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 I’m (also) into GIS and Web Mapping GIS 2go Internal apps for the Deutsche Bahn 3
Web Mapping means JavaScript 4
JavaScript speaks JSON { "type": "Feature", "geometry": { "type": "Point", "coordinates": [125.6, 10.1] }, [ˈdʒeɪsən] "properties": { "name": "Dinagat Islands" } } 5
GIS-Services are based on OGC Standards 6
… specified by XML Schemas More than 50 Schemas More than 100 individual versions Over 8MB and 800 individual XSD files 7
OGC Web Services speak XML GetCapabilities <wfs:WFS_Capabilities … /> GetFeature <wfs:FeatureCollection … /> Client WFS Server 8
JS Apps have a communication problem XSD <x:ml/> {”JS”:”ON”} ??? JS App WFS Server 9
Jsonix provides a solution Mapping XSD Jsonix <x:ml/> {”JS”:”ON”} JS App WFS Server 10
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 11
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 12
Jsonix Example Parse WMS Capabilities JSFiddle: http://bit.do/jsonix-001 13
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)); }); 14
Jsonix creates pretty JSON <WMS_Capabilities … { version="1.3.0" "WMS_Capabilities": { updateSequence="163"> "version": "1.3.0", <Service> … </Service> "updateSequence": "163", "service": { … }, <Capability> <Request> … </Request> "capability": { <Exception> … </Exception> "request": { … }, "exception": { … }, <Layer> "layer": { … , … "layer": [ … ] <Layer queryable="1"> … </Layer> } <Layer queryable="1" opaque="0"> … </Layer> } … } </Layer> } </Capability> </WMS_Capabilities> 15
… 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)); 16
The same thing without Jsonix? OpenLayers 3: ol.format. WMSCapabilities JSFiddle: http://bit.do/jsonix-002 17
Let’s take a closer look into the OL3 code ol.format.WMSCapabilities ca. 28 kB, ca 0.8 KLoC Type-safe Strongly-structured Only parsing Written manually Super exciting code 18
Why yet another XML-JS tool? 19
Jsonix is strongly-structured and type-safe And that’s unparalleled 20
What does “strongly-structured” mean? <WMS_Capabilities … How should the Layer element version="1.3.0" be represented in JSON? updateSequence="163"> <Service> … </Service> As a single element or as an array? <Capability> <Request> … </Request> <Exception> … </Exception> capability.layer. layer ? <Layer> … <!-- Just one Layer element --> capability.layer. layer[0] ? <Layer queryable="1"> … </Layer> </Layer> </Capability> You can’t decide it just based on </WMS_Capabilities> the XML instance You have to know the schema 21
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 22
Jsonix produces reliable structures <WMS_Capabilities … layer is an array: version="1.3.0" updateSequence="163"> <Service> … </Service> capability.layer. layer[0] <Capability> <Request> … </Request> <Exception> … </Exception> <Layer> … Because the mapping says so: <Layer queryable="1"> … </Layer> </Layer> </Capability> </WMS_Capabilities> 23
Strong, reliable structures === simpler code 24
What does “type-safe” mean? <WMS_Capabilities … Which type should version="1.3.0" queryable=”1” have in JSON? updateSequence="163"> <Service> … </Service> <Capability> <Request> … </Request> String? Number? Boolean? <Exception> … </Exception> <Layer> … <Layer queryable=" 1 "> … </Layer> </Layer> You can’t decide it just based on </Capability> the XML instance </WMS_Capabilities> You have to know the schema 25
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 26
Jsonix converts the types automatically <WMS_Capabilities … queryable is a boolean: version="1.3.0" updateSequence="163"> <Service> … </Service> { "queryable" : true, … } <Capability> <Request> … </Request> <Exception> … </Exception> <Layer> … Because the mappings says so: <Layer queryable=" 1 "> … </Layer> </Layer> </Capability> </WMS_Capabilities> 27
Safe, automatic type conversion === less work 28
Jsonix uses declarative mappings Jsonix mappings are just descriptions of structures, not imperative programs 29
Mappings are essential for strong structures and safe typing 30
Jsonix can generate mappings from XSDs Jsonix Schema Compiler XSD Mapping {“JS”:”ON”} <x:ml/> Jsonix Runtime 31
..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 32
Experimental: Generate JSON Schemas from XSDs Experimental java -jar jsonix-schema-compiler.jar -generateJsonSchema … JSON Jsonix Schema Compiler XSD Schema Mapping {“JS”:”ON”} <x:ml/> Jsonix Runtime 33
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); } [{ keyword: 'required', dataPath: '.value.capability.request.getCapabilities.dcpType[\'0\'].http', message: 'property .http is required' }, … ] 34
35 Compiling XSDs into mappings is hard 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
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 36
(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 37
(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 38
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