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Towards Interoperability between W3C Web of Things and NGSI-LD
1. Web of Things
The way towards interoperability with NGSI-LD
José Manuel Cantera Fonseca – FIWARE Foundation
May 2019 – FIWARE Summit Genoa
2. 1
WoT Introduction (I)
• The W3C Web of Things (WoT) has been created to enable
interoperability across IoT Platforms and application domains.
• WoT provides mechanisms to formally describe IoT
interfaces:
• allowing IoT devices and services to communicate with each other
• independent of the underlying implementation
• across multiple networking protocols
3. 2
WoT Interaction Model
• WoT applications use three types of Interaction Affordances:
• Properties
• Expose internal state of the Thing that can be directly accessed (read) and
optionally manipulated (write) or watched (observe) i.e. pull and push
communication styles.
• Actions
• allow to invoke a function or service exposed by the Thing or to trigger a process
• Events
• Thing-initiated notifications, discrete events, or streams of values sent
asynchronously to the receiver. Push communication style.
4. 3
WoT Building Blocks
• Thing Description (TD): it describes the network-facing
interface of a Thing. Each WoT Device MUST have a
corresponding TD. Normative
• Binding Templates can be used to describe multiple protocol
bindings, so that a Thing can communicate with different IoT
Platforms. Non-normative.
• Scripting API enables implementation of the application logic
of a Thing using a standardized contract for JavaScript. Non-
normative
• Status: Thing Description is expected to be a W3C
Recommendation in Q2 2019
• Note: This presentation is based on latest Editor’s Draft WoT
specifications.
• (Non substantial) changes may happen
Source: W3C
5. 4
W3C WoT Architectural elements & patterns
Source: W3C
Servient as a Thing
Servient as a Intermediary
• A Servient is a software stack that implements the WoT building blocks.
• Servients can host and expose Things and/or consume Things (i.e., host Consumers)
• Depending on the Protocol Binding, Servients can perform in both server and client role
6. 5
WoT Thing Description (WoT TD)
• It defines an information model based on a semantic vocabulary and a
serialized representation based on JSON. (but also allows JSON-LD
processing).
• The index.html for Things, human-readable and machine-understandable
metadata:
• Thing instances general metadata such as name, ID, descriptions
• Web Links to related Things or other documents.
• Interaction Affordance metadata (properties, actions, events)
• Public Security Configuration metadata
• Schemata of the information exchanged with the Thing (using JSON Schema).
• TD can be hosted locally by a Thing or externally (ex. by a Thing Catalogue)
11. 10
WoT NGSI-LD Interworking (IoW) principles
Overall Architecture:
NGSI-LD : Information Management Layer
WoT : IoT Service Layer
Proposed Interworking mechanism. An “NGSI-LD-WoT Intermediary” to play a dual role:
NGSI-LD Context Source
WoT Intermediary Servient
NGSI-LD-WoT Intermediary responsibilities:
Act as an Intermediary Servient to Things
Perform NGSI-LD mappings for WoT Events, Properties and Actions
Using extra semantic annotations added to the TD
”Similar process” to what currently the IoT Agents do
Expose WoT Properties as NGSI-LD Attributes (Context Source)
Propagate WoT Event data to an NGSI-LD Context Broker
(the intermediary does not have data storage capabilities itself)
Export WoT Actions as NGSI-LD Actuations (see next slides)
13. 12
WoT NGSI-LD Interworking (mappings)
Proposed Mappings
WoT Affordance NGSI-LD Element
Property Attribute (Property or Relationship)
Event Attribute (Property or Relationship)
Action "hasActuation" Relationship. NEW
"saref:hasCommand" Property. ETSI SAREF
"sosa:Actuation" Entity Type. W3C SSN
Mapping process can be guided through TD annotations expressed
using standard ontologies
W3C SSN
ETSI SAREF
….
15. 14
WoT Action NGSI-LD IoW Proposal
New terms introduced:
hasCommand Property (reused from SAREF). A collection of actuatable commands
hasActuation Relationship. NEW. A collection of actuations (sosa:Actuation)
sosa:Actuation Entity Type. (reused from W3C SSN)
For each WoT Action concerning an NGSI-LD Entity:
hasCommand contains the concerned command (ex. saref:ToggleCommand)
hasActuation has an NGSI-LD object of type “sosa:Actuation”
An Entity of type “sosa:Actuation” includes information about the execution status
of the last actuation (command execution) over such Entity.
Extended NGSI-LD API for launching Actuations over Entities
POST /ngsi-ld/v1/entities/{entityId}/actuations/{command}
ContextSourceRegistration API to allow registration of Actuation Executors
The “WoT-NGSI-LD Intermediary” is actually an Actuation Executor
18. 17
Issues and open points
Scalability
How to deal with millions of TDs and related annotations?
TD Templates (For further study).
Use JSON Hyperlinked Schema?
An Actuation Entity per Entity and command type … could be too much?
Semantic annotations suitability
Is the usage of W3C SSN / ETSI SAREF appropriate?.
Are we assuming the right semantics?
Specific Ontology for TD annotations?
What happens to WoT Binding Templates?
How to convey data transformations?
What happens to IoT Agents?
Can IoT Agents play a role in this architecture?
19. 18
Next Steps
Refine the proposed Architecture through a Proof of Concept
Study other alternatives for semantic annotation
Other concepts in SSN, SAREF, or custom ontology, etc.
Align with latest version of W3C WoT TD Specification
We hope to see it in CR state soon!!
Contribute to ETSI deliverables on NGSI-LD IoW with W3C WoT
Raise awareness of this work within the W3C WoT Community
20. 19
Conclusions
Interworking between W3C WoT and NGSI-LD is feasible
Semantic annotations are a powerful tool to bridge the Information
Management Layer and the IoT Service Layer
W3C WoT TD specification is very promising and flexible
W3C WoT TD as the lingua franca for semantic description of devices in the
Smart Domain
A great opportunity to improve the IoT Chapter of FIWARE!!