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+/*******************************************************************************
+ * Copyright (c) 2007, 2010 Association for Decentralized Information Management
+ * in Industry THTH ry.
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *     VTT Technical Research Centre of Finland - initial API and implementation
+ *******************************************************************************/
+package org.simantics.db.request;
+
+import org.simantics.db.ReadGraph;
+import org.simantics.db.Session;
+import org.simantics.db.exception.CancelTransactionException;
+import org.simantics.db.exception.DatabaseException;
+import org.simantics.db.procedure.AsyncMultiProcedure;
+
+/**
+ * The <code>GraphRequest</code> interface is used to create transaction
+ * requests to Simantics database implementations. Both read and write
+ * transaction requests use the same interface.
+ * 
+ * <p>
+ * The actual work carried out by the implemented request should be done in the
+ * <code>perform</code> method. It receives a <code>Graph</code> instance as
+ * the only argument which is the interface for actually reading and writing the
+ * graph data model.
+ * 
+ * <p>
+ * Transaction requests can be made to the database by creating your own
+ * <code>GraphRequest</code> instance and putting it in the request queue of
+ * the database session through the {@link Session} interface. The database
+ * session is responsible for executing the queued requests in a thread of its
+ * choice, or possibly/preferably multiple threads. The database session can
+ * allow multiple read-only requests to occur simultaneously, but read-write
+ * requests require exclusive database access. In other words only one
+ * read-write request can be in execution simultaneously.
+ * 
+ * <p>
+ * This interface also has two callbacks - <code>handleException</code> for
+ * allowing handling any exceptions thrown by <code>perform</code> and
+ * <code>requestCompleted</code> for performing actions after a request has
+ * been successfully completed.
+ * 
+ * <p>
+ * Clients of this interface are encouraged to extend the provided abstract
+ * implementations or this class or extend their own helper implementations for
+ * ones particular needs. The provided abstract implementations are:
+ * <ul>
+ * <li>{@link ReadGraphRequestAdapter} provides default implementations for
+ * everything except {@link #perform(ReadGraph)}.</li>
+ * <li>{@link SimpleGraphRequest} replaces {@link #perform(ReadGraph)} with
+ * {@link SimpleGraphRequest#run(ReadGraph)} for easier request implementation in
+ * simple cases.</li>
+ * <li>{@link GraphRequestWithResult} makes it easier for the user to return
+ * a single result Object from the request.</li>
+ * </ul>
+ * 
+ * @author Tuukka Lehtonen
+ * @see ReadGraphRequestAdapter
+ * @see GraphRequestRunner
+ * @see GraphRequestStatus
+ * @see GraphRequestWithResult
+ * @see Session
+ * @see SimpleGraphRequest
+ */
+public interface MultiRead<Result> {
+
+    /**
+     * When a <code>GraphRequest</code> is serviced by the database session
+     * the method <code>perform</code> is invoked.
+     * 
+     * <p>
+     * Perform receives an object instance implementing the <code>SyncReadGraph</code>
+     * interface which provides the only way to read/write the graph data model.
+     * The Graph instance must only be valid during the execution of the
+     * <code>perform</code> method and therefore should not be stored for use
+     * outside of its execution.
+     * 
+     * <p>
+     * The general contract of the method <code>perform</code> is that it may
+     * take any action whatsoever which involves reading or writing the data
+     * model through the received Graph instance.
+     * 
+     * @param g an interface for reading and writing the data model
+     * @return the result status of the request which affects how the
+     *         transaction proceeds, see GraphRequestStatus for more information
+     * @throws Exception when anything goes wrong inside the request thread
+     * @throws CancelTransactionException to indicate that the request needs to
+     *         be cancelled and any changes rolled back
+     */
+    void perform(ReadGraph graph, AsyncMultiProcedure<Result> callback) throws DatabaseException;
+    
+ }