Per OAuth2 terminology, a Resource Server is the server hosting the protected resources and capable of accepting and responding to protected resource requests.
Resource servers usually rely on some kind of information to decide whether access to a protected resource should be granted or not. For RESTful-based resource servers,
that information is usually carried in a security token, typically sent as a bearer token along with every request to the server. Web applications that rely on a session to
authenticate their users usually store that information in the user's session and retrieve it from there on every request.
A resource is part of the assets of an application and the organization. It can be a set of one or more endpoints, a classic web resource such as an HTML page, and so on.
Every single resource has a unique identifier which may represent a single resource or a set of resources. For instance, you may want to manage a _Banking Account Resource_ that represents and defines a set of authorization policies for all banking accounts.
But you may also have a different resource named _Alice's Banking Account_, which represents a single resource owned by a single customer, which may have its own set of authorization policies.
A permission associates the object being protected with the policies that must be evaluated in order to decide whether or not access should be granted.
{{book.project.name}} provides a rich platform for building a range of permission strategies ranging from simple to very complex, rule-based dynamic permissions. It provides great flexibility and helps to:
A policy defines the conditions that must be satisfied to grant access to an object. Different than permissions, you don't specify the object being protected
but the conditions that must be satisfied to get access to a given object (e.g., resource, scope, or both).
Policies are strongly related to the different _access control mechanisms_ that you can use to actually protect your resources.
With policies, you can implement strategies for ABAC, RBAC, Context-based Access Control or any combination of these.
Keycloak leverages the concept of policies and how you define them by providing the concept of *Aggregated Policies*, where you can build a "policy of policies" and still control the behavior of the evaluation.
Instead of writing a single and huge policy with all conditions that must be satisfied to get access to a given resource, the policies implementation in {{book.project.name}} {{book.project.module}} follows the *divide-and-conquer* technique.
That is, you can create individual policies, reuse them on different permissions, and build more complex policies by combining individual policies.
A Permission Ticket is a special type of token defined by the https://docs.kantarainitiative.org/uma/rec-uma-core.html[OAuth2's User-Managed Access (UMA) Profile] specification that provides an opaque structure whose form is determined by the authorization server. This
structure represents the resources and/or scopes being requested by a client as well a the policies that must be applied to a request for authorization data (requesting party token or RPT).
In UMA, permission tickets are crucial to support *person-to-person sharing* and also *person-to-organization sharing*. Using permission tickets for *authorization workflows* enables a range of scenarios from simple to complex,
where resource owners and resource servers have complete control over their resources based on fine-grained policies that govern the access to these resources.
During the UMA flow, permission tickets are issued by the authorization server to a resource server, which in turn returns it back to the client trying to access a protected resource. Once the client
receives the ticket, it can make a request for a requesting party token (RPT) (a final token holding authorization data) by sending the ticket back to the authorization server.
For more details, see link:../service/authorization/authorization-api.html[Authorization API] and the https://docs.kantarainitiative.org/uma/rec-uma-core.html[UMA] specification.
