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Merge "Add conceptual overview of the service catalog"

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..
Licensed under the Apache License, Version 2.0 (the "License"); you may not
use this file except in compliance with the License. You may obtain a copy
of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
License for the specific language governing permissions and limitations
under the License.
========================
Service Catalog Overview
========================
The OpenStack keystone service catalog allows API clients to dynamically
discover and navigate to cloud services. The service catalog may differ from
deployment-to-deployment, user-to-user, and project-to-project.
The service catalog is the first hurdle that API consumers will need to
understand after successfully authenticating with Keystone, making it a
critical focal point for the overall user experience of OpenStack.
*If you're integrating your OpenStack service with Keystone*, then please
follow the guidelines provided below.
*If you're writing an OpenStack client*, hopefully this helps you navigate the
service catalog that you're being presented so that you can quickly move on to
the business of consuming cloud services.
An example service catalog
==========================
The following is an example service catalog. It actually excludes several
common attributes such as ``id``, which are of no concern to end users,
``region_id``, which are a bit out of scope for this topic, and ``enabled``,
which is always ``true`` for end users.
This service catalog contains just one service, "Keystone", which is accessible
via a single endpoint URL:
.. code-block:: json
{
"catalog": [
{
"name": "Keystone",
"type": "identity",
"endpoints": [
{
"interface": "public",
"url": "https://identity.example.com:5000/"
}
]
}
]
}
The service catalog itself may appear in a token creation response (``POST
/v3/auth/tokens``), a token validation response (``GET /v3/auth/tokens``), or
as a standalone resource (``GET /v3/auth/catalog``).
Services
========
The service catalog itself is composed of a list of services.
Service entities represent web services in the OpenStack deployment. A service
may have zero or more endpoints associated with it, although a service with
zero endpoints is essentially useless in an OpenStack configuration.
In addition to the related endpoints, there are two attributes of services that
important to end users:
* ``name`` (string): user-facing name of the service
This attribute is not intended to be machine-parseable or otherwise meaningful
beyond branding or name-recognition for end users. Logical values might include
"Keystone" or maybe "Brand X Public Cloud Identity Service". Deployers should
be free to rename, and therefore rebrand, a service at will.
* ``type`` (string): describes the API implemented by the service. To support
future projects, the value should not be validated against a list.
An OpenStack-wide effort to standardize service types has been done outside of
Keystone and is known as the `service-types authority`_.
This should not convey the version of the API implemented by the service (as in
Cinder's ``volumev2`` service type) because both the ``volume`` service and
``volumev2`` service provide "block storage as a service" which is what the
service type is meant to convey. The underlying implementation is completely
irrelevant here.
In the general case, there should only be one service in a deployment per
service type, although Keystone does not enforce this today.
.. _service-types authority: https://service-types.openstack.org/
Endpoints
=========
Each service should have one or more related endpoints. An endpoint is
essentially a base URL for an API, along with some metadata about the endpoint
itself and represents a set of URL endpoints for OpenStack web services.
* ``interface`` (string): describes the visibility of the endpoint according to
one of three values (``public``, ``internal``, and ``admin``)
``public`` endpoints are intended for consumption by end users or other service
users, generally on a publicly available network interface.
``internal`` endpoints are intended for consumption by end users, generally on
an unmetered internal network interface.
``admin`` endpoints are intended only for consumption by those needing
administrative access to the service, generally on a secure network interface.
You might also think of each interface value as the result of a matrix of use
cases:
* **Public API** on a **public network**: use a ``public`` interface.
* **Public API** on an **internal network**: use an ``internal`` interface.
* **Privileged API** on a **public network**: unsupported! Use access controls
on your ``public`` endpoint instead.
* **Privileged API** on an **internal network**: ``admin`` interface, but use
access controls on your ``public`` endpoint instead. The notion of a
"privileged API" endpoint makes security-conscious developers instantly lazy
(security becomes someone else's problem), and is an obvious attack vector if
someone were to infiltrate your internal network. It also adds more
complexity to your API architecture which makes documentation, testing, and
API evolution that much more difficult.
* ``url`` (string): fully qualified URL of the service endpoint
This should be unversioned base URL for an API. Good examples include
``https://identity.example.com:5000/`` and ``https://keystone.example.com/``.
Conversely, ``https://identity.example.com:5000/v3/`` is an unfortunate example
because it directs all clients to connect to a versioned endpoint, regardless
of which API versions they understand. This makes it hard for services to do
any sort of API versioning, and for clients to dynamically discover additional
available versions.
For a period of time, keystone was stuck in a position where it implements a
``/v3/`` API, but for backwards compatibility with existing v2 clients, was
forced to continue advertising the ``/v2.0/`` endpoint in the service catalog
until it was reasonable to assume that all clients in the ecosystem are capable
of handling an unversioned URL. As a side effect, this has had a tremendous
impact on the awareness of, and thus adoption of, Keystone's Identity API v3
(which has been enabled by default — and stable — since the 2013.1 Grizzly
release). Don't put your project in that position!
Similarly, ``https://object-store.example.com/v1/KEY_\$(project_id)s`` (which
would ultimately be rendered to clients as a project-specific URL, such as
``https://object-store.example.com/v1/KEY_d12af07f4e2c4390a21acc31517ebec9``)
is an unfortunate example because not only does it hardcode an API version as
in the above example, but it also exposes the client's project ID directly to
the client. Instead, the operational scope or a request can be determined by
inspecting the user's token or consuming values populated by
``keystonemiddleware.auth_token``. It's also far less cacheable than a URL that
is neither project nor user specific, which is important given that every
client needs access to consume the service catalog prior to nearly every API
request.