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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]
},
"properties": {
"name": "Dinagat Islands"
}
}
[ˈdʒeɪsən]
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
JS App WFS Server
{”JS”:”ON”} ???
XSD
9
<x:ml/>
Jsonix provides a solution
Mapping
10
WFS Server
{”JS”:”ON”}
Jsonix
XSD
<x:ml/>
JS App
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"
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": [ … ]
}
}
}
}
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 …
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
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 …
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:
23
Strong, reliable structures
===
simpler code
24
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
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 …
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:
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 Runtime
XSD
Mapping
Jsonix Schema Compiler
{“JS”:”ON”} <x:ml/>
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
Jsonix Runtime
XSD
Mapping
Jsonix Schema Compiler
{“JS”:”ON”} <x:ml/>
33
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); }
34
[{
keyword: 'required',
dataPath: '.value.capability.request.getCapabilities.dcpType['0'].http',
message: 'property .http is required'
}, … ]
Compiling XSDs into mappings is hard
35
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
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
39
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
40
Jsonix can drastically simplify
XML processing in JavaScript apps
and thus save a lot of development effort
https://github.com/highsource/jsonix
41

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Talking to OGC Web Services in JSON with Jsonix

  • 1. Talking to OGC Web Services in JSON Or How I Learned to Stop Worrying and Love XML Processing in JavaScript 1
  • 2. Hi, my name is … Alexey Valikov http://xing.to/va https://github.com/highsource @orless 2
  • 3. … and I’m (also) into GIS and Web Mapping GIS 2go Internal apps for the Deutsche Bahn 3
  • 4. Web Mapping means JavaScript 4
  • 5. JavaScript speaks JSON { "type": "Feature", "geometry": { "type": "Point", "coordinates": [125.6, 10.1] }, "properties": { "name": "Dinagat Islands" } } [ˈdʒeɪsən] 5
  • 6. GIS-Services are based on OGC Standards 6
  • 7. … specified by XML Schemas More than 50 Schemas More than 100 individual versions Over 8MB and 800 individual XSD files 7
  • 8. OGC Web Services speak XML GetCapabilities <wfs:WFS_Capabilities … /> GetFeature <wfs:FeatureCollection … /> Client WFS Server 8
  • 9. JS Apps have a communication problem JS App WFS Server {”JS”:”ON”} ??? XSD 9 <x:ml/>
  • 10. Jsonix provides a solution Mapping 10 WFS Server {”JS”:”ON”} Jsonix XSD <x:ml/> JS App
  • 11. 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
  • 12. 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
  • 13. Jsonix Example Parse WMS Capabilities JSFiddle: http://bit.do/jsonix-001 13
  • 14. 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
  • 15. 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": [ … ] } } } } 15
  • 16. … 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
  • 17. The same thing without Jsonix? OpenLayers 3: ol.format. WMSCapabilities JSFiddle: http://bit.do/jsonix-002 17
  • 18. 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
  • 19. Why yet another XML-JS tool? 19
  • 21. 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 21
  • 22. 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
  • 23. 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: 23
  • 25. 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 25
  • 26. 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
  • 27. 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: 27
  • 28. Safe, automatic type conversion === less work 28
  • 29. Jsonix uses declarative mappings Jsonix mappings are just descriptions of structures, not imperative programs 29
  • 30. Mappings are essential for strong structures and safe typing 30
  • 31. Jsonix can generate mappings from XSDs Jsonix Runtime XSD Mapping Jsonix Schema Compiler {“JS”:”ON”} <x:ml/> 31
  • 32. ..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
  • 33. Experimental: Generate JSON Schemas from XSDs Jsonix Runtime XSD Mapping Jsonix Schema Compiler {“JS”:”ON”} <x:ml/> 33 JSON Schema Experimental java -jar jsonix-schema-compiler.jar -generateJsonSchema …
  • 34. 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); } 34 [{ keyword: 'required', dataPath: '.value.capability.request.getCapabilities.dcpType['0'].http', message: 'property .http is required' }, … ]
  • 35. Compiling XSDs into mappings is hard 35 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
  • 36. 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
  • 37. (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
  • 38. (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
  • 39. 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 39
  • 40. 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 40
  • 41. Jsonix can drastically simplify XML processing in JavaScript apps and thus save a lot of development effort https://github.com/highsource/jsonix 41