publishing-api: Publishing API's Model
This document serves as a broad introduction to the domain models used in the Publishing API and their respective purposes. They can be separated into 3 areas of concern:
- Content - Content that is stored in the Publishing API.
- Linking - Links between content that is stored.
- History - The storing of operations that may have altered content or links.
These areas are all interconnected through the use of shared
content_id is a UUID value that is used to identify distinct pieces
of content that are used on GOV.UK. It is generated from within a publishing
application and the same
content_id is used for content that is available in
multiple translations. Different iterations of the same piece of content all
share the same
Each piece of content stored in the Publishing API is associated with a
content_id, the links stored are relationships between
history is associated with a
The following is a high-level diagram that was generated with plantuml. The source that generated this diagram is checked into this repository.
A document represents all iterations of a piece of content in a particular locale. It is associated with multiple editions that represent distinct versions of a piece of content.
The concerns of a document are which iterations are represented on draft and live content stores; and the lock version for the content.
A document stores the
content_id, locale and lock version for
content. It is designed to be a simple model so that it can be used for
database level locking of concurrent requests.
An edition is a particular iteration of a piece of content. It stores most of the data that is used to represent content in the content store and is associated with a document. There are uniqueness constraints to ensure there are not conflicting Editions. Previously an Edition was named ContentItem.
Most of the fields stored on an edition are defined as part of the /put-content/:content_id API.
Key fields that are set internally by the Publishing API are:
state- where an edition is in its publishing workflow, can be "draft", "published", "unpublished" or "superseded".
user_facing_version- an integer that stores which iteration of a document an edition is.
content_store- indicates whether an edition is intended for draft, live or no content store.
Documents that have an edition with a "live"
content_store value will have
the corresponding edition presented on the live content store.
All documents where there is an edition with a "draft" or "live" value of
content_store are presented on the draft content store. With the draft
edition presented if available, otherwise the live one.
An edition can be in one of four states: "draft", "published", "unpublished" and "superseded".
At any one time a document can contain:
- 1 edition in a "draft" state
- 1 edition in a "published" or "unpublished" state
- any number of editions in a "superseded" state
When the first edition of a document is created it is in a "draft" state and available on the draft content store. The content can be updated any number of times before publishing.
Once an edition has been published it is possible to create a new edition of the draft - thereby having 1 draft edition and 1 published edition of a document.
A published edition can be unpublished, which will create an
unpublishing for the edition. The unpublished edition will
be represented on the live content store.
If a draft is published while there is already a published or unpublished
edition. The previous edition will have its
state updated to "superseded"
and will be replaced on the live content store with the newly published
There are uniqueness constraints to ensure conflicting editions cannot be stored:
No two editions can share the same
content_storevalues. This ensures there can't be multiple documents that are trying to use the same path on GOV.UK.
For a document there can't be two editions with the same
user_facing_version. This prevents there being two editions sharing the same version number.
- For a document there can't be two editions on the same content store. This prevents an edition being accidentally available in multiple versions in multiple places.
When creating and publishing editions an existing edition with the same base_path will be blocked due to uniqueness constraints. However when one of the items that conflicts is considered substitutable (typically a non-content type) the operation can continue and the blocking item will be discarded, in the case of a draft; or unpublished if it is published.
When an edition is unpublished an Unpublishing model is used to represent the type of unpublishing and associated meta data so that the unpublished edition can be represented correctly in the content store.
There are 5 types an unpublishing can be:
withdrawal- The edition will still be readable on GOV.UK but will have a withdrawn banner, provided with an
explanationand an optional
redirect- Attempts to access the edition on GOV.UK will be redirected to according to the
redirectshash, or a provided
gone- Attempts to access the edition on GOV.UK will receive a 410 Gone HTTP response.
vanish- Attempts to access the edition on GOV.UK will receive a 404 Not Found HTTP response.
substitute- This type cannot be set by a user and is automatically created when an edition is substituted.
An Edition can be associated with a ChangeNote, which stores a note describing the changes that have occurred between major editions of a Document and the time the changes occurred.
When presenting an edition of a Document to the content store, the change notes for that edition and all previous editions are combined to create a list of the change notes for the document.
AccessLimit is a concept that is associated with an Edition in a "draft" state. It is used to store a list of user id's (UIDs that represent users in signon) which will be the only users who can view the Edition in the draft environment.
A PathReservation is a model that associates a path (in the URI context of
https://gov.uk/<path>) with a publishing application. This model is used to
restrict the usage of paths to a particular publishing application.
These are created when content is created or moved, and can be created before content exists to ensure that no other app can use the path.
Associations between content in the Publishing API is stored through
these are used to indicate a relationship with the documents of one
content_id with the documents of a different
A LinkSet is a model that is used to represent the association of a
content_id and a collection of Links.
It stores a lock version number for usage in optimist locking.
A Link represents the association to another
content_id - known as the
link_type and ordering is also stored on a Link.
link_type is used to represent the relationship between the content of
the content_id. It is common for a link to have multiple relationships to
content of the same
link_type, the ordering field is used to store the order
in which the links of a certain
link_type was specified. The source of a link
can either be a
LinkSet (i.e. a
content_id), known as
link set links or an
Edition, known as
The Publishing API stores information on operations that change the state of data stored in the Publishing API. These are stored through the Event and Action models.
An Event is used to store the details of data that may change state within the Publishing API. It stores data that identifies the end user and web request that initiated the operation; which operation and which content will be affected; and the payload of the input. Only operations that successfully complete are stored as Events.
Events are used as a debugging and reference tool by developers of the Publishing API. As they generate large amounts of data the full details of them are not stored permanently.
Events older then a month are archived to S3, you can import these events back into your local DB by running the rake tasks in lib/tasks/events.rake, after you set up the relevant ENV variables. For example if you want to find all the events that are relevant for a particular content id you can run:
An Action is used to store the change history of a piece of content in the
Publishing API. They are associated with both a
an Edition. Requests that change the state in the Publishing API
create Actions that store which action was performed and the end user who
initiated the request.
Actions can be created by publishing applications to store additional data on the workflow of content.