/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.apache.hadoop.hbase.constraint;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.conf.Configurable;
import org.apache.hadoop.hbase.HTableDescriptor;
import org.apache.hadoop.hbase.client.Put;

/**
 * Apply a {@link Constraint} (in traditional database terminology) to a HTable.
 * Any number of {@link Constraint Constraints} can be added to the table, in
 * any order.
 * <p>
 * A {@link Constraint} must be added to a table before the table is loaded via
 * {@link Constraints#add(HTableDescriptor, Class...)} or
 * {@link Constraints#add(HTableDescriptor, org.apache.hadoop.hbase.util.Pair...)}
 * (if you want to add a configuration with the {@link Constraint}). Constraints
 * will be run in the order that they are added. Further, a Constraint will be
 * configured before it is run (on load).
 * <p>
 * See {@link Constraints#enableConstraint(HTableDescriptor, Class)} and
 * {@link Constraints#disableConstraint(HTableDescriptor, Class)} for
 * enabling/disabling of a given {@link Constraint} after it has been added.
 * <p>
 * If a {@link Put} is invalid, the Constraint should throw some sort of
 * {@link ConstraintException}, indicating that the {@link Put} has failed. When
 * this exception is thrown, not further retries of the {@link Put} are
 * attempted nor are any other {@link Constraint Constraints} attempted (the
 * {@link Put} is clearly not valid). Therefore, there are performance
 * implications in the order in which {@link BaseConstraint Constraints} are
 * specified.
 * <p>
 * If a {@link Constraint} fails to fail the {@link Put} via a
 * {@link ConstraintException}, but instead throws a {@link RuntimeException},
 * the entire constraint processing mechanism ({@link ConstraintProcessor}) will
 * be unloaded from the table. This ensures that the region server is still
 * functional, but no more {@link Put Puts} will be checked via
 * {@link Constraint Constraints}.
 * <p>
 * Further, {@link Constraint Constraints} should probably not be used to
 * enforce cross-table references as it will cause tremendous write slowdowns,
 * but it is possible.
 * <p>
 * NOTE: Implementing classes must have a nullary (no-args) constructor
 *
 * @see BaseConstraint
 * @see Constraints
 */
@InterfaceAudience.Private
public interface Constraint extends Configurable {

  /**
   * Check a {@link Put} to ensure it is valid for the table. If the {@link Put}
   * is valid, then just return from the method. Otherwise, throw an
   * {@link Exception} specifying what happened. This {@link Exception} is
   * propagated back to the client so you can see what caused the {@link Put} to
   * fail.
   * @param p {@link Put} to check
   * @throws ConstraintException when the {@link Put} does not match the
   *         constraint.
   */
  public void check(Put p) throws ConstraintException;

}