Consuming Service Catalog

This document describes the process to correctly find a service’s endpoint from the Service Catalog.

Note

The process described in this document is compatible with all known OpenStack Public Clouds and also matches the behavior of the python library keystoneauth, which is the reference implementation of authenticating with keystone and consuming information from the catalog. In some places an argument can be made for a different process, but given keystoneauth’s wide use and reference nature, we’ve chosen to keep backwards compatibility with keystoneauth’s behavior rather than design a new perfect process. keystoneauth itself notes internally places where it kept backwards compatibility with the libraries that predate it. Notes have been left about stricter behavior a library or framework could choose to impose.

Note

The use of the word “object” in this document refers to a JSON object, not an Object from any particular programming language.

User Request

The ultimate goal of this process is for a user to find the information about an endpoint for a service given some inputs. The user will start the process knowing some number of these parameters. Each additional input expected from the user without an answer of “where do they learn this information” will increase the difficulty of a user consuming services, so client libraries and utilities are strongly encouraged to do whatever they can to be extra helpful in helping the user ask the right question.

Note

Be liberal with what you accept and strict with what you emit.

There is one piece of information that is absolutely required that the user know:

service-type
The official name of the service, such as compute, image or block-storage as listed in the OpenStack Service Types Authority. Required. It is impossible for a user to consume service discovery without knowing what service they want to discover.

The user may also wish to express an alteration to the general algorithm:

be-strict
Forgo lenient backwards compatibility concessions and be more strict in input and output validation.

There are several optional pieces of information that the user might know, or additional constraints the user might wish to express.

region-name
The region of the service the user desires to work with. May be optional, depending on whether the cloud has more than one region. Services all exist within regions, but some clouds only have one region. If {be-strict} has been given, {region-name} is required.

Note

It is highly recommended that {region-name} always be required to protect against single-region clouds adding a region in the future. However, keystoneauth today allows region-name to be omitted and there are a large number of clouds in existence with a single region named RegionOne. For completely new libraries or major versions where breaking behavior is acceptable, requiring region-name by default would be preferred.

interface
Which API interface, such as public, internal, or admin the user wants to use. A user can also request a list of interfaces they find acceptable in the order of their preference, such as ['internal', 'public'] (Optional, defaults to public.)
service-name
Arbitrary name given to the service by the deployer. Optional.

Note

In all except the most extreme cases this should never be needed and its use as a meaningful identifier by Deployers is strongly discouraged. However, the Consumer has no way to otherwise mitigate the situation if their Deployer has provided them with a catalog where a service-name must be used, so service-name must be accepted as input. If {be-strict} has been requested, supplying {service-name} should be an error.

service-id
Unique identifier for an endpoint in the catalog. Optional.

Note

On clouds with well-formed catalogs service-id should never be needed. If {be-strict} has been requested, supplying {service-id} should be an error.

endpoint-override
An endpoint for the service that the user has procured from some other source. (Optional, defaults to omitted.)

At the end of the discovery process, the user should know the {service-endpoint}, which is the endpoint to use as the root of the service, and the {interface} of the endpoint that was found.

In the description that follows, each of the above inputs and outputs will be referred to like {endpoint-override} so that it is clear whether a user supplied input to the process or one of the expected outputs is being discussed. Other values that are fetched at one point in the process and referred to at a later point are similarly referred to like {service-catalog}. Names will not be reused within the process to hold different content at different times.

It is also assumed that the user has an {auth-url} and authentication information. The authentication process itself is out of the scope of this document.

Discovery Algorithm

Services should be registered in the {service-catalog} using their {service-type} from the OpenStack Service Types Authority. However, for historical reasons there are some services that have old service types found in the wild. To facilitate moving forward with the correct {service-type} names, but also support existing users and installations, the OpenStack Service Types Authority contains a list of historical aliases for such services. See Consuming Service Types Authority for information on the data itself.

Clients will need a copy of the data published in the OpenStack Service Types Authority to be able to complete the full Discovery Algorithm. A client library could either keep a local copy or fetch the data from https://service-types.openstack.org/service-types.json and potentially cache it. It is recommended that client libraries handle consumption of the historical data for their users but also allow some mechanism for the user to provide a more up to date verison of the data if necessary. See Consuming Service Types Authority for information on how to fetch the data.

The basic process is:

  1. If the user has provided {endpoint-override}, STOP. This is the {service-endpoint}.
  2. Authenticate to keystone at the {auth-url}, retreiving a token which contains the {service-catalog}.
  3. Retrieve {catalog-endpoint} from the {service-catalog} given some combination of {service-type}, {interface}, {service-name}, {region-name} and {service-id}. (See Endpoint from Catalog.)

Endpoint from Catalog

The {service-catalog} can be found in the token returned from keystone authentication.

If v3 auth is used, the catalog will be in the catalog property of the top-level token object. Such as:

{
  'token': {
    'catalog': {}
  }
}

If v2 auth is used it will be in the serviceCatalog property of the top-level access object. Such as:

{
  'access': {
    'serviceCatalog': {}
  }
}

In both cases, the catalog content itself is a list of objects. Each object has two main keys that concern discovery:

type
Matches {service-type}
endpoints
List of endpoint objects for that service

Additionally, for backwards compatibility reasons, the following keys may need to be checked.

name
Matches {service-name}
id
Matches {service-id}

The list of endpoints has a different format depending on whether v2 or v3 auth was used. For both versions each endpoint object has a region key, which should match {region-name} if one was given.

In v2 auth the endpoint object has three keys publicURL, internalURL, adminURL. The endpoint for the {interface} requested by the user is found in the key with the name matching {interface} plus the string URL.

In v3 auth the endpoint object has a url that is the endpoint that is being requested if the value of interface matches {interface}.

Concrete examples of tokens with catalogs:

V3 Catalog Objects:

{
  "token": {
    "catalog": [
        {
            "endpoints": [
                {
                    "id": "39dc322ce86c4111b4f06c2eeae0841b",
                    "interface": "public",
                    "region": "RegionOne",
                    "url": "https://identity.example.com"
                },
                {
                    "id": "ec642f27474842e78bf059f6c48f4e99",
                    "interface": "internal",
                    "region": "RegionOne",
                    "url": "https://identity.example.com"
                },
                {
                    "id": "c609fc430175452290b62a4242e8a7e8",
                    "interface": "admin",
                    "region": "RegionOne",
                    "url": "https://identity.example.com"
                }
            ],
            "id": "4363ae44bdf34a3981fde3b823cb9aa2",
            "type": "identity",
            "name": "keystone"
        }
    ],
}

V2 Catalog Objects:

{
  "access": {
    "serviceCatalog": [
      {
        "endpoints_links": [],
        "endpoints": [
          {
            "adminURL": "https://identity.example.com/v2.0",
            "region": "RegionOne",
            "publicURL": "https://identity.example.com/v2.0",
            "internalURL": "https://identity.example.com/v2.0",
            "id": "4deb4d0504a044a395d4480741ba628c"
          }
        ],
        "type": "identity",
        "name": "keystone"
      },
    ]
  }
}

The algorithm is:

  1. Find the objects in the {service-catalog} that match the requested {service-type}. (See Match Candidate Entries.)
  2. If {service-name} was given and the objects remaining have a name field, keep only the ones where name matches {service-name}.

Note

Catalogs from Keystone v3 before v3.3 do not have a name field. If {be-strict} was not requested and the catalog does not have a name field, {service-name} should be ignored.

  1. If {service-id} was given and the objects remaining have a id field, keep only the ones where id matches {service-id}.

Note

Catalogs from Keystone v2 do not have an id field. If {be-strict} was not requested and the catalog does not have a id field, {service-id} should be ignored.

The list of remaining objects are the {candidate-catalog-objects}. If there are no endpoints, return an error that there are no endpoints matching {service-type} and {service-name}.

Use {candidate-catalog-objects} to produce the list of {candidate-endpoints}.

For each endpoint object in each of the {candidate-catalog-objects}:

  1. If v2, if there is no key of the form {interface}URL for any of the the {interface} values given, discard the endpoint.
  2. If v3, if interface does not match any of the {interface} values given, discard the endpoint.

If there are no endpoints left, return an error that there are no endpoints matching any of the {interface} values, preferrably including the list of interfaces that were found.

For each remaining endpoint in {candidate-endpoints}:

  1. If {region_name} was given and does not match either of region or region_id, discard the endpoint.

If there are no remaining endpoints, return an error that there are no endpoints matching {region_name}, preferrably including the list of regions that were found.

  1. From the set of remaining candidate endpoints, find the ones that best matches the requested {service-type}. (See Find Endpoint Matching Best Service Type.)

The remaining {candidate-endpoints} match the request. If there is more than one of them, use the first, but emit a warning to the user that more than one endpoint was left. If {be-strict} has been requested, return an error instead with information about each of the endpoints left in the list.

Note

It would be more correct to raise an error if there is more than one endpoint left, but the keystoneauth library returns the first and changing that would break a large number of existing users. If one is writing a completely new library from scratch, or a new major version where behavior change is acceptable, it is preferable to raise an error here if there is more than one endpoint left.

  1. If v2, the {catalog-endpoint} is the value of {interface}URL.
  2. If v3, the {catalog-endpoint} is the value of url.

Match Candidate Entries

For every entry in the catalog:

  1. If the entry’s type matches the requested {service-type}, it is a candidate.
  2. If the requested type is an official type from the OpenStack Service Types Authority that has aliases and one of the aliases matches the entry’s type, it is a candidate.
  3. If the requested type is an alias of an official type from the OpenStack Service Types Authority and the entry’s type matches the official type, it is a candidate.

Note

Requesting one alias and finding a different alias is not supported at this point because most aliases carry implied information about major versions as well. A subsequent spec adds the process for version discovery at which point it can be safe to attempt to return an endpoint listed under an alias different than what was requested.

Find Endpoint Matching Best Service Type

Given a list of candidate endpoints that have matched the other criteria:

  1. Check the list of candidate endpoints to see if one of them matches the requested {service-type}. If any are an exact match, Find Endpoint Matching Best Interface.
  2. If the requested {service-type} is an official type in the OpenStack Service Types Authority that has aliases, check each alias in order of preference as listed in the Authority to see if it has a matching endpoint from the candidate endpoints. For all endpoints that match the first alias with matching endpoints, Find Endpoint Matching Best Interface.
  3. If the requested {service-type} is an alias of an official type in the OpenStack Service Types Authority and any endpoints match the official type, Find Endpoint Matching Best Interface.

Find Endpoint Matching Best Interface

Given a list of candidate endpoints that have matched the other criteria:

  1. In order of preference of {interface} list, return all endpoints that match the first {interface} with matching endpoints.

For example, given the following catalog:

{
  "token": {
    "catalog": [
        {
            "endpoints": [
                {
                    "interface": "public",
                    "region": "RegionOne",
                    "url": "https://block-storage.example.com/v3"
                }
            ],
            "id": "4363ae44bdf34a3981fde3b823cb9aa3",
            "type": "volumev3",
            "name": "cinder"
        },
        {
            "endpoints": [
                {
                    "interface": "public",
                    "region": "RegionOne",
                    "url": "https://block-storage.example.com/v2"
                }
            ],
            "id": "4363ae44bdf34a3981fde3b823cb9aa2",
            "type": "volumev2",
            "name": "cinder"
        }
    ],
}

Then the following:

service_type = 'block-storage'
# block-storage is not found, get list of aliases
# volumev3 is found, return it

service_type = 'volumev2'
# volumev2 not an official type in authority, but is in catalog
# return volumev2 entry

service_type = 'volume'
# volume not in authority or catalog
# volume is an alias of block-storage
# block-storage is not found. Return error.

Given the following catalog:

{
  "token": {
    "catalog": [
        {
            "endpoints": [
                {
                    "interface": "public",
                    "region": "RegionOne",
                    "url": "https://block-storage.example.com"
                }
            ],
            "id": "4363ae44bdf34a3981fde3b823cb9aa3",
            "type": "block-storage",
            "name": "cinder"
        }
    ],
}

Then the following:

service_type = 'block-storage'
# block-storage is found, return it

service_type = 'volumev2'
# volumev2 not in authority, is an alias for block-storage
# block-storage is in the catalog, return it

Given the following catalog:

{
  "token": {
    "catalog": [
        {
            "endpoints": [
                {
                    "interface": "public",
                    "region": "RegionOne",
                    "url": "https://block-storage.example.com"
                }
            ],
            "id": "4363ae44bdf34a3981fde3b823cb9aa3",
            "type": "block-storage",
            "name": "cinder"
        },
        {
            "endpoints": [
                {
                    "interface": "public",
                    "region": "RegionOne",
                    "url": "https://block-storage.example.com/v2"
                },
                {
                    "interface": "internal",
                    "region": "RegionOne",
                    "url": "https://block-storage.example.int/v2"
                }
            ],
            "id": "4363ae44bdf34a3981fde3b823cb9aa2",
            "type": "volumev2",
            "name": "cinder"
        }
    ],
}

Then the following:

service_type = 'block-storage'
interface = ['internal', 'public']
# block-storage is found
# block-storage does not have internal, but has public
# return block-storage public

service_type = 'volumev2'
interface = ['internal', 'public']
# volumev2 not an official type in authority, but is in catalog
# volumev2 has an internal interface
# return volumev2 internal entry

Consuming Service Types Authority

The OpenStack Service Types Authority is data about official service type names and historical service type names commonly in use from before there was an official list. It is made available to allow libraries and other client API consumers to be able to provide a consistent interface based on the official list but still support existing names. Providing this support is highly recommended, but is ultimately optional. The first step in the matching process is always to return direct matches between the catalog and the user request, so the existing consumption models from before the existence of the authority should always work.

In order to consume the information in the OpenStack Service Types Authority it is important to know a few things:

  1. The data is maintained in YAML format in git. This is the ultimately authoritative source code for the list.
  2. The data is published in JSON format at https://service-types.openstack.org/service-types.json and has a JSONSchema at https://service-types.openstack.org/published-schema.json.
  3. The published data contains a version which is date based in ISO Date Time Format, a sha which contains the git sha of the commit the published data was built from, and pre-built forward and reverse mappings between official types and aliases.
  4. The JSON file is served with ETag support and should be considered highly cacheable.
  5. The current version of the JSON file should always be the preferred file to use.
  6. The JSON file is similar to timezone data. It should not be considered versioned such that stable releases of distros should provide a frozen version of it. Distro packages should instead update for all active releases when a new version of the file is published.