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+/.idea
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+OCFL Community Extension NNNN: Object Version Properties
+=========================================
+
+* **Extension Name:** NNNN-object-version-properties
+* **Authors:** Linda Reijnhoudt, Jan van Mansum
+* **Minimum OCFL Version:** 1.0
+* **OCFL Community Extensions Version:** 1.0
+* **Obsoletes:** n/a
+* **Obsoleted by:** n/a
+
+Overview
+--------
+
+This extension provides a way to record properties of an OCFL object version beyond the standard properties defined by OCFL itself (`created`, `message`, etc.).
+This may be useful for cases where additional information about the version is needed, which cannot conveniently be stored in the object itself in a standard
+way. Another use-case may be a property that is not available at the time the version content is created.
+
+This extension only describes how the properties should be recorded, it does not prescribe what properties should be recorded or their semantics. One way to
+define the semantics of properties is to use the [Property Registry extension](../property-registry/property-registry.md).
+
+The object-version-properties extension is implemented by creating and maintaining the following items:
+
+* A `object_version_properties.json` file containing the properties for each version of the OCFL object.
+* A sidecar file `object_version_properties.json.sha512` (or other configured digest) containing the digest of the `object_version_properties.json` file, in
+  the same manner as the OCFL inventory files. The digest algorithm must be the same as the one used for the OCFL inventory files.
+
+The object_version_properties.json file
+---------------------------------------
+
+The object version properties are stored in a JSON file named `object_version_properties.json` in the extensions directory of the OCFL object. This file must
+contain entries for all versions in this object. Each version entry has as key the version identifier (e.g. `v1`, `v2`, etc.) and as value a JSON object with
+the properties for that version. If no properties are recorded for a version, the entry for that version must be an empty JSON object.
+
+The properties recorded under a version key pertain to that version only, so when a new version is created, properties that do not change must be copied from
+the previous version. Otherwise, this extension does not restrict the properties that can be recorded, nor does it define their semantics.
+
+Implementation
+--------------
+
+Clients that implement this extension must validate that the `object_version_properties.json` file is well-formed JSON and that it contains an entry for each
+version in the object as part of the validation of the OCFL object.
+
+Clients may support storing the version properties when creating a new version of an OCFL object, by passing the properties map as an extra parameter to the
+version creation method. Likewise, clients may support retrieving the properties for a specific version of an OCFL object as a properties map. Other optional
+features may include the ability to search for object versions based on their properties.
+
+Examples
+--------
+
+### 1. Simple property
+
+If the repository wants to record a simple property, for instance the User-Agent that made the request to create this version, this could be done as follows.
+
+```text
+[storage_root]
+  ├── 0=ocfl_1.0
+  ├── ocfl_1.0.txt
+  ├── ocfl_layout.json
+  ├── object-version-properties.md
+  ├── 0de
+  |   └── 45c
+  |       └── f24
+  |           └── item1
+  │               └── 0=ocfl_object_1.0
+  │                   ├── inventory.json
+  │                   ├── inventory.json.sha512
+  |                   └── extensions
+  │                        └── object-version-properties/
+  │                            ├── object_version_properties.json
+  │                            └── object_version_properties.json.sha512
+  └────────────────── v1/
+                      ├── inventory.json
+                      ├── inventory.json.sha512
+                      └── content/
+                          └── ... files ...
+```
+
+The `object-version-properties.json` of the OCFL object could have the following entry:
+
+```json
+{
+  "v1": {
+    "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64)"
+  }
+}
+```
+
+### 2. Complex property
+
+If the repository wants to record a complex object for a version, for instance that it has been deaccessioned, this could be done with a similar structure. The
+`object-version-properties.json` of the OCFL object might have the following entry:
+
+```json
+{
+  "v1": {
+    "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64)"
+  },
+  "v2": {
+    "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64)",
+    "deaccessioned": {
+      "datetime": "2025-10-15T13:19:00",
+      "reason": "Deaccessioned because dataset was deleted in Easy"
+    }
+  }
+}
+```
+
+Note, that this example also demonstrates that a property that remains unchanged between versions (in this case "User-Agent") must be copied from the previous
+version.