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Last updated: 11 Jun 2024

publishing-api: Links and Link Expansion

Contents

Introduction

Publishing API stores links between content items so that frontend applications can render links to related content on GOV.UK, such as the organisation responsible for publishing the page. These links contain only a reference to the related content item - they don't contain any information about the related content item until they are "expanded".

Link expansion is the process of converting the stored links of an edition into a JSON representation containing more information about the linked content items. The expansion process occurs immediately before sending an edition downstream to the Content Store.

A closely related process to this is dependency resolution, which runs immediately after a document has been presented to Content Store. This is roughly the opposite of link expansion. When a document is updated, Publishing API works out which documents link to the updated document, and presents the linked documents to Content Store with the new expanded link content.

Example Output

Below is an abridged example of an edition represented as JSON after link expansion has occurred.

{
  "base_path": "/government/organisations/department-for-transport/about/welsh-language-scheme",
  "content_id": "5f54d009-7631-11e4-a3cb-005056011aef",
  ...
  "links": {
    "organisations": [{
      "analytics_identifier": "D9",
      "api_path": "/api/content/government/organisations/department-for-transport",
      "base_path": "/government/organisations/department-for-transport",
      "content_id": "4c717efc-f47b-478e-a76d-ce1ae0af1946",
      "description": null,
      "document_type": "organisation",
      "locale": "en",
      "public_updated_at": "2015-06-03T13:12:51Z",
      "schema_name": "organisation",
      "title": "Department for Transport",
      "details": {
        "brand": "department-for-transport",
        "logo": {
          "formatted_title": "Department\u003cbr/\u003efor Transport",
          "crest": "single-identity"
        }
      },
      "links": {}
    }],
    "available_translations": [{
      "analytics_identifier": null,
      "api_url": "https://www.gov.uk/api/content/government/organisations/department-for-transport/about/welsh-language-scheme",
      "base_path": "/government/organisations/department-for-transport/about/welsh-language-scheme",
      "content_id": "5f54d009-7631-11e4-a3cb-005056011aef",
      "description": "When conducting public business in Wales, English and Welsh languages are treated equally.",
      "document_type": "welsh_language_scheme",
      "locale": "en",
      "public_updated_at": "2013-06-21T13:22:34Z",
      "schema_name": "corporate_information_page",
      "title": "Welsh language scheme",
      "links": {}
    }, {
      "analytics_identifier": null,
      "api_url": "https://www.gov.uk/api/content/government/organisations/department-for-transport/about/welsh-language-scheme.cy",
      "base_path": "/government/organisations/department-for-transport/about/welsh-language-scheme.cy",
      "content_id": "5f54d009-7631-11e4-a3cb-005056011aef",
      "description": "Wrth gynnal busnes cyhoeddus yng Nghymru, ieithoedd Cymraeg a Saesneg yn cael eu trin yn gyfartal.",
      "document_type": "welsh_language_scheme",
      "locale": "cy",
      "public_updated_at": "2013-06-21T13:22:34Z",
      "schema_name": "corporate_information_page",
      "title": "Cynllun iaith Gymraeg",
      "links": {}
    }]
  },
}

Within a links JSON object there are keys which indicate the type of link (link_type), and at the value of those keys is an array of all links of that type. In the above example there are two types of link: organisations and available_translations which contain 1 and 2 links respectively.

There are two categories of link with two separate API routes: edition links and link set links.

put-content - Edition links

These links are added via the put-content endpoint. They are associated with a single edition of a document.

In cases when there are edition links and link set links which have the same link_type, the edition links will take precedence during link expansion.

Edition links should be the default approach to link creation, because link set links have the disadvantage of applying to all editions and locales of a document, which is generally not what is required in most use cases. However, edition links have a substantial limitation, which is that recursive link expansion is not applied to them. If a document needs to access data from documents more than one link away, then link set links must be used.

There was an attempt to allow recursive expansion of edition links, but we weren't able to complete it. See https://github.com/alphagov/publishing-api/pull/2605.

These links are added via the patch-link-set endpoint. They are associated with a content_id rather than an edition_id, which therefore associates them with all of the editions and all of the locales of a document.

This is important because it means when they are updated they will immediately apply to live editions of a document.

Theoretically there isn't a need for link set links to exist as edition links can serve all of the same use cases. However, edition links are missing the crucial feature of recursive expansion, so we cannot retire link set links. Link set links pre-date edition links.

Available translations

A document can be available in multiple translations. This will be determined by documents sharing the same content_id and having different locale values.

The links for an edition will automatically include the available translations of editions of documents matching the same content_id, including the current locale.

Example

For item E which exists in English ("en") and Welsh ("cy") the available translations links include a link to the "en" and "cy" variations of the document.

These are typical links which have been added via patch-link-set or put-content. They are presented with the link_type provided when creating the link.

These links may be recursive depending on their link_type.

Certain link_types are considered the reverse of others, as configured in the expansion link rules.

A reverse link corresponds with a direct link and has a reverse name. For instance parent has a reverse name of children.

Example

For item A which has a link to B which is a reverse link_type of parent. A will be presented with a link to B of type parent, whereas B will be presented with a link to A of type children.

A quirk of reverse links is that they are presented with their corresponding link. For example when presenting a collection of links with a type of children, each one of those links will have a link of type parent which links to the original content.

These links may be recursive dependant on their link_type, they are defined in LinkExpansion::Rules.

Some link types are considered recursive. These types are used to present a tree structure of links and are frequently a source of confusion.

Recursive links are used in cases where multiple levels of links are needed to render content. A common use case for this is breadcrumbs, where we may want to know the hierarchy from the root / page to the page we are on. Breadcrumbs are represented by using a recursive parent link type.

Example

Consider the page Apprenticeship Standards which has breadcrumbs of "Home > Further education and skills > Apprenticeships"

  • "Apprenticeship Standards" would have a link to "Apprenticeships" of type parent.
  • "Apprenticeships" would have a link to "Further education and skills" of type parent.
  • "Further education and skills" could have a link to "Home" of type parent.

As parent is a recursive link type each link would include a link to its subsequent parent forming a graph of:

"Apprenticeship Standards" -parent-> "Apprenticeships" -parent-> "Further education and skills" -parent-> "Home"

From which breadcrumbs can be generated.

Recursive links can be defined as a path of link types, which means that only a structure of links that matches the defined path will be included in link expansion.

An example of this is the path of ordered_related_items, mainstream_browse_pages and parent. Only a tree of links that match this path would be included in link expansion.

This path would be included in the links representation for an item A:

"Item A" -ordered_related_items-> "Item B" -mainstream_browse_pages-> "Item C" -parent-> "Item D"

However this path would not be:

"Item A" -mainstream_browse_pages-> "Item B" -ordered_related_items-> "Item C" -parent-> "Item D"

An item in a path of link types can be marked as recurring. This means that there can be many items of this type in the path.

For the path ordered_related_items, mainstream_browse_pages and parent.recurring there be any number of parent items, and only 1 instance of ordered_related_items and mainstream_browse_pages.

This is a valid path for ordered_related_items, mainstream_browse_pages and parent:

"Item A" -ordered_related_items-> "Item B" -mainstream_browse_pages-> "Item C" -parent-> "Item D" -parent-> "Item E" -parent-> "Item F"

Yet this is invalid:

"Item A" -ordered_related_items-> "Item B" -mainstream_browse_pages-> "Item C" -mainstream_browse_pages-> "Item D" -parent-> "Item E" -parent-> "Item F"

The rules for recursive link types are defined in LinkExpansion::Rules.

Whether a link is included during link expansion depends on which state the linked item is in.

Edition has a state of published or unpublished

Links are included when the linked edition exists in a published state. Editions that are in an unpublished state with type withdrawn may be linked to depending on their link_type. Editions that are unpublished but not withdrawn are not linked.

The link_types that define whether a withdrawn edition is linked to are defined in LinkExpansion.

Edition has a state of draft

Links to draft editions are only included if the item which is having its links expanded is also in the draft state.

Links are presented as a JSON object where the keys of the object define the link types and for each link type there is an array of links.

The ordering of links is determined by the order in which the links were added via patch-link-set. Automatic link types (e.g. translations) do not have a specific ordering.

Fields

By default links contain the following fields:

  • analytics_identifier - Used to track a content item in analytics software
  • api_path - The path to the JSON representation of this item
  • base_path - The public path to this item
  • content_id - A UUID to represents the document
  • description - A short description of the content
  • document_type - This describes a type of document used on GOV.UK and allowed by the schema
  • locale - The language this document is written in
  • public_updated_at - The date/time that this document was last changed
  • schema_name - The schema (in content_schemas) that this edition conforms to
  • title - The title of the edition
  • links - Any recursive links that are presented with a link representation of an edition

The fields can be customised per link_type. These customisations are defined in LinkExpansion::Rules.

Developer gotchas

Link expansion is complicated and thus can be challenging for developers to understand. This section attempts to cover some of the common questions, we always welcome any suggestions to simplify link expansion.

To understand why a link is presented the following things should be considered:

  • If the link is an available_translations link it will be a translation of the document;
  • The link could be a direct link from the Edition or LinkSet;
  • The link could be a link defined on a different Edition or LinkSet and be a reverse link and represented reciprocally;
  • If the link is defined inside a different link it will be either be a recursive link or it will be the counterpart of a reverse link.

A link that you expect to appear might not be appearing for one of the following reasons:

  • The item to be linked to might not be available in a linkable state;
  • If it is a link set link, there could be Edition links that are defined with the same link_type which means the edition links will take precedence.
  • If a recursive link is expected it may not be following a valid recursive link path.

Links for a specific link_type can be defined to return different fields as part of link expansion. These are defined in LinkExpansion::Rules.

Debugging

You can explore link expansion in the rails console by creating a LinkExpansion instance.

> link_expansion = LinkExpansion.by_content_id(content_id, locale: :en, with_drafts: true)

You can then print the link_graph of the link expansion to view the links.

> link_expansion.link_graph.to_h
=> {:organisations=>
  [{:content_id=>"2e7868a8-38f5-4ff6-b62f-9a15d1c22d28", :links=>{}},
   {:content_id=>"b548a09f-8b35-4104-89f4-f1a40bf3136d", :links=>{}},
   {:content_id=>"de4e9dc6-cca4-43af-a594-682023b84d6c", :links=>{}},
   {:content_id=>"e8fae147-6232-4163-a3f1-1c15b755a8a4", :links=>{}}],
 :related=>[{:content_id=>"78cedbfe-d3aa-41c3-b8c0-aeb5d9035d6a", :links=>{}}]}

You can navigate through the link_graph object for further debugging information.