Talking to OGC Web Servicesin JSON

Or How I Learned to Stop Worryingand Love XML Processing in JavaScript

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Hi, my name is …

Alexey Valikovhttp://xing.to/vahttps://github.com/highsource@orless

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

GIS 2go

Internal apps for the Deutsche Bahn3

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 SchemasMore than 100 individual versionsOver 8MB and 800 individual XSD files

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OGC Web Services speak XMLGetCapabilities

<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/>

Jsonix provides a solution

Mapping

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WFS Server

{”JS”:”ON”} Jsonix

XSD

<x:ml/>

JS App

What is Jsonix?

Jsonix is a powerful Open-Source JavaScript libraryfor XML↔JS conversionhttps://github.com/highsource/jsonixBidirectional, type-safe, strongly-structuredDriven by declarative XML↔JS mappings…… which can be generated from XML schemas automatically

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Further Jsonix features

Works in browsersWorks in Node.jsWorks with AMD, CommonJS and without (globals)Namespace-awareSupports almost all of the XML Schema simple types (including binary types and QNames)Supports xsi:typeAnd 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-Throughvar getCapabilitiesUrl = …;

// First we create a context for XLink and WMS mappingsvar 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 URLunmarshaller.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 Contextvar 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

ol.format.WMSCapabilities

ca. 28 kB, ca 0.8 KLoCType-safeStrongly-structuredOnly parsingWritten manuallySuper exciting code

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Why yet another XML-JS tool?

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Jsonixis 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 elementbe 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 instanceYou have to know the schema

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Jsonix is strongly-structuredJsonix 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 shouldqueryable=”1” have in JSON?

String? Number? Boolean?

You can’t decide it just based on the XML instanceYou have to know the schema

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Jsonix is type-safeJsonix 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 mappingsJsonix mappings are just descriptions of structures, not imperative programs

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Mappings are essentialfor 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 followsjava -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 …

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'

}, … ]

Compiling XSDs into mappings is hard

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By A

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Compiling XSDs into mappings is hard

We’ve huge number of schemas with complex interdependenciesSchemas sometimes have errorsWe have to resolve naming collisionsWe’re often forced to customize generation or even patch schemas

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(Unofficial) OGC Schemas Project

https://github.com/highsource/ogc-schemasPre-generated mapping for OGC Schemas:JS↔XML: Jsonix mappingsJava↔XML: JAXB classesAvailable via npmjs.org and Maven Central

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(Unofficial) OGC Schemas ProjectCompiles 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 mappingsvar 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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TakeawayJsonix is a powerful Open-Source JavaScript library for XML↔JS conversionWorks in browsers as well as Node.jsBidirectional, strongly-structured and type-safeUses declarative XML↔JS mappingsMappings can be generated from XML SchemasThe (unofficial) OGC Schemas Project provides ready-to-use pre-generated mapping for many OGC Schemas

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Jsonix can drastically simplifyXML processing in JavaScript apps

and thus save a lot of development effort

https://github.com/highsource/jsonix

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