Metadata

Metadata

This topic document serves to provide guidance on how to work with metadata in OpenStack REST APIs.

Metadata is sometimes confused with tags. While they have some things in common, the main function of metadata is to attach additional information, in the form of key-value pairs, to entities. Tags, on the other side, are used to classify entities in groups. A separate guidelines document exists for tags.

For background on the REST guidelines referenced here, see the topic documents on Naming Conventions and HTTP Guidelines.

Metadata Representation

A Python dictionary is used as representation of metadata for a resource. This dictionary is added as an additional field in the representation of the parent resource with name metadata.

Example request using a server resource:

GET /servers/1234567890

Response:

{
    'id': '1234567890',

    ... other server resource properties ...

    'metadata': {
        "foo": "Foo Value",
        "bar": "Bar Value",
        "baz": "Baz Value"
    }
}

Updates to the metadata are issued in accordance to the standard HTTP request methods, issued directly against the parent resource, so for example, to update the metadata dictionary of a resource, a PUT request should be sent to the resource, including not only the metadata, but the complete resource representation in the body. The update in this case does not need to be limited to metadata, other properties can be updated at the same time.

For resources that have a representation that is not in JSON, a separate endpoint must be created to expose the metadata. See the “Metadata Resource URLs” section below for more information.

Character Encoding for Metadata Keys and Values

Per RFC 7159#section-8.1, JSON documents shall be encoded in UTF-8, UTF-16, or UTF-32, with UTF-8 being the default and the recommended encoding for maximum interoperability.

Since the entire metadata representation is a JSON document, the encoding of the keys and values must match the encoding of the parent document. The use of UTF-8 encoding is strongly recommended.

Metadata Resource URLs

Sometimes it may be inconvenient to work with the metadata portion of a resource using the complete resource representation, so metadata can also be exposed as a stand-alone resource. The root resource URL for metadata management must be the URL of the resource to which the metadata belongs, followed by /metadata (for APIs that use user-generated URLs with varying number of components the /metadata URL component can be added as a prefix instead of a suffix).

For example, the resource identified by URL http://example.com:8774/servers/1234567890 must expose its metadata with root URL http://example.com:8774/servers/1234567890/metadata.

Obtaining Metadata

To obtain the metadata for a resource, a GET request must be sent to the root metadata URL. On success, the server responds with a 200 status code and the complete set of metadata items in the response body.

Example request:

GET /servers/1234567890/metadata

Response:

{
    "metadata": {
        "foo": "Foo Value",
        "bar": "Bar Value",
        "baz": "Baz Value"
    }
}

Modifying Metadata

To add, remove, or change metadata items, a PUT request must be sent to the root metadata URL, with the updated complete list of metadata items in the body of the request. On success, the server responds with a 200 status code and the complete updated metadata block in the response body.

Note

A PUT request should use etags to avoid the lost update problem.

Example request (updates “foo”, removes “bar”, adds “qux” and leaves “baz” untouched):

PUT /servers/1234567890/metadata
{
    "metadata": {
        "foo": "Foo Value Updated",
        "baz": "Baz Value",
        "qux": "Qux Value"
    }
}

Response:

{
    "metadata": {
        "foo": "Foo Value Updated",
        "baz": "Baz Value",
        "qux": "Qux Value"
    }
}

Deleting Metadata

To delete the entire metadata block associated with a resource, a DELETE request must be sent to the root metadata URL. On success, the server responds with a 204 status code.

Example request:

DELETE /servers/1234567890/metadata

To delete multiple metadata items without affecting the remaining ones, a PUT request must be sent to the root metadata URL with the updated complete list of metadata items (without items to delete) in the body of the request. On success, the server responds with a 200 status code.

Example request (removes “foo” and “qux”):

PUT /servers/1234567890/metadata
{
    "metadata": {
        "baz": "Baz Value"
    }
}

Response:

{
    "metadata": {
        "baz": "Baz Value"
    }
}

To delete a single metadata item see below.

Addressing Individual Metadata Items

As an optional extension to the above, an API can elect to expose additional endpoints to give clients the ability to work with individual metadata items. If a project decides to implement this option, then each metadata key-value pair should be accessed individually at a URL formed by appending the key name to the root metadata URL. Note that this option is not available for APIs that use user-generated URLs.

To insert a single metadata item without having to send the entire metadata block, the client can send a POST request to the root metadata URL, and include the individual metadata item representation in the request body. On success, the server responds with a 201 status code and includes the new metadata item’s URL in the Location header in the response.

Example request:

POST /servers/1234567890/metadata
{
    "key": "qux",
    "value": "Qux Value"
}

Response:

Location: http://example.com:8774/servers/1234567890/metadata/qux
{
    "key": "qux",
    "value": "Qux Value",
}

As shown in the above example, metadata items can be accessed individually by appending the key name to the root metatadata URL. The representation includes the key and the value. This format gives APIs the option to include additional properties that describe a metadata item, such as an expiration date.

To modify an item, a PUT request is sent to the metadata item’s URL. On success, the server responds with a 200 status code and the updated representation of the metadata item in the response body.

Example request:

PUT /servers/1234567890/metadata/qux
{
    "key": "qux",
    "value": "Qux Value Updated"
}

Response:

{
    "key": "qux",
    "value": "Qux Value Updated"
}

To delete a single metadata item without affecting the remaining ones, a DELETE request is sent to the metadata item URL. On success, the server responds with a 204 status code.

Example request:

DELETE /servers/1234567890/metadata/qux
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