Allow simple string tagging of instances

Allow simple string tagging of instances

https://blueprints.launchpad.net/nova/+spec/tag-instances

This blueprint aims to add support for a simple string tagging mechanism for the instance object in the Nova domain model.

Problem description

In most popular REST API interfaces, objects in the domain model can be “tagged” with zero or more simple strings. These strings may then be used to group and categorize objects in the domain model.

In order to align Nova’s REST API with the Internet’s common understanding of resource tagging, we can add a API microversion that allows normal users to add, remove and list tags for an instance.

Use Cases

A typical end-user would like to attach a set of strings to an instance. The user does not wish to use key/value pairs to tag the instance with some simple strings.

Proposed change

No changes to existing metadata, system_metadata or extra_specs functionality are being proposed. This is specifically for adding a new API for normal users to be able to tag their instances with simple strings.

Add a API microversion that allows a user to add, remove, and list tags for an instance.

Add a API microversion to allow searching for instances based on one or more string tags.

Alternatives

Alternatives to simple string tagging are already available in Nova through the instance metadata key/value pairs API extension. But it’s not quite right to use metadata for tagging. 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.

Data model impact

The nova.objects.instance.Instance object would have a new tags field of type nova.objects.fields.ListOfStrings that would be populated on-demand (i.e. lazy-loaded).

A tag shall be defined as a Unicode bytestring no longer than 60 bytes in length. (This length is entirely arbitrary and could be reduced or expanded depending on review discussion…)

The tag is an opaque string and is not intended to be interpreted or even read by the virt drivers. In the REST API changes below, non-URL-safe characters in tags will need to be urlencoded if referred in the URI (for example, doing a DELETE /servers/{server}/tags/{tag}, the {tag} would need to be urlencoded.

Note

Glance already has object tagging functionality, and the database schema in that project uses a VARCHAR(255) length for the tag value. I would greatly prefer to keep a shorter-than-255 length. There are a number of performance reasons (including the fact that MySQL converts all varchar columns to fixed-width columns when doing aggregation and temporary tables containing the varchar columns). In addition, if the tags are UTF-8 (as proposed above), the 255 width will actually be 765 bytes wide (which exacerbates the fixed-width problems on MySQL).

For the database schema, the following table constructs would suffice

CREATE TABLE tags (
    resource_id CHAR(32) NOT NULL PRIMARY KEY,
    tag VARCHAR(60) NOT NULL CHARACTER SET utf8
     COLLATION utf8_ci PRIMARY KEY,
    CONSTRAINT resource_tag_constraint UNIQUE (resource_id, tag),
    deleted_at DATETIME,
    deleted INT DEFAULT 0
);

There shall be a new hard-coded limit of 50 for the number of tags a user can use on a server. No need to make this configurable or use the quota system at this point.

REST API impact

This proposal would add a API microversion for retrieving and setting tags against an instance. In addition, it would add a API microversion to allow the searching/listing of instances based on one or more string tags.

The tag CRUD operations API microversion would look like the following:

A list of tags for the specified server returns with the server details information

GET /servers/{server_id}

Response

{
    'id': {server_id},

    ... other server resource properties ...

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

A servers list detail request returns details information about each server, including a list of tags for each server

GET /servers/detail

Response

{
    'servers': [
        {
            'id': {server1_id},

            ... other server resource properties ...

            'tags': ['foo', 'bar', 'baz']
        },
        {
            'id': {server2_id},

            ... other server resource properties ...

            'tags': ['one', 'two']
        }
}

Get only a list of tags for the specified server

GET /servers/{server_id}/tags

Response

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

Replace set of tags on a server

PUT /servers/{server_id}/tags

with request payload

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

Response

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

If the number of tags exceeds the limit of tags per server, shall return a 400 Bad Request

Add a single tag on a server

PUT /servers/{server_id}/tags/{tag}

Returns 201 Created.

If the tag already exists, no error is raised, it just returns the 204 No Content

If the number of tags would exceed the per-server limit, shall return a 400 Bad Request

Check if a tag exists or not on a server

GET /servers/{server_id}/tags/{tag}

Returns 204 No Content if tag exist on a server.

Returns 404 Not Found if tag doesn’t exist on a server.

Remove a single tag on a server

DELETE /servers/{server_id}/tags/{tag}

Returns 204 No Content upon success. Returns a 404 Not Found if you attempt to delete a tag that does not exist.

Remove all tags on a server

DELETE /servers/{server_id}/tags

Returns 204 No Content.

The API microversion that would allow searching/filtering of the GET /servers REST API call would add the following query parameters:

  • tags

  • tags-any

  • not-tags

  • not-tags-any

To request the list of servers 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 servers 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 a server 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 servers 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 server 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 servers 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 servers 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 servers 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 servers 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”.

Complex queries may have contradictory parameters. Example:

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

In this case we should let Nova find these servers. Obviously there are no such servers and Nova will return an empty list.

No change is needed to the JSON response for the GET /servers/ call.

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

None, though REGEXP-based querying on some fields might be modified to use a faster tag-list filtering query.

Other deployer impact

None

Developer impact

None

Implementation

See Work Items section below.

Assignee(s)

Primary assignee:

snikitin

Other contributors:

jaypipes

Work Items

Changes would be made, in order, to:

  1. the database API layer to add support for CRUD operations on instance tags (Done)

  2. the database API layer to add tag-list filtering support to instance_get_all_by_filters (Done for ‘tags’ and ‘tags-any’ filters)

  3. the nova.objects layer to add support for a tags field of the Instance object (Done)

  4. the API microversion for CRUD operations on the tag list

Dependencies

None.

Testing

Would need new Tempest and unit tests.

Documentation Impact

Docs needed for new API microversion and usage.

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nova-specs