== Evaluation API

When writing rule-based policies such as when you are using Javascript or JBoss Drools, Keycloak provides an *Evaluation API* from where you
can obtain useful information.

This API consists of a few interfaces that provides you access to information such as:

* Information about the identity asking for a permission. Here you can obtain the identity identifier (eg.: username) or any other claim/attribute about it.
* Information about the runtime environment and any other attribute associated with the execution context.

The main interface is *org.keycloak.authorization.policy.evaluation.Evaluation*, which defines the following contract:

```java
public interface Evaluation {

    /**
     * Returns the {@link ResourcePermission} to be evaluated.
     *
     * @return the permission to be evaluated
     */
    ResourcePermission getPermission();

    /**
     * Returns the {@link EvaluationContext}. Which provides access to the whole evaluation runtime context.
     *
     * @return the evaluation context
     */
    EvaluationContext getContext();

    /**
     * Grants the requested permission to the caller.
     */
    void grant();

    /**
     * Denies the requested permission.
     */
    void deny();
}
```

For full instructions on using the Evaluation API refer to JavaDocs.

=== The Evaluation Context

The evaluation context provides useful information to policies during their evaluation.

```java
public interface EvaluationContext {

    /**
     * Returns the {@link Identity} that represents an entity (person or non-person) to which the permissions must be granted, or not.
     *
     * @return the identity to which the permissions must be granted, or not
     */
    Identity getIdentity();

    /**
     * Returns all attributes within the current execution and runtime environment.
     *
     * @return the attributes within the current execution and runtime environment
     */
    Attributes getAttributes();
}
```

From this interface, policies can obtain:

* The authenticated *Identity*
* Information about the execution context and runtime environment

Before evaluating policyes, {{book.project.name}} builds an *EvaluationContext* based on:

* All claims obtained from the OAuth2 Access Token that was sent along with the authorization request
+
In this case, the *Identity* object provides the same claims as define by the access token. For instance, if you are using a _Protocol Mapper_ to include custom claim
to a oAuth2 Access Token you can also access this claim from a policy and use it to build your conditions
+
* Some built-in environment attributes such as:
+
** kc.authz.context.time.date_time, holding the current time
** kc.authz.context.client.network.ip_address, holding the IP address of the client requesting permissions
** kc.authz.context.client.network.host, holding the host name of the client requesting permissions