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Semantic Calendaring

Semantic Calendaring explores how interoperability of calendaring and scheduling applications may be improved via the combination of the Semantic Web with calendaring standards.

Introduction

The Internet was designed to connect multiple disparate nodes spread across many different networks. And whilst it was intended to promote decentralisation of data, we increasingly see data siloed in applications with little or no interoperability with other systems.

Common protocols for interoperability are often an afterthought for many modern applications, instead focusing on their own custom APIs and protocols. Whilst these APIs may be useful for single client/server connectivity, it becomes impractical to implement support for multiple applications using different APIs.

Open Standards

To maximise the usefulness of data, and protect user rights to data portability, we must strive to implement and evolve open standards, such as those defined by the Internet Engineering Task Force (IETF) and the World Wide Web Consortium (W3C). In this way we ensure greater system interoperability and data portability.

The Semantic Web

The W3C and IETF have defined many standards to promote data sharing via common formats, such as the Resource Definition Framework (RDF), JSON Linked Data (JSON-LD) and the ATOM Syndication Format. Such standards define a general model for both what data can be shared and how it can be shared.

Whilst such general purpose models are useful for enabling data sharing, often we find more specific data formats and protocols can be more suited for specific types of data. One such example is the use of iCalendar for events and other temporal-based information.

iCalendar

Calendaring standards such as iCalendar have increasing support for Semantic Web concepts via updated and extensions to the original specifications.

This enables calendaring software to participate in interoperability initiatives designed for the Web. Some of these initiatives include:

  • Web Linking
  • Linked Data (e.g. JSON-LD, RDFa, etc.)
  • Microformats
  • WebSub
  • WebMention
  • Linked Data Notifications

In applying these standards natively to a calendaring context, the Semantic Web provides support for a powerful graph of calendaring information.

Principles

The principles of semantic calendaring can be divided into four areas: rich content, semantic metadata, linking and publishing.

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Rich Content

Calendar interoperability is both strengthened and limited by it's approach to backwards compatibility. Whilst catering to the lowest common denominator has ensured maximum compatibility across calendaring clients, it has meant that simple features such as rich content have struggled to gain traction.

Whilst the introduction of the STYLED-DESCRIPTION property was intended to solve the issue of rich content support, it's implementation has been slow due to unclear guidelines on how it should be used.

Semcal proposes the following principles around using rich content in semantic calendar objects:

  1. Every semantic calendar object SHOULD include a STYLED-DESCRIPTION property of content type text/html. This property SHOULD also be derived content (indicated via the DERIVED=TRUE parameter) to promote consistency of HTML styling and formatting.
  2. A semantic calendar object SHOULD only be maintained via a single calendar user agent (CUA). This is to ensure that the rich content in each STYLED-DESCRIPTION property is consistent with the rest of the object (e.g. we explicitly want to avoid updating the DESCRIPTION property without updating the corresponding STYLED-DESCRIPTION. CUAs that don't support semantic calendaring SHOULD remove any STYLED-DESCRIPTION properties if they update the DESCRIPTION to avoid propogating inconsistent data.
  3. A semantic calendar object SHOULD include a LINK property referring to a URL for editing/maintaining the object. This is specified via the LINKREL=edit parameter, which should be incorporated into an update function in supporting CUAs. This is to support simultaneous access to a calendar object (e.g. via CalDAV) using multiple clients.

Metadata

Templates are often used to apply sensible defaults and semantic metadata when creating new events and other objects. iCalendar has recently introduced two properties supporting semantic metadata: CONCEPT and STRUCTURED-DATA. Using CONCEPT we are able to provide more granular type information that allows CUAs to recognise and process objects differently.

STRUCTURED-DATA allows us to capture or generate additional data formats beyond the scope of the iCalendar specification. We can embed agreed formats (such as JSON-LD) to enhance CUAs' interoperability.

The following principles for semantic data are proposed by semcal:

  1. Each semantic calendar object MUST include a CONCEPT property. This is used to specify the type of event, to-do, journal, etc. represented by the calendar object. An example of a type could be a Meeting event, a Service Request to-do, or a Metric journal. By specifying type information a richer interaction is facilitated between calendar user agents. NOTE: "vanilla" typing MUST also be supported by providing types that represent event, to-do, journal, etc. without further specificity.
  2. A semantic calendar object SHOULD include linked data via the STRUCTURED-DATA property. Linked data is typically defined via JSON-LD or RDFa formats (JSON and XML respectively), and allows for interactions with non-calendaring systems. For example, a Web search engine may be able to parse the STRUCTURED-DATA property of semantic calendar objects in order to index the object for improved search results.

Linking

The LINK property allows for additional linking between semantic calendar objects to support construction of a larger semantic graph.

The following semcal principles demonstrate additional linking relationships:

  1. For recurring events, individual occurrences may be customized with additional information specifically for that instance. With semcal, when customising such an instance the CUA SHOULD include a LINK property referring to the previous instance via the LINKREL=prev parameter.
  2. Semantic calendaring objects SHOULD include authoring information via one or more LINK properties with a LINKREL=author parameter.

Publishing

Whilst many calendaring clients support subscribing to remote calendars, the reality is that most published events are statically imported to internal calendars either via Web links or email attachments.

The following principles propose how to support subscriptions for updating individual events:

  1. A CUA supporting semantic calendar SHOULD create a topic named from the event UID value for each published event via a WebSub hub. This hub is to be linked in the event via a LINK with LINKREL=hub. A recipient CUA SHOULD check for such LINK properties and automatically subscribe to the topic corresponding to the event UID. The subscription duration should be at least until the event has passed, but typically with an additional grace period to allow for updates including media related to the event.
  2. Optionally, a CUA may choose to support reciprocal links for specific events via the Webmention standard. To support reciprocal links a semantic calendar object MUST include a LINK property referring to a callback URL for posting a new link. This property is identified via the LINKREL=webmention parameter. When creating new calendar objects that refer to another, CUAs may check for a Webmention link in the related object and POST a link to the new event. The receiving CUA may optionally require approval prior to updating the original event with the new link, but once approved would add a new LINK property for the URL using the LINKREL=replies parameter.