src/main/java/com/google/gwtorm/server/Schema.java

// Copyright 2008 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.gwtorm.server;

/**
 * Application database definition and top-level schema access.
 *
 * <p>Applications should extend this interface and declare relation methods for each entity/table
 * used. Relation methods must be marked with the {@link Relation} annotation and return an
 * interface extending {@link Access}. At runtime the application extension of Schema will be
 * automatically implemented with a generated class, providing implementations of the Access
 * extensions from each of the declared relation methods.
 *
 * <p>Instances of a schema should be obtained through the {@link com.google.gwtorm.jdbc.Database}
 * class on a pure-JDBC implementation and through <code>GWT.create()</code> on the GWT client side.
 *
 * <p>In the JDBC implementation each Schema instance wraps around a single JDBC Connection object.
 * Therefore a Schema instance has a 1:1 relationship with an active database handle.
 *
 * <p>A Schema instance (as well as its returned Access instances) is not thread safe. Applications
 * must provide their own synchronization, or ensure that at most 1 thread access a Schema instance
 * (or any returned Access instance) at a time. The safest mapping is 1 schema instance per thread,
 * never shared.
 *
 * <p>For example the OurDb schema creates two tables (identical structure) named <code>someFoos
 * </code> and <code>otherFoos</code>:
 *
 * <pre>
 * public interface FooAccess extends Access&lt;Foo, Foo.Key&gt; {
 *   &#064;PrimaryKey(&quot;key&quot;)
 *   Foo byKey(Foo.Key k) throws OrmException;
 * }
 * public interface OurDb extends Schema {
 *   &#064;Relation
 *   FooAccess someFoos();
 *
 *   &#064;Relation
 *   FooAccess otherFoos();
 * }
 * </pre>
 */
public interface Schema extends AutoCloseable {
  /** Commit a pending transaction. */
  public void commit() throws OrmException;

  /** Abort a pending transaction, if one is already in progress, otherwise nop. */
  public void rollback() throws OrmException;

  /**
   * Add any missing columns, create any missing tables or sequences.
   *
   * <p>This method does not drop any unused columns or tables, leaving them intact for applications
   * to continue to query after the update. Any unused columns that are NOT NULL are altered to
   * accept NULL.
   *
   * @param e executor to perform (or log) the statements.
   * @throws OrmException one or more objects could not be added to the schema.
   */
  public void updateSchema(StatementExecutor e) throws OrmException;

  /**
   * Drop any unused columns, tables, or sequences.
   *
   * <p>This method destroys data, as columns may be removed entirely.
   *
   * @param e executor to perform (or log) the statements.
   * @throws OrmException one or more drops could not be completed.
   */
  public void pruneSchema(StatementExecutor e) throws OrmException;

  /** @return access interface for each declared relation. */
  public Access<?, ?>[] allRelations();

  /** Close the schema and release all resources. */
  @Override
  public void close();
}