## Policies

updated: 2020-10-17 18:54

# Policies

A policy is a wrapper of a collection of rules that will be checked/enforced when Senlin performs some operations on the objects it manages. The design goals of policy support in Senlin are flexibility and customizability. We strive to make the policies flexible so that we can accommodate diverse types of policies for various usage scenarios. We also want to make policy type development an easier task for developers to introduce new policies and/or customize existing ones for their needs.

## Policy Properties

A policy object has the following properties:

- `id`: a string containing the globally unique ID for the object;

- `name`: a string containing the name of the policy object;

- `type`: a string containing the name of the policy type;

- `spec`: a map containing the validated specification for the object;

- `created_at`: timestamp of the object creation;

- `updated_at`: timestamp of last update to the object;

- `data`: a map containing some private data for the policy object;

## Creating a Policy

When the Senlin API receives a request to create a policy object, it first checks if the JSON body contains a map named `policy` that has the `name` and `spec` keys and values associated with them. If any of these keys are missing, the request will be treated as an invalid one and rejected.

After the preliminary request validation done at the Senlin API layer, Senlin engine will further check whether the specified policy type does exist and whether the specified `spec` can pass the validation logic in the policy type implementation. If this phase of validation is successful, a policy object will be created and saved into the database, then a map containing the details of the object will be returned to the requester. If any of these validations fail, an error message will be returned to the requester instead.

## Listing Policies

Policy objects can be listed using the Senlin API. When querying the policy objects, a user can specify the following query parameters, individually or combined:

- `filters`: a map containing key-value pairs that will be used for matching policy records. Records that fail to match this criteria will be filtered out. The following strings are valid keys:

  - `name`: name of policies to list, can be a string or a list of strings;

  - `type`: type name of policies, can be a string or a list of strings;

- `limit`: a number that restricts the maximum number of records to be returned from the query. It is useful for displaying the records in pages where the page size can be specified as the limit.

- `marker`: A string that represents the last seen UUID of policies in previous queries. This query will only return results appearing after the specified UUID. This is useful for displaying records in pages.

- `sort`: A string to enforce sorting of the results. It can accept a list of known property names as sorting keys separated by commas. For each sorting key, you can append either `:asc` or `:desc` as its sorting order. By default, `:asc` is assumed to be the sorting direction.

- `global_project`: A boolean indicating whether policy listing should be done in a tenant-safe way. When this value is specified as False (the default), only policies from the current project that match the other criteria will be returned. When this value is specified as True, policies that matching all other criteria would be returned, no matter in which project a policy was created. Only a user with admin privilege is permitted to do a global listing.

The Senlin API performs some basic checks on the data type and values of the provided parameters and then passes the request to Senlin engine. When there are policy objects matching the query criteria, a list of policy objects is returned to the requester. If there is no matching record, the result will be an empty list.

## Getting a Policy

A user can provide one of the UUID, the name or the short ID of policy object to the Senlin API `policy_show` to retrieve the details about a policy.

If a policy object matching the criteria is found, Senlin API returns the object details in a map; if more than one object is found, Senlin API returns an error message telling users that there are multiple choices; if no object is found matching the criteria, a different error message will be returned to the requester.

## Updating a Policy

After a policy is created, a user can send requests to the Senlin API for changing some of its properties. To avoid potential state conflicts inside the Senlin engine, we currently don’t allow changes to the `spec` property of a policy. However, changing the `name` property is permitted.

When validating the requester provided parameters, Senlin API will check if the values are of valid data types and whether the values fall in allowed ranges. After this validation, the request is forwarded to Senlin engine for processing.

Senlin engine will try to find the policy using the specified policy identity as the UUID, the name or a short ID of the policy object. When no matching object is found or more than one object is found, an error message is returned to the user. Otherwise, the engine updates the object property and returns the object details in a map.

## Deleting a Policy

A user can specify the UUID, the name or the short ID of a policy object when sending a `policy_delete` request to the Senlin API.

Senlin engine will try to find the matching policy object using the specified identity as the UUID, the name or a short ID of the policy object. When no matching object is found or more than one object is found, an error message is returned. Otherwise, the API returns a 204 status to the requester indicating that the deletion was successful.

To prevent deletion of policies that are still in use by any clusters, the Senlin engine will try to find if any bindings exist between the specified policy and a cluster. An error message will be returned to the requester if such a binding is found.

updated: 2020-10-17 18:54

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