the internet of things vs the web of thingscs.unibo.it/projects/iot/iot_wot.pdf · 7 the web of...

TheInternetofThingsvsTheWebofThings

[email protected]

MASTERDEGREEINCOMPUTERSCIENCEDEPARTMENTOFCOMPUTERSCIENCEANDENGINEERING,UNIVERSITYOFBOLOGNA,ITALY

[email protected]

Coursewebsite:h8p://www.cs.unibo.it/projects/iot/

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IoTProtocolStack

THEWEBOFTHINGS(WoT) L.BONONI,M.DiFELICE,DEPARTMENTOFCOMPUTERSCIENCEANDENGINEERING,UNIVERSITYOFBOLOGNA,ITALY

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IoTProtocolStack

IEEE802.3 IEEE802.11 IEEE802.15 IEEE802.16

IPv4andIPv6+6LoWPAN

TCP UDP

MQTT CoAP AMQP HTTP OTHERS

PHY/MACPROTOCOLS

NETWORKPROTOCOLS

TRANSPORTPROTOCOLS

MESSAGINGPROTOCOLS

OPENANDPROPRIETARY

STACKS

OTHERS

TCP/IPSTACK


4

TheWebofThings(WoT)q  Overviewq  Backgroundq WebThing:CharacterisVcsq WebThing:ArchitecturesandTechnologiesq  Findabilityproblem:TheWebThingModelandthe

semanVcWebq  ImplemenVngtheWoTwithNode.js


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TheWebofThings(WoT)


BuildingtheWebofThingsD.D.GuinardandV.M.TrifaMANNINGEdiVons,2016

h8ps://webo\hings.org/book/

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TheWebofThings(WoT)q  IDEA:UsetheWorldWideWeb(WWW)ecosystem

andinfrastructuretobuildapplicaVonsfortheIoT.²  InteractwithThingsviawebbrowsers.²  ExploretheWebofThingsassurfingtheweb.²  Retrieve, process and display sensor data by using web

technologies,likeJavaScript,JSON,WebSockets.

²  Novelparadigm,butalsocomplementarytotheIoT.²  The term appeared first in 2007, at present several research

groupsworkingoncloselyrelatedconcepts(e.g.ThePhysicalWeb).THEWEBOFTHINGS(WoT)

L.BONONI,M.DiFELICE,DEPARTMENTOFCOMPUTERSCIENCEANDENGINEERING,UNIVERSITYOFBOLOGNA,ITALY

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TheWebofThings(WoT)q  ADVANTAGESoftheWoT

²  Abstractthecomplexityandvarietyoflower-layerprotocolbehindthesimplemodeloftheWWW.

²  FacilitatetheintegraVonofallsortsofIoTdevices.²  EasetheapplicaVondeploymentandmaintenance.²  Relyonwidelyusedsecurityandprivacymechanisms.

q  SHORTCOMINGSoftheWoT²  WoTdevicesmustsupporttheTCP/IPstack.²  Performanceonresource-constraineddevices.


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TheWebofThings(WoT)q  TheWoTisimplementedontopoftheTCP/IPstack

(i.e.attheApplica[onLayer).


TechnologiesenablingtheconnecVonamongThings.

MakeThingsdiscoverableandusablebyWebapps.

SharetheWoTdatainasecureway.

Createmesh-updataapplicaVonsinvolvingmulVpleWebThingsandexternalWebservices.

TheWoTarchitectureisnotcomposedoflayersinthestrictsense,rathersoflevelsaddingfuncVonaliVes.

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Reference:TheWWWq  Internetapplica[on,Client-serverarchitecture


WEBSERVER WEBCLIENT

Managetheresources(e.g.HTMLdocuments)

QuerytheServerinordertoperformoperaVonsonthe

resources(e.g.GETanHTMLdoc)

REQUEST

RESPONSE

q BasedontheHTTP(HypertextTransferProtocol)Protocol²  Stateless,textual,request-responseprotocol²  Versions:HTTP/1.1,HTTP/1.2,HTTP/2²  LimitedsetofoperaVons:GET,POST,HEAD,PUT,OPTIONS,…

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Reference:TheRESTPrinciplesq  Representa[onalStateTransfer(REST)àsetof

architecturalprinciplesfordistributedsystems.1.  ClientServeràInteracVonsbasedonarequest-response

communicaVonpa8ern.2.  UniformInterfacesàUnambigousstandardinterfacefor

accessingtheresources(e.g.theURI).3.  Statelessàclientcontextandstatearenotstoredontheserver.4.  Cacheableàdataarecachedbyclientsandintermediaries.5.  LayeredSystemàintermediatecomponentscanhidewhatis

behindthem(e.g.contentdeliverynetworks).THEWEBOFTHINGS(WoT)


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Reference:TheJSONLanguage


q JavaScriptObjectNota[onàDatadescrip[onformat² AJSONfileisalsocalleda“Document”.² JSONdocumentcanbeeasilyparsedbymachines.² Generalsyntax,appliableonthousandsofdifferentuse-cases² Favoursystemintegra[onandinteroperabilityamongthird-partyso\warecomponents.ü AJSONdocumentissurroundedbybrackets{}ü Eachdataentryisa<key,value>couple.

{givenname:“mario”}{givenname:“mario”,lastname:“rossi”}

JSONW3CTutorial:h8ps://www.w3schools.com/js/js_json_intro.asp

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² ValueàNumber,integerorreal{name:“mario”,age:15,score:13.45}²  ValueàString,surroundedbyquotes{givenname:“mario”,lastname:“rossi”}²  ValueàBoolean,i.e.trueorfalse{name:“mario”,employed:true}²  ValueàArray,surroundedbysquarebrackets{name:“mario”,codes:[“134”,”042”]}²  ValueàJSONObject,surroundedbybrackets{name:“mario”,address:{city:“bologna”,nation:“italy”}}

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{givenname:“Mario”,lastname:“Rossi”,age:45,employed:true,salary:1200.00,phones:[“0243434”,“064334343”],office:[{name:“A”,street:Zamboni,number:7},{name:“B”,street:Irnerio,number:49}]}

JSONDocument

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TheWebofThings(WoT)q  Overviewq  Backgroundq WebThing:Characteris[csq WebThing:ArchitecturesandTechnologiesq  Findabilityproblem:TheWebThingModelandthe



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WebThing:Characteris[csq  UniforminterfaceàThingsfollowthesamerules

ofthewebRESTfulcomponents,i.e.:.²  Addressableresources.Everyresourcemusthaveaunique

idenVfierandshouldbeaddressableusingauniquemechanism.²  RepresentaVonofresources.ServerscanmanagemulVple

representaVonoftheresources;clientscanqueryforaspecificrepresentaVonoftheavailableresources.

²  Self-descripVvemessages.ClientsmustuseandimplementonlythemethodsprovidedbytheHTTPprotocol.

²  HypermediaastheengineoftheapplicaVonstate(HATEOS)THEWEBOFTHINGS(WoT)


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q  EverydeviceontheWebofThingsmusthavearootURLcorrespondingtoitsnetworkaddress.


<scheme>”:”<authority><path>[“?”query][“#”fragment]

http://gateway.api.com/devices/TVhttps://kitchen:3000/fridge/http://192.168.1.23/buildings/devices/raspberryPI

² WebThingsmustbeanHTTPserver.² WebThingsshouldusesecureHTTPconnecVons(HTTPS).² WebThingsmustexposetheirproperVesusingahierarchicalstructure.

WebThing:Characteris[cs

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q  ResourcesontheWoTcanbeorganizedinahierarchydefinedbyaURLpath(talkmorelater).


http://gateway.api.com/devices/raspi

actuators/

sensors/

temperature/

gyroscope/

accelerometer/

RootDeviceURL

Actuators

Sensors

Temperature

Gyroscope

Accelerometer

led/

LEDs 1/

1/

1/

2/

1/

2/


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q  UniforminterfaceàThingsfollowthesamerulesofthewebRESTfulcomponents,i.e.:.²  Addressableresources.Everyresourcemusthaveaunique







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q  AWebThingcansupportmul[plerepresenta[ons(=mulVpledataformats)ofitsresources.

q  ClientcanrequestapreferredrepresentaVonthroughtheHTTPcontentnego[a[onmechanism.


GET/piHost:devices.webofthings.ioAccept:application/json

200OKContent-Type:application/json…

HTTPREQUESTHEADER

HTTPRESPONSEHEADER


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q  AWebThingscanprovidebasicHTTP-basedopera[onsonresources:GET,POST,PUT,DELETE.


GET/pi/sensors/temperature/valueHost:devices.webofthings.ioAccept:application/json

200OKHTTP/1.1Content-Type:application/json{“temperature”:”25”}

²  Readthevalueofaresource.²  SafeandidempotentoperaVon.

GEToperaVon

HTTPREQUEST

HTTPRESPONSE


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POST/pi/display/messagesHTTP/1.1Host:devices.webofthings.ioContent-Type:application/json{“Content”:”Helloworld”,“duration”:10}

201CreatedHTTP/1.1Location:devices.webofthings.io/pi/display

/messages/2210

²  Createanewinstanceofsomethingthatdoesn’thaveitsownURL.²  Unsafeandnon-idempotentoperaVon.

POSToperaVon

HTTPREQUEST

HTTPRESPONSE


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PUT/pi/leds/4HTTP/1.1Host:devices.webofthings.ioContent-Type:application/json{“red”:0,“green”:123”,“blue”:123}

200OKHTTP/1.1

²  UpdatesomethingthatalreadyexistsandhasalreadyitsownURL.²  UnsafeandidempotentoperaVon.

PUToperaVon

HTTPREQUEST

HTTPRESPONSE


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DELETE/rules/24HTTP/1.1Host:devices.webofthings.io

200OKHTTP/1.1

²  PermanentlyremovearesourcefromaThing.²  UnsafeandidempotentoperaVon.

DELETEoperaVon

HTTPREQUEST

HTTPRESPONSE


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HTTPdefinesalistofstandardstatuscodestobereturnedbytheserveruponrecepVonofeveryrequest:

² 200OK(SuccessfulcompleVVonofarequest)² 201CREATED(Returneda\erthecreaVonofaresource)² 202ACCEPTED(ReturnedbysynchoperaVonsa\errequest)² 401UNAUTHORIZED(AuthorizaVonfailedornotissued)² 404NOTFOUND(Resourceordocumenthasnotbeenfound)² 500INTERNALSERVERERROR(Errorinprocessingtherequest)² 501SERVICEUNAVAILABLE(Servercan’thandletherequest)


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q  AWebThingscaninformtheclientsaboutthelistofoperaVonspermi8edonaspecificresource,byusingtheOPTIONSHTTPcommand.


OPTIONSpi/sensors/humidity/HTTP/1.1Host:devices.webofthings.io

204NoContentHTTP/1.1Content-Length:0Allow:GET,OPTIONSAccept-Ranges:bytes

HTTPREQUEST

HTTPRESPONSE


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q  TheWoTmodeldefinedin[1]statesthateachWebThingMUSTmeettheserequirements:


1)  AWebThingMUSTatleastbeanHTTP/1.1server.

2)  AWebThingMUSThavearootresourceaccessibleviaanHTTPURL.

3)  AWebThingMUSTsupportGET,PUT,POST,andDELETEHTTPcommands.

4)  AWebThingMUSTimplementHTTPstatuscodes:200,400and500.

5)  AWebThingMUSTsupportJSONasdefaultrepresentaVon.

6)  AWebThingMUSTsupportGETonitsrootURL.

[1]WebThingmodel,COMPOSEEUproject:h8p://model.webo\hings.io


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q  TheWoTmodeldefinedin[1]statesthateachWebThingSHOULDmeettheserequirements:


1)  AWebThingSHOULDusesecureHTTPconnecVons(HTTPS).

2)  AWebThingSHOULDimplementtheWebSocketProtocol.3)  AWebThingSHOULDsupporttheWebThingsmodel(seelater).

4)  AWebThingSHOULDreturna204codeforallwriteoperaVons.

5)  AWebThingSHOULDprovideadefaulthuman-readabledocumenta[on.



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q  TheWoTmodeldefinedin[1]statesthateachWebThingMAYmeettheserequirements:


1)  AWebThingMAYsupporttheHTTPOPTIONSverboneachresource.2)  AWebThingMAYofferaHTML-baseduserinterface.

3)  AWebThingMAYprovidepreciseinformaVonabouttheintendedmeaningofindividualpartsofthemodel(e.g.semanVcWebtechniques)



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WebThing:EnablingTechnologiesq  PROBLEM.HTTPimplementsarequest-response

communicaVonpa8ern.Whataboutpush-basedIoTapplicaVons?

²  Usepollingmechanism²  UseWebhooks/HTTPcallbacks²  Uselong-pollingmechanism²  UseWebSockets


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q WebHooksàTheWebThingandtheWebclientservebothasHTTPclientandasHTTPserver.


ApplicaVon

Thing

Subscribe

ApplicaVon

Thing

Publish

POST/pi/sensors/humidity/subsContent-Type:application/json{“callback”:”https://url-of-client/pubs”}

POST/pubsHTTP/1.1Host:https://url-of-client/Content-Type:application/json{“humidity”:50}

WebThing:EnablingTechnologies

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q  PROBLEM.HTTPimplementsarequest-responsecommunicaVonpa8ern.Whataboutpush-basedIoTapplicaVons?




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q  LongPollingàAclientsendstheHTTPrequesttotheserver;theserverholdstherequestVllanewvalueoftheresourceisavailable,thenitsendsaresponse.


Client WebThing SensorviaGPIORequest

NewvalueResponse

Request

NewvalueResponse

AssoonastheClientreceivestheresponse,itissuesanewrequest.


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q  PROBLEM.HTTPimplementsarequest-responsecommunicaVonpa8ern.Whataboutpush-basedIoTapplicaVons?




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q WebSocketsenablefull-duplex(bidirecVonal)communicaVonoverasingleTCPconnecVon.²  PartoftheHTML5specificaVon²  Novelprotocol,alternaVvetotheHTTP²  Muchshorterheader(2bytes)thanHTTP


1.  ClientsendsanHTTPrequesttotheserver,askingforanupgradetoWebSockets.2.  TheserverreplieswithCode101SwitchingProtocolsifitsupportsWebSockets3.  AbidirecVonalTCPsocketisopenandusedforthedatatransfer.4.  TheTCPsocketislong-living,i.e.terminedonlywhenClientorServertransmitaCloseframe.

WEBSOCKETSPROTOCOLHANDSHAKE


WebSocketProtocolSpecifica[on:h8ps://tools.iet.org/html/rfc6455

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q WebSocketsenablefull-duplex(bidirecVonal)communicaVonoverasingleTCPconnecVon.


GET/pi/sensors/humidity/HTTP/1.1Host:devices.webofthings.ioUpgrade:websocketConnection:Upgrade

HTTP/1.1101SwitchingProtocolsConnection:Upgrade[]Upgrade:websocketAccess-Control-Allow-Origin:http://localhost:63342


REQUESTforWEBSOCKETSUPGRADE

CONFIRMATIONofWEBSOCKETSUPGRADE



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q WebSocketsenablefull-duplex(bidirecVonal)communicaVonoverasingleTCPconnecVon.


GET/pi/sensors/humidity/HTTP/1.1Host:devices.webofthings.ioUpgrade:websocketConnection:Upgrade

HTTP/1.1101SwitchingProtocolsConnection:Upgrade[]Upgrade:websocketAccess-Control-Allow-Origin:http://localhost:63342


REQUESTforWEBSOCKETSUPGRADE

CONFIRMATIONofWEBSOCKETSUPGRADE



41THEWEBOFTHINGS(WoT)


WebThing:Architectures


² DirectConnec[vityWoTScenario²  WebClientsandWeb

Thingscanbelongtothethesamenetworkortodifferentnetworks.

²  EachWebThingimplementsanHTTPserverandtheWoTAPI.

²  TheRouterisnotaWebThingObject.





² Gateway-basedConnec[vityWoTScenario

²  NotalltheThingsareabletoimplementtheWoTAPIandtosupporttheWebSockets.

²  TheGatewayisaWebThingObject,andworksasproxyfortheotherThings.





² Cloud-basedConnec[vityWoTScenario²  Asinthepreviousscenario,

notalltheThingsareabletoimplementtheWoTAPIandtosupporttheWebSockets.

²  Differentlyfromthepreviousscenario,thegateway/proxyisacloudserviceandnotanotherdevicelocatedwithinthesamenetwork.

44

TheWebofThings(WoT)q  Overviewq  Backgroundq WebThing:CharacterisVcsq WebThing:ArchitecturesandTechnologiesq  Findabilityproblem:TheWebThingModeland

theseman[cWebq  ImplemenVngtheWoTwithNode.js


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q  FindabilityàcapabilityofeasilydiscoverandunderstandanyenVtyoftheWebofThings.Threeseparatesub-problems:q  HowtodiscoverWebThings.q  Howtoknowwhatcommandstosendandhow.q  Howtounderstandthemeaningofdatabeing

exchangedwiththeWebThing.


WoTFindabilityProblem

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q  ThereareseveraldiscoveryprotocolsforLANs:mDNS,DLNA,UPnP,AppleBonjour,…


serviceup:{interfaceIndex:4type:{ name:‘http’ protocol:‘tcp’ fullyQualified:true}fullname;‘Man\\032MFC-8520DN._http._tcp.local.’host:‘EVT-BW-MAN.local’port:80addresses:[‘192.168.0.6’]….

}

mDNSmessage ² ClientslistenfornewmDNSmessagesandpopulatethelocalDNStable.

AserviceoftypeHTTP/TCPhasbeendiscovered.

Theserviceisreachableat:hep://evt-bw-man.local


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h8p://model.webo\hings.io


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q WebThingModelàconceptual,uniformdescripVonofaThingandofitscapabiliVes.²  Flexibility:itshouldbeabletorepresentallsortsof

devicesandproducts,aswellasallsortsofinteracVons.²  Viability:itshouldensurethatclientapplicaVonscan

interactwithnewThingsautomaVcally(withoutanyhumanintheloop)²  Severalapproachesproposed,fewconsolidatedsoluVons.²  Wefollowthemodelproposedin: hep://model.webofhings.io



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q  AlltheWebofThingshavearootURL,andimplementalogicaltreestructureforresources:²  /modelàmetadatasuchasThingname,descripVonsor

configuraVons(aGETofthemodelwillreturnitscompletedescripVon).²  /propertiesàinternalstateofaThing,expressedbyalist

of<key,value>tuples,wherethevaluecanbeanyJSONvalue.²  /actionsàfuncVonsofferedbytheThingtoclients.²  /ThingsàlistofThingsproxiedbythecurrentdevice.²  /subscriptionsàlistofacVvesubscripVons(incase

publish-subscribeparadigmisimplemented).THEWEBOFTHINGS(WoT)



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q  AlltheWebofThingshavearootURL,andimplementalogicalresourcetreestructure:


RootResourceoftheWebThingh8p://wt.mypi.local

Model/model

ProperVes/properties

AcVons/actions

Things/Things

Temperature/temperature

LEDs/leds

ledStateON/1234ledStateOFF/1235BLESpeaker/speaker


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q  InresponsetoaHTTPGETrequest(ontheURL),aWebofThingmustreturnaJSONrepresentaVonlikethis:


Fieldname Type Descrip[on

id String RelaVveURLoftheresource

createdAt String Timestampwhentheresourcewascreated

updatedAt String Timestampwhentheresourcewaslastupdated

name String Shorthuman-readablenameofaresource

description String Human-readabledescripVonofaresource

tags String Arrayoftags

customFields Objects JSONobjectwithkey-valuepairs

links Objects JSONObjectwiththelistofsub-resources




{"id":"myCar","name":"Mygreatcar","description":"Thisissuchagreatcar.","createdAt":"2012-08-24T17:29:11.683Z","updatedAt":"2012-08-24T17:29:11.683Z","tags":["cart","device","test"],"customFields":{"size":"20","color":"blue"},

"links":{"model":{"link":"model/","title":"ModelthisWebThing."},"properties":{"link":"properties/","title":"PropertiesofthisWebThing."},"actions":{"link":"actions/","title":"ActionsofthisWebThing."},...}}


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q  Eachresourcemaylinktodifferentsub-resources.q  Eachlinkisdefinedbyarela[ontype(the"link

type"),theactualURLofthesub-resource(the"link"),andahuman-readableiden[fierfortherelaVon(the"Vtle").

q  Linksshouldbeexposedintwoways:²  Usingthelinks:fieldoftheJSONpayload.²  UsingtheHTTPLinkheaderfield.



55

q  TheWTModelincludesthefollowinglinktypes:


Rela[ontype Descrip[on

model AlinktotheresourcedescripVon.

properties TheproperVesofthisresource.

actions TheacVonsthatthisresourcecanperform.

things TheWebthingsproxiedbythisresource.

subscriptions TheendpointtomanagesubscripVonstothisresource.

type TheinstanceoftheresourceidenVfiedbyatargetexternalURL.

product AlinktoauthoritaVveproductinformaVonforthisWebThing.

help AlinktotheonlinemanualpageforthisWebThing.

ui AlinktotheHTML-baseduserinterfaceforthisWebThing.


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q  EXAMPLES:LinksincludedintheJSONformat:


“links”:{"model":{"link":"model/","title":"ModelofthisWebThing."},"properties":{"link":"properties/","title":"PropertiesofthisThing."},"actions":{"link":"actions/","title":"ActionsofthisWebThing."},….

{..."links":{"<relType>":{"link":"<String>","title":"<String>"}"<_customRelType>":{"link":"<String>","title":"<String>"},...}}


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q  EXAMPLES:LinksincludedintheHTTPHeader:


-->REQUESTGET/http://wt.mypi.local<--RESPONSE200OKLink:<model/>;rel="model"Link:<properties/>;rel="properties"Link:<actions/>;rel="actions"Link:<product/>;rel="product"Link:<type/>;rel="type"Link:<help/>;rel="help"Link:<ui/>;rel="ui"Link:<_myCustomLinkRelType/>;rel="_myCustomLinkRelType”

…HereitfollowstheJSONrepresentaVonoftheWebThing….


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q  EXAMPLES:LinksincludedintheHTTPHeader:


-->REQUESTGET/http://wt.mypi.local<--RESPONSE200OKLink:<model/>;rel="model"Link:<properties/>;rel="properties"Link:<actions/>;rel="actions"Link:<product/>;rel="product"Link:<type/>;rel="type"Link:<help/>;rel="help"Link:<ui/>;rel="ui"Link:<_myCustomLinkRelType/>;rel=“myCustomLinkRelType”

…HereitfollowstheJSONrepresentaVonoftheWebThing….


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q  OPERATIONEXAMPLE:RetrieveproperVes’values


-->REQUESTGEThttp://wt.mypi.local/properties<--RESPONSE200OKLink:<model/>;rel=”model”[{"id":"temperature","name":"KitchenTemperatureSensor","values":{"temp":22,"timestamp":"2015-06-14T14:30:00.000Z"},

…]


60

q  OPERATIONEXAMPLE:Updateaproperty


-->REQUESTPUT{wt}/properties/temperature/[{"temp":24}]<--RESPONSE204NOCONTENTLocation:{wt}/properties/temperature/


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q  OPERATIONEXAMPLE:RetrievethelistofacVons


-->REQUESTGEThttp://wt.mypi.local/properties<--RESPONSE200OKLink:<http://webofthings.org/actions/upgradefirmware>;rel="type”[{"id":"upgradeFirmware","name":"UpgradeDeviceFirmware"},{"id":"reboot","name":"Reboot"}]


62

q  OPERATIONEXAMPLE:ExecuteanacVon


-->REQUESTPOSThttp://wt.mypi.local/actions/reboot{"delay":50,"mode":"debug"}<--RESPONSE204NORESPONSELocation:{wt}/actions/reboot/233


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q  OPERATIONEXAMPLE:RetrievetheacVonstatus


-->REQUESTGEThttp://wt.mypi.local/actions/reboot/233<--RESPONSE200OK{"id":"233","value":{"delay":50,"mode":"debug"},"status":"executing","timestamp":"2015-06-14T14:30:00.000Z"}


64







h8p://model.webo\hings.io

² TheWebThingModeldescribedsofarprovidestheabstracVonofanExtendedWebThing(i.e.Level3).

² ClientscandiscoverthewaytointeractwithWebThings…howevertheycannotinferthemeaningofdata,andrelaVonshipsamongdifferentdataentries.




² Seman[cWebreferstoasetoftechniquestoeasethefinding,sharingandprocessofwebcontentsthankstoacommonandextendibledatadescripVonandinterchangeformat.² MeaningisassociatedwithdataenVVesbyannotaVngthemeta-

datawithVocabularyelements.² Vocabularyelementscanalsohaverela[onshipswitheachother.² AreasonercanbeusedtoinferaddiVonalproperVesor

relaVonships.




² JavaScriptObjectNota[onforLinkedData(JSON-LD)²  LightweightsyntaxtoserializeLinkedDatainJSON.²  100%compaVblewiththeJSONlanguage.²  InaddiVon,itintroducessemanVcfeaturessuchas:

q  Auniversaliden[fiermechanismforJSONobjectsviatheuseofIRIs.q  AmechanisminwhichavalueinaJSONobjectmayrefertoaJSON

objectonadifferentsiteontheWeb.q  Theabilitytoannotatestringswiththeirlanguage.q  AwaytoassociatedatatypeswithvaluessuchasdatesandVmes.


JSON-LD1.0W3CSpecifica[ons:h8p://www.w3.org/TR/json-ld/



² JavaScriptObjectNota[onforLinkedData(JSON-LD)

ThecompletedescripVonofJSON-LDsyntaxtokensandkeywordscanbefoundat:http://www.w3.org/TR/json-ld/

Keyword Descrip[on Example

@context UsedtodefinethevocabolaryusedthroughoutaJSON-LDdocument

@id UsedtouniquelyidenVfythingswithIRI

@type Usedtosetthedatatypeofanode

@language UsedtospecifythelanguageforaparVcularstringvalue




² W3CThingDescrip[on(TD)h8ps://w3c.github.io/wot-thing-descripVon/ThingisanabstracVonofaphysicalenVty.

AThingDescripVondescribesthemetadataandinterfacesofThings.Itconsistsof:

²  SemanVcmetadatafortheThing.

²  Anarrow-waistinterac[onmodel.

²  Seman[cschematomakedatamodelsmachine-understandable.

²  FeaturesforWebLinkingtoexpressrelaVonsamongThings.




² W3CThingDescrip[on(TD)h8ps://w3c.github.io/wot-thing-descripVon/Threeinterac[onmodelsaredefinedintheW3CTDmodel:

²  Property:readableand/orwriteabledata.

²  Action:changesorprocessesonaThingthattakeacertainVmetocomplete.

²  Event:mechanismforeventstobenoVfiedbyaThingonacertaincondiVon.




² ThingDescrip[oninstancesareserializedinJSON-LD{"@context":["http://w3c.github.io/wot/w3c-wot-td-context.jsonld"],"@type":["Thing"],"name":"MyLampThing","interaction":[{"@type":["Property"],"name":"status","outputData":{"type":"string"},"writable":false,"observable":true,"link":[{"href":"coaps://mylamp.example.com:5683/status","mediaType":"application/json“




}]},{"@type":["Action"],"name":"toggle","link":[{"href":"coaps://mylamp.example.com:5683/toggle","mediaType":"application/json"}]},{"@type":["Event"],"name":"overheating","outputData":{"type":"string"},"link":[{"href":"coaps://mylamp.example.com:5683/oh","mediaType":"application/json"}]}


73


semanVcWebq  Implemen[ngtheWoTwithNode.js


74

TheWebofThings(WoT)


WEBCLIENT WEBTHING(RaspberryPI)

ARDUINONANO+DHT22TEMPERATURE

SENSOR

HTTP/WebSocketsviaWi-FI

75

Reference:Node.jsFrameworkq  Javascriptisthemostpopularprogramming

language,accordingtothenumberofpublicrepositoriesinGitHub.


h8ps://github.com/madnight/githut

76

q  Node.jsFramework²  Opensourceserverframeworkfordeployinghigh-

performanceserver-sideapplicaVons.²  Node.jsapplicaVonsaredeployedinJavascript.²  Single-threaded,non-blockingwebservers.²  Asynchronousprogramming.²  Highlymodular,basedonthenpmpacketmanager.


Reference:Node.jsFramework

77

q  Node.jsFramework


varhttp=require('http');http.createServer(function(req,res){res.writeHead(200,{'Content-Type':'text/html'});res.end('HelloWorld!');}).listen(8080);console.log(‘WebServerstartedonport8080);

first.jsCreateaserverandpassitafuncVontobe

calledwheneveranewclientsendsanHTTPrequest.

Theserverlistensonport8080

/home/raspi/$nodefirst.jsWebServerstartedonport8080

StarVngtheserverviacommand-line


78

q  Node.jsFramework


varhttp=require(’request');http.createServer(function(req,res){…

first.jsIncludean

externalModulenamedrequest.

/home/raspi/$npminstallrequest--saveInstalladdiVonalPackages

throughthenpmtool

first.jsrequest-modules/package.json

APPLICATIONSTRUCTURE


79

q  SynchronousServer-sideProgramming(e.g.PHP)


Request

Response


Thread1

Request

Response

Thread2

Request

Response

Thread3

varresult=database.query(“SELECT ThingsFROM DeviceTable”);

console.log(result);

²  Scriptworksinasequen[almanner.²  ScriptexecuVonisblockedVllcurrentI/O

operaVonsarecompleted.²  (INTHECODEABOVE)Themessageiswri8en

ontheconsoleAFTERthedatabasequeryhasbeencompleted.

80

q  AsynchronousServer-sideProgramming


Request

Response


Request

Response

Request

Response

database.query(“SELECT ThingsFROM DeviceTable”, functionresult{ //dosomethingwithresults

}};console.log(result);

²  AsynchronousI/OoperaVons.²  Anonymouscallbacksareexecutedoncea

requesthasbeencompleted.²  (INTHECODEABOVE)Themessagemightbe

wri8enontheconsolebeforethequeryhasbeencompleted.

EVENTLOOP(onethread)

81

q  LearnmoreaboutNode.jsprogramming



W3CTutorial:https://www.w3schools.com/nodejs/

OfficialPage:https://nodejs.org/en/

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