Modified the code to initiate the connection before the first action and

to ensure that we are connected before every database operation. This
allows our connection to transiently fail even after the first operation,
but then be properly established once the database is again available.


git-svn-id: https://samskivert.googlecode.com/svn/trunk@116 6335cc39-0255-0410-8fd6-9bcaacd3b74c

projects/samskivert/src/java/com/samskivert/jdbc/MySQLRepository.java

@@ -1,5 +1,5 @@
 //
-// $Id: MySQLRepository.java,v 1.3 2001/03/19 22:59:08 mdb Exp $
+// $Id: MySQLRepository.java,v 1.4 2001/05/26 03:22:24 mdb Exp $
 
 package com.samskivert.jdbc;
 
@@ -29,6 +29,9 @@ public abstract class MySQLRepository extends Repository
     protected int lastInsertedId ()
 	throws SQLException
     {
+        // make sure we've got a connection
+        ensureConnection();
+
 	// we have to do this by hand. alas all is not roses.
 	Statement stmt = _session.connection.createStatement();
 	ResultSet rs = stmt.executeQuery("select LAST_INSERT_ID()");

projects/samskivert/src/java/com/samskivert/jdbc/Repository.java

@@ -1,5 +1,5 @@
 //
-// $Id: Repository.java,v 1.4 2001/03/19 22:59:08 mdb Exp $
+// $Id: Repository.java,v 1.5 2001/05/26 03:22:25 mdb Exp $
 
 package com.samskivert.jdbc;
 
@@ -22,8 +22,8 @@ import com.samskivert.jdbc.jora.*;
 public abstract class Repository
 {
     /**
-     * Creates the repository and opens music database. A properties
-     * object should be supplied with the following fields:
+     * Creates and initializes the repository. A properties object should
+     * be supplied with the following fields:
      *
      * <pre>
      * driver=[jdbc driver class]
@@ -32,6 +32,15 @@ public abstract class Repository
      * password=[jdbc password]
      * </pre>
      *
+     * <p> The connection to the database will not be established until
+     * the first operation is executed. It is important that database
+     * operations be invoked via <code>execute()</code> rather than
+     * directly so that the repository can manage the database session
+     * properly (reestablishing the session on transient failures and so
+     * on). If a derived class wishes to perform operations directly, it
+     * should call <code>ensureConnection()</code> before performing any
+     * database activity.
+     *
      * @param props a properties object containing the configuration
      * parameters for the repository.
      */
@@ -48,19 +57,29 @@ public abstract class Repository
 	_password =
 	    requireProp(props, "password", "No driver password specified.");
 
-        // establish a connection to the database
-        establishConnection();
+        // we attempt to reestablish the connection if there's a transient
+        // failure (like the database is down or our old connection gets
+        // closed by the database), but we don't want to get too crazy
+        // about connecting to the database automatically, so we limit our
+        // attempts to twice every thirty seconds
+        _throttle = new Throttle(2, 30);
     }
 
     /** Establishes a connection to the database. */
-    protected void establishConnection ()
+    protected void ensureConnection ()
         throws SQLException
     {
-        // bail out if we've already got a connection
+        // nothing doing if we've already got a connection
         if (_session != null) {
             return;
         }
 
+        // make sure we don't try to reestablish the connection
+        // overzealously in times of lengthy database outage
+        if (_throttle.throttleOp()) {
+            throw new SQLException("Connection attempt throttled.");
+        }
+
         try {
             // create our JORA session
             _session = new Session(_dclass);
@@ -105,15 +124,15 @@ public abstract class Repository
         // first close the old connection
         shutdown();
         // then open a new one
-        establishConnection();
+        ensureConnection();
     }
 
     /**
      * After the database session is begun, this function will be called
      * to give the repository implementation the opportunity to create its
-     * table objects. As this is done during the initialization of the
-     * repository, the implementation has a chance to fail the entire
-     * repository creation process if the necessary tables do not exist.
+     * table objects. If the database session fails, a new session will be
+     * established and this function will be called again for the new
+     * session.
      */
     protected abstract void createTables () throws SQLException;
 
@@ -153,8 +172,9 @@ public abstract class Repository
     }
 
     /**
-     * Executes the supplied operation without attempting to reestablish
-     * the database connection in the event of a transient failure.
+     * Executes the supplied operation. In the event of a transient
+     * failure, the repository will attempt to reestablish the database
+     * connection and try the operation again.
      */
     protected void execute (Operation op)
 	throws SQLException
@@ -168,7 +188,7 @@ public abstract class Repository
      * which case a call to rollback() is executed on the session.
      *
      * @param retryOnTransientFailure if true and the operation fails due
-     * to an transient failure (like losing the connection to the database
+     * to a transient failure (like losing the connection to the database
      * or deadlock detection), the connection to the database will be
      * reestablished and the operation attempted once more.
      */
@@ -176,7 +196,7 @@ public abstract class Repository
 	throws SQLException
     {
         // make sure our connection is established
-        establishConnection();
+        ensureConnection();
 
 	try {
 	    // invoke the operation
@@ -260,4 +280,7 @@ public abstract class Repository
     protected String _url;
     protected String _username;
     protected String _password;
+
+    // we use this to prevent too many database connection attempts
+    protected Throttle _throttle;
 }