+# Valid database indexes
+# Operation-based indexes
+define( 'DB_SLAVE', -1 ); # Read from the slave (or only server)
+define( 'DB_MASTER', -2 ); # Write to master (or only server)
+define( 'DB_LAST', -3 ); # Whatever database was used last
+
+# Obsolete aliases
+define( 'DB_READ', -1 );
+define( 'DB_WRITE', -2 );
+
+