Linked Data Event Streams in Solid LDP containers

Wout Slabbinck, Ruben Dedecker, Sindhu Vasireddy, Ruben Verborgh and Pieter Colpaert

DEPARTMENT ELIS

RESEARCH GROUP IDLab Technology

What is Solid?

A protocol to give people control over their data

• Cross app interoperability
• Separating the data from applications

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Storage in Solid

Linked Data Platform (LDP)

• Container-resource pattern�<container> a ldp:Container;� ldp:contains <container/resource1>;� ldp:contains <container/resource2>;

Storage in Solid

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Linked Data Platform (LDP)

• Container-resource pattern�<container> a ldp:Container;� ldp:contains <container/resource1>;� ldp:contains <container/resource2>;
• HTTP-based CRUD operations for interacting with resources

Core components of Solid

WebID

• Identity of the agent

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Solid Open ID Connect

• Authenticate a person

Core components of Solid

Web Access Control (WAC)

• Provide access control over resources
• Through Access Control Lists (ACLs)

Core components of Solid

• Interacting with data: LDP
• Authentication: WebID and Solid-OIDC
• Authorization: WAC

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Let’s build an application!

Solid in practice: Location application

Solid in practice: Location application

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51° 0' 46.90'' N

3° 42' 29.48'' E

Location application example

Writing

HTTP PUT

Pod/private/location.ttl

Location application example

Reading

HTTP GET

Pod/private/location.ttl

Location application example

It works. We are done!

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Location application example

Remember cross app interoperability

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• Data and application are not separated
• E.g. Solid app that visualizes routes won’t work with the location data as we only have latest value

Let’s take a step back

Event Sourcing to the rescue

In an Event Source every action is written as an immutable event in an append-only event log

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Event Sourcing

Event log

App

Writing

App

Snapshot

State

Reading

Location event t0

Location event t1

Location event t2

Event Sourcing

Location event t0

Location event t1

Location event t2

Event log

App

App

Complex query

Writing

Append-only Event Log

Linked Data Event Streams (LDES)

• Event Stream: collection of immutable objects
• Hypermedia interface

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https://w3id.org/ldes/specification

View

Fragments

Event Stream

Event Sourcing in Solid

LDES in LDP Protocol

• Linked Data Event Streams (LDES): collection of immutable objects
• First explain LDES very briefly
• Take images from slide deck LDES Write for the REAL protocol

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https://w3id.org/ldes/specification

Append-only Event Log in Solid

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LDES in a Solid Container

LDES in a Solid Container

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https://woutslabbinck.github.io/LDESinLDP/

LDES in a Solid Container

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https://woutslabbinck.github.io/LDESinLDP/

Event Stream

View

Fragments

Location application: revisited

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Append location

Event log

Location application: revisited

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Read latest location

App

App

Read routes

App

…

Conclusion

Event sourcing in Solid is possible

• Strong separation of data and apps
• Allows for multiple views over data

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But, some limitations using the Solid LDP Protocol

Re-balancing is impossible

• Dealing with a large number of resources
• We proposed B+ Tree for optimisation
• With slash semantics, impossible to move resources as their IRI would change

Not strictly following the LDES in LDP protocol

• May lead to a corrupt event source
• LDES view invalid
• Unable to retrieve correct objects

Retention policies are client managed

• Clients enforcing these policies is not efficient
• Solid server should indicate retention policies based on its capacity

Let us have a discussion

Should there be a new kind of append-only container, managed by the server, in the Solid Specification?

wout.slabbinck@ugent.be