# Decentralized Hypermedia Ecosystems

Olivier Boissier, Pierre-Antoine Champin, Amit K. Chopra, Nicoletta Fornara, Fabien Gandon, Ruben Taelman, Jan van den Bussche

► To cite this version:

Olivier Boissier, Pierre-Antoine Champin, Amit K. Chopra, Nicoletta Fornara, Fabien Gandon, et al. Decentralized Hypermedia Ecosystems. Agents on the Web 2023, Feb 2023, Wadern, Germany. pp.123-126. emse-04240553

HAL Id: emse-04240553

https://hal-emse.ccsd.cnrs.fr/emse-04240553v1

Submitted on 27 Sep 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>
## 5.1 Decentralized Hypermedia Ecosystems

Olivier Boissier (Ecole des Mines – St. Étienne, FR)

Pierre-Antoine Champin (INRIA – Sophia Antipolis, FR)

Amit K. Chopra (Lancaster University, GB)

Nicoletta Fornara (University of Lugano, CH)

Fabien Gandon (INRIA – Sophia Antipolis, FR)

Ruben Taelman (Ghent University, BE)

Jan Van den Bussche (Hasselt University, BE)

Inspired by the recent focus of decentralized data for Web applications, as supported in infrastructures such as SOLID, this group discussed the idea of decentralized hypermedia applications in general. In particular, this working group focused on understanding how to integrate the technologies and models used and proposed by three areas of research targeted by the seminar.

1. The notion of Hypermedia, Hypermedia as the Engine of Application State (HATEOAS), and ecosystems developed by the Web of Things community;

2. The notion of Linked Data Platform (a W3C recommendation) together with open specifications such as SOLID proposed by the Semantic Web and Linked Data community.

3. Decentralized abstractions studied in multiagent systems. These include the concept of agents, norms or policies, protocols, artifact, workspace, and organization together with processes of regimenting and enforcing norms.

This integration was investigated by first discussing a list of crucial terms used in each of the communities involved, in order to achieve a good level of mutual understanding. Subsequently, by discussing and specifying a scenario in which all the technologies and concepts proposed by each of the communities involved can be used at different levels of abstraction.

## Crucial Terms

- **Hypermedia**: a non-linear multimedia medium where resources are linked by hyperlinks i.e. a digital reference to data that the can be followed to discover new data

- **Hypermedia as the Engine of Application State (HATEOAS)**: is a constraint of the REST application architecture where a client interacts with a network application whose application servers provide information dynamically through hypermedia.

- **Decentralized System**: is a type of system where every node makes its own decision, there is not a central coordinating or governing unit or server.
- SOLID: Solid is a set of standards that enables a decentralization of data, with interoperable applications over this decentralized data.

- SOLID ecosystem: is a specific deployment of a community of users and agents using a given set of applications on top of their Solid pods, and complying with a set of technical and social norms.

- SOLID pods: Solid is a specification that lets people store their data securely in decentralized personal online data stores called pods. Pods are like secure personal web servers for data. When data is stored in someone's Pod, they control which people and applications can access it.

- Pody: is a term forged during the Dagstuhl Seminar to identify the Pod of an agent that embodies some of its features. This approach is referred to as the empodiment of the agent.

- Web ID: URI that identify a person / an agent

- Agents: autonomous entities able to perceive, act and interact with other agents, making decisions, subject to norms.

- Norms: Capture social expectations, they are by definition violable. Norm types include commitment (obligation), prohibition, permission and so on.

- Artifacts: non autonomous entities.

- Workspace: a way to organize the artifacts, the artifacts are situated in workspace, agents join workspaces, workspace scope the observability for an agent.

- Enforcing: applying sanctions (rewards or fines) when norms are fulfilled or not fulfilled

- Regimenting: hindering the violations of norms by agents.

- ODRL: the Open Digital Rights Language (ODRL) is a W3C recommendation defining a policy expression language. ODRL policies are used to represent permitted and prohibited actions over a certain asset, and obligations required to be met by stakeholders.

## Scenario: Social Media Sharing Application in a Decentralized Hypermedia Ecosystem

We designed and described a plausible decentralized architecture that broadly combines above themes with reference to a social media sharing application. In contrast to the traditional approach of realizing an application as a Web service that mediates interactions between its users, here each user is represented by its own agent who interacts with other agents based on an understanding of applicable norms.

## The Scene

Alice is a person sharing her photos on the social network FaceUnBook running this app in her browser and using her pod to host her data and her agent to manage her photo sharing preferences (among other tasks). Bob is a friend of Alice and is following her activity stream on FaceUnBook. His agent is in charge of collecting the activity of all his friends, which will be summarized in the pod of Bob. This summary can then improve discovery when Bob uses the FaceUnBook app, which is backed by the personal query engine of Bob. It combines all search capabilities it can identify to plan and solve this aggregation query. Alice attended the seminar 23081 at Schloss Dagstuhl. Among other things the agent of that organization enforces the norms of the organization that are stored in the organization's pod. The agent also collects pictures taken by the Camera42. This happens in Europe which is an organization with norms. The agent GDPR agent is in charge of enforcing the GDPR norms for every member of Europe. To make all members in Europe findable by the GDPR agent, Europe has a norm saying that all pods in Europe must be registered to Europe's
<figure><img src="image_2.png" /></figure>

pod. **Friedrich** is a kid accompanying Alice in Dagstuhl. The agents, pods and resources in general are hosted on different platforms but discover and interact with each other through the hypermedia fabric and its standards. To support the discovery of resources, pods and their contents, **Q** is a query service available in the environment as one possibility to read and write within the hypermedia fabric, used by either software agents, or by human agents through client-side apps. This scenario follows the empodiment principle i.e. the agents communicate through their pods (pody) both through the inbox container of the pody or through other containers of the pody.

**An Interaction to Share Photos May Process as Follows:**

Alice took photos at Dagstuhl, in which herself and Friedrich are depicted. She runs the FaceUnBook application in her browser, and uses it to share the photo with her friends. The application posts the photo in Alice's pod, and (HTTP) posts a notification in Bob's inbox (a specific container in his pod). Dagstuhl publishes on its pod ODRL policies about photo sharing that apply to all seminar participants. Alice tags the photo with the tag "Dagstuhl Seminar 23081", then the FaceUnbook application also reads the policy and asks Alice to confirm that she complies with that policy (including the rule that no photo of a children must be published without consent from a caretaker – in that case, Alice is the caretaker and consents). Then the application (HTTP) posts a notification in Dagstuhl's inbox. Bob likes Alice's photo; he improves the contrast in an image editor, and republishes it on his own pod (notifying Bob's friends, including Alice). Alice's agent notices that the republishing violates Alice's preferences and (HTTP) posts a warning to the pods of Alice and Bob.

**Moving Forward**

People in the group have independently worked on different aspects of the above architecture. A way forward would be to actually realize this application as broadly outlined above, see how things fit together and identify the gaps. Although in the above description Alice acts in accordance with the norms, in another situation, she may act in violation of the norms, e.g., if she posts a photo of a child who needs medical attention. FaceUnbook must not prevent
such sharing but must support recording her violation, based on which the child's parent, for example, may demand an explanation from her. This brings in accountability and a richer notion of social processes than currently possible.