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Runtime APIs of Tools4j Config
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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.
 */
package org.deephacks.tools4j.config;

import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Id identify a configuration instance similar to how primary key are used in a database and are unique 
 * with respect to instance and schema.  
 * 
 * 
 * Every configurable class must annotate one (and only one) field with this annotation so 
 * that specific instances can be addressed.  
 * 
 * 
 * Instance ids are assigned to instances by an administrator and may not change after they have been created. 
 * Instance ids are also used by administrators for indicating relationships between instances. Applications use 
 * instance ids to read instances at runtime.  
 * 
 * 
 * Configurable classes can be singletons. This means only one instance and instantiation/removal are restricted.
 * Singleton instances does not have an id annotation but is instead identified through its schema name.  
 * Properties, though, are allowed to change. The instance is automatically created when the class is registered. 
 * If the id already exist, creation will be silently ignored (i.e. no attempt merge or update). Singeltons are 
 * read preferably using{@link RuntimeContext#singleton(Class)}
 * 
 * 
 * 
 * Id field must only have a single value of {@link java.lang.String} type.
 * Id field can be static final. This is a singleton.
 * Id field are not allowed to be non-final static.
 * Id field are not allowed to be non-static final.
 * 
 * 
 * @author Kristoffer Sjogren
 */
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.FIELD })
@Inherited
public @interface Id {
    /**
     * A name can have a real world meaning (like social security numbers, account numbers etc) 
     * or be entirely artificial. The name should reflect this intent.
     * 
     * Name must be unique for all configuration classes of same type.
     * 
     */
    String name() default "";

    /**
     * Description can be used to inform if identification is intended to use attributes 
     * that exist in the real world or relate to already eastablished system architecture 
     * aspects/concepts.
     * 
     * @return
     */
    String desc();
}