keystone/doc/source/devref/api_change_tutorial.rst

API Change Tutorial

This document will guide you through the process of proposing and submitting an API change to keystone.

Prerequisites

In order to follow this tutorial, it is assumed that you have read our development_best_practices and ../architecture documents.

Proposing a change

You need to create a blueprint, submit a specification against the keystone-specs repository and bring it up to discussion with the keystone core team for agreement.

Create

1. Create a blueprint in launchpad;
2. git clone https://git.openstack.org/openstack/keystone-specs;
3. cp specs/template.rst specs/backlog/<feature>.rst;
4. Write the spec based on the template. Ensure the BP link points to the one created in step 1;
5. Also update the documentation at api/v3/identity-api-v3.rst to reflect the proposed API changes;
6. Push to gerrit for review;
7. Propose agenda items to the keystone meeting, and make sure someone who understands the subject can attend the meeting to answer questions.

Agreement

The keystone core team will evaluate the specification and vote on accepting it or not. If accepted, the proposal will be targeted to a release; otherwise, the specification will be abandoned.

As soon as there is an agreement on the specification, the change may start rolling out.

Implementing a change

In this section, let's assume that a specification proposing the addition of a description field to the roles API was accepted. In the next subsections, you will find a detailed explanation on the needed code changes to the keystone code to implement such change.

Architectural Recapitulation

As you saw in the ../architecture document, there are four logical levels of code at which a successful request calls: router, controller, manager and driver.

For the role backend, they can be found in the directory keystone/assignment, in the following paths, respectively: routers.py, controllers.py, core.py and role_backends/sql.py (currently only the SQL driver is supported).

Changing the SQL Model and Driver

First, you need to change the role model to include the description attribute. Go to keystone/assignment/role_backends/sql.py and update it like:

class RoleTable(sql.ModelBase, sql.DictBase):

    attributes = ['id', 'name', 'domain_id', 'description']
    description = sql.Column(sql.String(255), nullable=True)
    ...

Now, when keystone runs, the table will be created with the new attribute. However, what about existing deployments which already have the role table created? You need to migrate their database schema!

The directory keystone/common/sql/migrate_repo/versions owns all the migrations since keystone day 1. Create a new file there with the next migration number. For example, if the latest migration number there is 101, create yours as 102_add_role_description.py, which will look like:

def upgrade(migrate_engine):
    meta = sql.MetaData()
    meta.bind = migrate_engine

    role_table = sql.Table('role', meta, autoload=True)
    description = sql.Column('description', sql.String(255),
                             nullable=True)
    role_table.create_column(description)

Do not forget to add tests for your migration at keystone/tests/unit/test_sql_upgrade.py, you may take other tests as example and learn how to develop yours. In this case, you would need to upgrade to 102 check the migration has added the description column to the role table.

Changing the role driver itself in keystone/assignment/role_backends/sql.py will not be necessary, because the driver handles the role entities as Python dictionaries, thus the new attribute will be handled automatically.

Changing the Manager

Managers handle the business logic. Keystone provides the basic CRUD for role entities, that means that the role manager simply calls the driver with the arguments received from the controller, and then returns the driver's result back to controller. Additionally, it handles the cache management.

Thus, there is no manager change needed to make it able to operate role entities with the new description attribute.

However, you should add tests for the role CRUD operations with the new attribute to keystone/tests/unit/assignment/test_core.py.

When trying to determine whether a change goes in the driver or in the manager, the test is whether the code is business logic and/or needs to be executed for each driver. Both common and business logics go in the manager, while backend specific logic goes in the drivers.

Changing the Controller and Router

Business logic should not go in the controller. The controller should be viewed as a binding between the business logic and the HTTP protocol. Thus, it is in charge of calling the appropriate manager call and wrapping responses into HTTP format.

Controllers use JSON schemas do determine whether a provided role is a valid representation or not. Role create and role update schemas are available at keystone/assignment/schema.py.

You will need to update their properties to include a description attribute:

_role_properties = {
    'name': parameter_types.name,
    'description': parameter_types.description
}

Besides doing the entity validation using such schemas, controllers pass and accept all the attributes to and from the manager. Thus, there is no further change needed at the controller level.

Furthermore, as role entities are passed in the request body to keystone calls, the role routes do not need to be changed; i.e the routes still are:

POST /v3/roles
GET /v3/roles/{id}
HEAD /v3/roles/{id}
PATCH /v3/roles/{id}
DELETE /v3/roles/{id}

Conclusion

At this point, keystone role entities contain a description attribute. In order to make it happen, you have learned how the keystone architecture is, what is the responsibility of each layer, how database migrations occur and the way entities are represented into tables.

The pattern of the change made in this tutorial applies to other keystone subsystems as well, such as resource and identity.