Content Fragments Support in AEM Assets HTTP API

Overview

Learn about support for Content Fragments in the Assets HTTP API, an important piece of AEM’s headless delivery feature.

The Assets REST API allows developers for Adobe Experience Manager to access content (stored in AEM) directly over the HTTP API, via CRUD operations (Create, Read, Update, Delete).

The API lets you operate Adobe Experience Manager as a headless CMS (Content Management System) by providing Content Services to a JavaScript front end application. Or any other application that can execute HTTP requests and handle JSON responses.

For example, Single Page Applications (SPA), framework-based or custom, require content provided over the HTTP API, often in JSON format.

While AEM Core Components provide a very comprehensive, flexible and customizable API that can serve required Read operations for this purpose, and whose JSON output can be customized, they do require AEM WCM (Web Content Management) know-how for implementation as they must be hosted in pages that are based on dedicated AEM templates. Not every SPA development organization has direct access to such knowledge.

This is when the Assets REST API can be used. It allows developers to access assets (for example, images and content fragments) directly, without the need to first embed them in a page, and deliver their content in serialized JSON format.

The Assets REST API also allows developers to modify content - by creating new, updating, or deleting existing assets, content fragments and folders.

The Assets REST API:

• follows the HATEOAS principle

• implements the SIREN format

Prerequisites

The Assets REST API is available on each out-of-the-box install of a recent AEM version.

Key Concepts

The Assets REST API offers REST-style access to assets stored within an AEM instance.

It uses the /api/assets endpoint and requires the path of the asset to access it (without the leading /content/dam).

• This means that to access the asset at:
  • /content/dam/path/to/asset
• You need to request:
  • /api/assets/path/to/asset

For example, to access /content/dam/wknd/en/adventures/cycling-tuscany, request /api/assets/wknd/en/adventures/cycling-tuscany.json

The HTTP method determines the operation to be executed:

• GET - to retrieve a JSON representation of an asset or a folder
• POST - to create new assets or folders
• PUT - to update the properties of an asset or folder
• DELETE - to delete an asset or folder

The exact format of supported requests is defined in the API Reference documentation.

Transactional Behavior

All requests are atomic.

This means that subsequent (write) requests cannot be combined into a single transaction that could succeed or fail as a single entity.

AEM (Assets) REST API versus AEM Components

Security

If the Assets REST API is used within an environment without specific authentication requirements, AEM’s CORS filter needs to be configured correctly.

In environments with specific authentication requirements, OAuth is recommended.

Available Features

Content Fragments are a specific type of Asset, see Working with Content Fragments.

For further information about features available through the API see:

• The Assets REST API
• Entity Types, where the features specific to each supported type (as relevant to Content Fragments) are explained

Paging

The Assets REST API supports paging (for GET requests) via the URL parameters:

• offset - the number of the first (child) entity to retrieve
• limit - the maximum number of entities returned

The response will contain paging information as part of the properties section of the SIREN output. This srn:paging property contains the total number of (child) entities ( total), the offset and the limit ( offset, limit) as specified in the request.

Example: Paging

GET /api/assets.json?offset=2&limit=3

...
"properties": {
    ...
    "srn:paging": {
        "total": 7,
        "offset": 2,
        "limit": 3
    }
    ...
}
...

Entity Types

Folders

Folders act as containers for assets and other folders. They reflect the structure of the AEM content repository.

The Assets REST API exposes access to the properties of a folder; for example, its name, title, and so on. Assets are exposed as child entities of folders, and sub-folders.

Assets

If an asset is requested, the response will return its metadata; such as title, name and other information as defined by the respective asset schema.

The binary data of an asset is exposed as a SIREN link of type content.

Assets can have multiple renditions. These are typically exposed as child entities, one exception being a thumbnail rendition, which is exposed as a link of type thumbnail ( rel="thumbnail").

Content Fragments

A content fragment is a special type of asset. They can be used to access structured data, such as texts, numbers, dates, among others.

As there are several differences to standard assets (such as images or audio), some additional rules apply to handling them.

Representation

Content fragments:

• Do not expose any binary data.

• Are completely contained in the JSON output (within the properties property).

• Are also considered atomic, that is, the elements and variations are exposed as part of the fragment’s properties vs. as links or child entities. This allows for efficient access to the payload of a fragment.

Content Models and Content Fragments

Currently the models that define the structure of a content fragment are not exposed through an HTTP API. Therefore the consumer needs to know about the model of a fragment (at least a minimum) - although most information can be inferred from the payload; as data types, and so on. are part of the definition.

To create a content fragment, the (internal repository) path of the model has to be provided.

Associated Content

Associated content is currently not exposed.

Using

Usage can differ depending on whether you are using an AEM author or publish environment, together with your specific use case.

• It is strongly recommended that creation is bound to an author instance (and currently there is no means to replicate a fragment to publish using this API).

• Delivery is possible from both, as AEM serves requested content in JSON format only.

  • Storage and delivery from an AEM author instance should suffice for behind-the-firewall, media library applications.

  • For live web delivery, an AEM publish instance is recommended.

Read/Delivery

Usage is via:

GET /{cfParentPath}/{cfName}.json

For example:

http://<host>/api/assets/wknd/en/adventures/cycling-tuscany.json

The response is serialized JSON with the content structured as in the content fragment. References are delivered as reference URLs.

Two types of read operations are possible:

• Reading a specific content fragment by path, this returns the JSON representation of the content fragment.
• Reading a folder of content fragments by path: this returns the JSON representations of all content fragments within the folder.

Create

Usage is via:

POST /{cfParentPath}/{cfName}

The body has to contain a JSON representation of the content fragment to be created, including any initial content that should be set on the content fragment elements. It is mandatory to set the cq:model property and it must point to a valid content fragment model. Failing to do so will result in an error. It is also necessary to add a header Content-Type which is set to application/json.

Update

Usage is via

PUT /{cfParentPath}/{cfName}

The body has to contain a JSON representation of what is to be updated for the given content fragment.

This can simply be the title or description of a content fragment, or a single element, or all element values and/or metadata.

Delete

Usage is via:

DELETE /{cfParentPath}/{cfName}

Limitations

There are a few limitations:

• Content fragment models are currently not supported: they cannot be read or created. To be able to create a content fragment, or update an existing one, developers have to know the correct path to the content fragment model. Currently the only method to get an overview of these is through the administration UI.
• References are ignored. Currently there are no checks on whether an existing content fragment is referenced. Therefore, for example, deleting a content fragment might result in issues on a page that contains a reference to the deleted Content Fragment.
• JSON data type The REST API output of the JSON data type is currently string based output.

Status Codes and Error Messages

The following status codes can be seen in the relevant circumstances:

• 200 (OK)
  Returned when:

  • requesting a content fragment via GET
  • successfully updating a content fragment via PUT

• 201 (Created)
  Returned when:

  • successfully creating a content fragment via POST

• 404 (Not found)
  Returned when:

  • the requested content fragment does not exist

• 500 (Internal server error)

  NOTE

  This error is returned:

  • when an error that cannot be identified with a specific code has happened
  • when the given payload was not valid

  The following lists common scenarios when this error status is returned, together with the error message (monospace) generated:

  • Parent folder does not exist (when creating a content fragment via POST)

  • No content fragment model is supplied (cq:model is missing), cannot be read (due to an invalid path or a permission problem) or there is no valid fragment model:

    • No content fragment model specified
    • Cannot create a resource of given model '/foo/bar/qux'

  • The content fragment could not be created (potentially a permission problem):

    • Could not create content fragment

  • Title and or description could not be updated:

    • Could not set value on content fragment

  • Metadata could not be set:

    • Could not set metadata on content fragment

  • Content element could not be found or could not be updated

    • Could not update content element
    • Could not update fragment data of element

  The detailed error messages are usually returned in the following manner:

  {
    "class": "core/response",
    "properties": {
      "path": "/api/assets/foo/bar/qux",
      "location": "/api/assets/foo/bar/qux.json",
      "parentLocation": "/api/assets/foo/bar.json",
      "status.code": 500,
      "status.message": "...{error message}.."
    }
  }
  

API Reference

See here for detailed API references:

• Adobe Experience Manager Assets API - Content Fragments

• Assets HTTP API

  • Available Features

Additional Resources

For further information see:

• Assets HTTP API documentation
• AEM Gem session: OAuth