/*
 * Copyright (C) 2010 Google, Inc.
 *
 * 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 com.google.inject.persist;

/**
 * This interface is used to gain manual control over the unit of work. This is mostly to do work in
 * non-request, non-transactional threads. Or where more fine-grained control over the unit of work
 * is required. Starting and ending a unit of work directly corresponds to opening and closing a
 * {@code Session}, {@code EntityManager} or {@code ObjectContainer} respectively.
 *
 * <p>The Unit of Work referred to by UnitOfWork will always be local to the calling thread. Be
 * careful to end() in a finally block. Neither JPA, nor Hibernate supports threadsafe sessions
 * (reasoning behind thread-locality of Unit of Work semantics).
 *
 * <ul>
 * <li>Using UnitOfWork with the PersistFilter inside a request is not recommended.
 * <li>Using UnitOfWork with session-per-txn strategy is not terribly clever either.
 * <li>Using UnitOfWork with session-per-request strategy but *outside* a request (i.e. in a
 *     background or bootstrap thread) is probably a good use case.
 * </ul>
 *
 * @author Dhanji R. Prasanna (dhanji@gmail com)
 */
public interface UnitOfWork {

  /**
   * Starts a Unit Of Work. Underneath, causes a session to the data layer to be opened. If there is
   * already one open, the invocation will do nothing. In this way, you can define arbitrary
   * units-of-work that nest within one another safely.
   *
   * <p>Transaction semantics are not affected.
   */
  void begin();

  /**
   * Declares an end to the current Unit of Work. Underneath, causes any open session to the data
   * layer to close. If there is no Unit of work open, then the call returns silently. You can
   * safely invoke end() repeatedly.
   *
   * <p>Transaction semantics are not affected.
   */
  void end();
}