Tags

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

Tags are often confused with metadata. While the two have an intersection, the main function of tags is to classify a collection of entities in groups, while metadata is used to attach additional information to entities. A separate guideline document exists for metadata.

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

Tags Representation

Tags are strings attached to an entity with the purpose of classification into groups.

An entity can have zero, one or more tags associated with it, for that reason the recommended representation within the parent entity is a list.

Example request using a server resource:

GET /servers/1234567890

Response:

{
    'id': '1234567890',

    ... other server resource properties ...

    'tags': ['foo', 'bar', 'baz']
}

Updates to the tags are issued in accordance to the standard HTTP request methods, issued directly against the parent resource. To update the tag list of a resource, a PUT request should be sent to the resource, including not only the updated tag list, but the complete resource representation in the body. The update in this case does not need to be limited to tags, other properties can be updated at the same time. Note that by using a PUT request it is possible to add and/or remove multiple tags in one single operation, simply by sending the updated tag list with the resource representation.

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

Tags Restrictions

Tags are strings with the following basic restrictions:

  • Tags are case sensitive.
  • ‘/’ is not allowed to be in a tag name
  • Comma is not allowed to be in a tag name in order to simplify requests that specify lists of tags
  • All other characters are allowed to be in a tag name

Note

The ‘/’ character is forbidden because some servers have a problem with encoding this character. The problem is that the server will handle ‘%2F’ as ‘/’ even though ‘/’ is encoded. It’s a problem of poor server implementation. To avoid problems with handling URLs character ‘/’ is forbidden in tag names.

Character Encoding for Tags

Per RFC 7159, 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 tags are part of a JSON document, the encoding of the tag names must match the encoding of the parent document. The use of UTF-8 encoding is strongly recommended.

Tags Resource URLs

Sometimes it may be inconvenient to work with the tags portion of a resource using the complete resource representation, so tags can optionally be exposed as a stand-alone resource as well. If a project decides to provide this functionality, then the root resource URL for tag management should be the URL of the resource to which the tags belong, followed by /tags (for APIs that use user-generated URLs with varying number of components the tags/ 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 tags with root URL http://example.com:8774/servers/1234567890/tags.

Obtaining the Tag List

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

Example request:

GET /servers/1234567890/tags

Response:

{
    "tags": ['foo', 'bar', 'baz']
}

Note that this representation differs from the one adopted by Nova. One reason is that with this structure it is possible to add additional metadata to the request body. A secondary reason is that JSON arrays as a top level entity have been found to expose vulnerabilities in browsers, as reported by the following articles:

Modifying the Tag List

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

Example request (removes “bar” and adds “qux”):

PUT /servers/1234567890/tags
{
    "tags": ['foo', 'baz', 'qux']
}

Response:

{
    "tags": ['foo', 'baz', 'qux']
}

If the number of tags exceeds the limit allowed by the API, the return code should be 400 Bad Request as the HTTP Guidelines describe. To achieve request success, the client should change the requested number of tags to be less than the API limit.

Deleting Tags

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

Example request:

DELETE /servers/1234567890/tags

Addressing Individual Tags

To provide even more fine-grained access to tags, another optional extension is to expose resource URLs for individual tags. If a project decides to implement this option, then each tag should be accessed individually at a URL formed by appending the tag name to the root tag URL. Note that this option is not available for APIs that use user-generated URLs.

To insert a single tag without having to send the entire tag list, the client should send a PUT request to the inidividual tag URL. On success, the server responds with a 201 status code and includes the new tag’s URL in the Location header in the response.

Example request:

PUT /servers/1234567890/tags/qux
<no body>

Response:

Location: http://example.com:8774/servers/1234567890/tags/qux
<no body>

To check if a tag exists or not, the client should send a GET request to the individual tag URL. If the tag exists, the server responds with a status code 204 and no response body. If the tag does not exist, the server responds with a status code 404.

To delete a single tag without affecting the remaining ones, a DELETE request is sent to the individual tag URL. On success, the server responds with a 204 status code. If an invalid tag is given, a 404 response is returned.

Example request:

DELETE /servers/1234567890/tags/qux

Filtering and Searching by Tags

To search the collection of entities by their tags, the client should send a GET request to the collection URL, and include query string parameters that define the query. These arguments can be combined with other arguments, such as those that perform additional filtering outside of tags, pagination, sorting, etc. The recommended query string arguments for filtering tags are tags, tags-any, not-tags and not-tags-any.

Note that once again this is different than the nova specification, which uses repeated tag query arguments to specify a list of tags. The preference here is to be consistent with the sorting guideline document, for which it was decided that repeating query string arguments is not a good idea due to not having good support among web clients and servers.

To request the list of entities that have a single tag, tags argument should be set to the desired tag name. Example:

GET /servers?tags=red

To request the list of entities that have two or more tags, the tags argument should be set to the list of tags, separated by commas. In this situation the tags given must all be present for an entity to be included in the query result. Example that returns servers that have the “red” and “blue” tags:

GET /servers?tags=red,blue

To request the list of entities that have one or more of a list of given tags, the tags-any argument should be set to the list of tags, separated by commas. In this situation as long as one of the given tags is present the entity will be included in the query result. Example that returns the servers that have the “red” or the “blue” tag:

GET /servers?tags-any=red,blue

To request the list of entities that do not have one or more tags, the not-tags argument should be set to the list of tags, separated by commas. In this situation only the entities that do not have any of the given tags will be included in the query results. Example that returns the servers that do not have the “red” nor the “blue” tag:

GET /servers?not-tags=red,blue

To request the list of entities that do not have at least one of a list of tags, the not-tags-any argument should be set to the list of tags, separated by commas. In this situation only the entities that do not have at least one of the given tags will be included in the query result. Example that returns the servers that do not have the “red” tag, or do not have the “blue” tag:

GET /servers?not-tags-any=red,blue

The tags, tags-any, not-tags and not-tags-any arguments can be combined to build more complex queries. Example:

GET /servers?tags=red,blue&tags-any=green,orange

The above example returns any servers that have the “red” and “blue” tags, plus at least one of “green” and “orange”.

It is possible to create a request which is self-contradictory. Example:

GET /servers?tags=red&not-tags=red

This should be treated as a valid request (ie not a client error), and should return an empty result-set with a 2xx status code.