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Layout of a Fedora OCFL Object

This document shows a series of diagrams depicting the structure of the three varieties of Fedora resources compared with their non-Fedora OCFL representations. Following the diagrams are detailed descriptions of the Fedora naming conventions.
The three varieties of Fedora resources are:

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Finally, a detailed description is provided of the internal contents of the Fedora-specific files.

For more information on OCFL, see the specification.

Atomic Resources

An atomic resource is either a single binary or a single container that maps to its own OCFL object. Below  Below is a comparison of how Fedora atomic resources are represented in OCFLbetween a plain OCFL representation of a single item object and Fedora's atomic resource representation.

Single-binary OCFL Object

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• The container user-properties: the filename is always "fcr-container.nt".
• The container's ACL: the filename is always "fcr-container~fcr-acl.nt".
• The container's header file: the filename is always "fcr-root.json". This allows Fedora to find the header file without know if the resource is a binary, container or archival group.
• The container ACL's header file: the filename is always "fcr-root~fcr-acl.json".

Archival Group Resources

A Fedora archival group resource is container that contains a potentially nested hierarchy of zero or more children containers and/or binaries.

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Below is a comparison between a plain OCFL representation of a compound object and Fedora's archival group resource representation.

Compound OCFL Object

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The following is an example of an OCFL object that contains one version, consisting of two binaries, one of a hierarchy of files and directories. For more information on OCFL, see the specificationwhich is nested within a child container.

No Format
[object root] ├── 0=ocfl_object_1.0 ├── inventory.json ├── inventory.json.sha512 └── v1/ ├── inventory.json ├── inventory.json.sha512 └── content/ ├── emptyimage.txttiff ├──└── foo/ │ └── bar.xml └── image.tiff

Basic Fedora OCFL Structure

Fedora Archival Group

Archival groups are a way to collect several Fedora resources into a single OCFL object. The constituent Fedora resources (archival parts) can be any combination of containers and binaries.The following is how Fedora (as an archival group) would represent the above example:

[object root]
    ├── 0=ocfl_object_1.0
    ├── inventory.json
    ├── inventory.json.sha512
    └── v1/
        ├── inventory.json
        ├── inventory.json.sha512
        └── content/
            ├── .fcrepo/
            │   ├── fcr-root.json              <-- Required "header" file holding system metadata about the archival group.
            │   ├── empty.txt.json fcr-root~fcr-acl.json      <-- Optional, only present if this Fedora resource has its own ACL.
            │   ├── image.tiff.json            <-- Required "header" file holding system metadata about the binary.
            │   ├── image.tiff~fcr-desc.json   <-- Required "header" file holding system metadata about the binary's description.
            │   ├── image.tiff~fcr-acl.json    <-- Optional, only present if this Fedora resource has its own ACL.
            │   └── foo/                       <-- Required nested structure within .fcrepo/ mirrors content structure
            │       ├── foo.json               <-- Required "header" file holding system metadata about the nested container.
            │       ├── foo~fcr-acl.json       <-- Optional, only present if this Fedora resource has its own ACL.
            │       └──├── bar.xml.json           <-- Required "header" file holding system metadata about the binary.
            │       ├── foo.jsonbar.xml~fcr-desc.json  <-- Required "header" file holding system metadata about the binary's description.
            │       └── imagebar.tiff.jsonxml~fcr-acl.json   <-- Optional, only present if this Fedora resource has its own ACL.
            ├── fcr-container.nt               <-- Required file for holding user-properties describing the archival group container.
            ├── fcr-container~fcr-acl.nt       <-- Optional, only present if this Fedora resource has its own ACL.
            ├── empty.txt image.tiff
            ├── image.tiff~fcr-desc.nt         <-- Required "binary description".
            ├── image.tiff~fcr-acl.nt          <-- Optional, only present if this Fedora resource has its own ACL.
            └── foo/
                ├── fcr-container.nt           <--  │   └── bar.xml
Required file for holding user-properties describing the archival part container.
                └── image.tiff

├── fcr-container~fcr-acl.nt   <-- Optional, only present if this Fedora resource has its own ACL.
                ├── bar.xml
                ├── bar.xml~fcr-desc.nt        <-- Required "binary description".
                └── bar.xml~fcr-acl.nt         <-- Optional, only present if this Fedora resource has its own ACL.

Naming conventions

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• The container user-properties: the filename is always "fcr-container.nt".
• The container's ACL: the filename is always "fcr-container~fcr-acl.nt".
• The container's header file: the filename is always "fcr-root.json". This allows Fedora to find the header file without know if the resource is a binary, container or archival group.
• The container ACL's header file: the filename is always "fcr-root~fcr-acl.json".
• The binary: the filename provided during ingest of the binary is retained in the OCFL persistence. ("image.tiff", in this example)
• The binary's description: the filename is the name of the binary with the addition of the suffix, "~fcr-desc.nt". ("image.tiff~fcr-desc.nt", in this example)
• The binary's ACL: the filename is the name of the binary with the addition of the suffix, "~fcr-acl.nt". ("image.tiff~fcr-acl.nt", in this example)
• The binary's header file: the filename is always "fcr-root.json". This allows Fedora to find the header file without know if the resource is a binary, container or archival group.
• The binary description's header file: the filename is always "fcr-root~fcr-desc.json".
• The binary ACL's header file: the filename is always "fcr-root~fcr-acl.json".

General

Every Fedora OCFL object contains a .fcrepo directory in its root. This directory contains JSON files, header files, that describe the resources contained in the OCFL object. The specifics of the contents of the header files is not discussed here, just their naming conventions. They contain information that enables Fedora to know how it is supposed to interact with a resource. Header files must be identifiable by Fedora without knowing anything about a resource other than its IDs.

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