Add mechanism to avoid overloading database server when it's restarting.

git-svn-id: svn+ssh://svn.gna.org/svn/gnustep/libs/sqlclient/trunk@27024 72102866-910b-0410-8b05-ffd578937521

ChangeLog

@@ -1,3 +1,10 @@
+2008-11-12 Richard Frith-Macdonald  <rfm@gnu.org>
+
+	* SQLClient.h:
+	* SQLClient.m:
+	Add support for tracking the number of consecutive connection failures
+	and imposing a delay between connection attempts.
+
 2008-07-19  Nicola Pero <nicola.pero@meta-innovation.com>
 
 	* configure.ac: Documented the --with-additional-include=,

SQLClient.h

@@ -374,12 +374,15 @@ extern unsigned	SQLClientTimeTick();
    * Timestamp of last operation.<br />
    * Maintained by -simpleExecute: -simpleQuery:recordType:listType:
    * and -cache:simpleQuery:recordType:listType:
+   * Also set for a failed connection attempt, but not reported by the
+   * -lastOperation method in that case.
    */
   NSTimeInterval	_lastOperation;	
   NSTimeInterval	_duration;
   unsigned int		_debugging;	/** The current debugging level */
   GSCache		*_cache;	/** The cache for query results */
   NSThread		*_cacheThread;	/** Thread for cache queries */
+  unsigned		_connectFails;	/** The count of connection failures */
 }
 
 /**
@@ -529,7 +532,14 @@ extern unsigned	SQLClientTimeTick();
  * If the <em>connected</em> instance variable is NO, this method
  * calls -backendConnect to ensure that there is a connection to the
  * database server established. Returns the result.<br />
- * Performs any necessary locking for thread safety.
+ * Performs any necessary locking for thread safety.<br />
+ * This method also counts the number of consecutive failed connection
+ * attempts.  A delay is enforced between each connection attempt, with
+ * the length of the delay growing with each failure.  This ensures
+ * that applications which fail to deal with connection failures, and
+ * just keep trying to reconnect, will not overload the system/server.<br />
+ * The maximum delay is 30 seconds, so when the database server is restarted,
+ * the application can reconnect reasonably quickly.
  */
 - (BOOL) connect;
 
@@ -712,6 +722,11 @@ extern unsigned	SQLClientTimeTick();
  */
 - (NSString*) quotef: (NSString*)fmt, ...;
 
+/**
+ * Convert a big (64 bit) integer to a string suitable for use in an SQL query.
+ */
+- (NSString*) quoteBigInteger: (int64_t)i;
+
 /**
  * Convert a 'C' string to a string suitable for use in an SQL query
  * by using -quoteString: to convert it to a literal string format.<br />

SQLClient.m

@@ -1141,13 +1141,33 @@ static unsigned int	maxConnections = 8;
       [lock lock];
       if (connected == NO)
 	{
+	  if (_connectFails > 1)
+	    {
+	      NSTimeInterval	delay;
+	      NSTimeInterval	elapsed;
+
+	      /* If we have repeated connection failures, we enforce a
+	       * delay fo up to 30 seconds between connection attempts
+	       * to avoid overloading the system with too frequent
+	       * connection attempts.
+	       */
+	      delay = (_connectFails < 30) ? _connectFails : 30;
+	      elapsed = GSTickerTimeNow() - _lastOperation;
+	      if (elapsed < delay)
+		{
+		  [NSThread sleepForTimeInterval: delay - elapsed];
+		}
+	    }
 	  NS_DURING
 	    {
 	      [self backendConnect];
+	      _connectFails = 0;
 	    }
 	  NS_HANDLER
 	    {
 	      [lock unlock];
+	      _lastOperation = GSTickerTimeNow();
+	      _connectFails++;
 	      [localException raise];
 	    }
 	  NS_ENDHANDLER
@@ -1347,7 +1367,7 @@ static unsigned int	maxConnections = 8;
 
 - (NSDate*) lastOperation
 {
-  if (_lastOperation > 0.0)
+  if (_lastOperation > 0.0 && _connectFails == 0)
     {
       return [NSDate dateWithTimeIntervalSinceReferenceDate: _lastOperation];
     }
@@ -1494,6 +1514,11 @@ static unsigned int	maxConnections = 8;
   return quoted;
 }
 
+- (NSString*) quoteBigInteger: (int64_t)i
+{
+  return [NSString stringWithFormat: @"%lld", i];
+}
+
 - (NSString*) quoteCString: (const char *)s
 {
   NSString	*str;