Service> <Capability> <Request> … </Request> <Exception> … </Exception> <Layer> … <Layer queryable="1"> … </Layer> </Layer> </Capability> </WMS_Capabilities>

Which type should queryable=”1” have in JSON? String? Number? Boolean? You can’t decide it just based on the XML instance You have to know the schema

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Jsonix is type-safe

Jsonix knows types of elements and attributes from the mapping … and converts strings from/to these types automatically Jsonix always produces values in JSON and XML according to the types defined in the mapping

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Jsonix converts the types automatically

<WMS_Capabilities … version="1.3.0" updateSequence="163"> <Service> … </Service> <Capability> <Request> … </Request> <Exception> … </Exception> <Layer> … <Layer queryable="1"> … </Layer> </Layer> </Capability> </WMS_Capabilities>

queryable is a boolean: { "queryable" : true, … } Because the mappings says so:

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Safe, automatic type conversion === less work

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Jsonix uses declarative mappings

Jsonix mappings are just descriptions of structures, not imperative programs

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Mappings are essential for strong structures and safe typing

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Jsonix can generate mappings from XSDs

Jsonix Runtime XSD Mapping Jsonix Schema Compiler {“JS”:”ON”} <x:ml/>

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..as follows

java -jar jsonix-schema-compiler.jar

  • catalog schemas/catalog.cat

schemas/ogc/wms/1.3.0/capabilities_1_3_0.xsd

  • b schemas
  • d mappings

Generates Jsonix mappings for WMS 1.3.0: WMS_1_3_0_Full.js GitHub: http://bit.do/jsonix-006

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Experimental: Generate JSON Schemas from XSDs

Jsonix Runtime XSD Mapping Jsonix Schema Compiler {“JS”:”ON”} <x:ml/>

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JSON Schema

Experimental

java -jar jsonix-schema-compiler.jar

  • generateJsonSchema …
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Experimental: Validate JSON against generated JSON Schemas Using the great “Another JSON Schema Validator” AJV

// Load JSON Schemas var ajv = new Ajv(); ajv.addSchema(XMLSchemaJsonSchema, … ); ajv.addSchema(JsonixJsonSchema, … ); ajv.addSchema(XLink_1_0JsonSchema, 'http://www.w3.org/1999/xlink'); // Compile the validation function and validate var validate = ajv.compile(WMS_1_3_0JsonSchema); if (!validate(capabilitiesElement)) { console.log(validate.errors); }

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[{ keyword: 'required', dataPath: '.value.capability.request.getCapabilities.dcpType[\'0\'].http', message: 'property .http is required' }, … ]

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Compiling XSDs into mappings is hard

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By Alex E. Proimos (http://www.flickr. com/photos/proimos/4199675334/) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

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Compiling XSDs into mappings is hard

We’ve huge number of schemas with complex interdependencies Schemas sometimes have errors We have to resolve naming collisions We’re often forced to customize generation or even patch schemas

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(Unofficial) OGC Schemas Project

https://github.com/highsource/ogc-schemas Pre-generated mapping for OGC Schemas: JS↔XML: Jsonix mappings Java↔XML: JAXB classes Available via npmjs.org and Maven Central

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(Unofficial) OGC Schemas Project

Compiles over 30 OGC Schemas with more than 80 single versions OWS, WMS, SLD, GML, Filter, WFS, WPS, WCS, WMC, SWE, SPS, SOS, OM, SensorML, KML, ISO 19139, CSW, and many more… … ready-to-use with Jsonix

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Using pre-generated mappings

var mappings = [XLink_1_0, SMIL_2_0, SMIL_2_0_Language, GML_3_1_1, OWS_1_0_0, Filter_1_1_0, DC_1_1, DCT, CSW_2_0_2]; // Create a Jsonix context var context = new Jsonix.Context(mappings, { namespacePrefixes: { "http://www.opengis.net/cat/csw/2.0.2": "csw", "http://www.opengis.net/ogc": "ogc", "http://www.opengis.net/gml": "gml" } });

JSFiddle: http://bit.do/jsonix-007

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Takeaway

Jsonix is a powerful Open-Source JavaScript library for XML↔JS conversion Works in browsers as well as Node.js Bidirectional, strongly-structured and type-safe Uses declarative XML↔JS mappings Mappings can be generated from XML Schemas The (unofficial) OGC Schemas Project provides ready-to-use pre-generated mapping for many OGC Schemas

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Jsonix can drastically simplify XML processing in JavaScript apps and thus save a lot of development effort https://github.com/highsource/jsonix

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