Add failover host

README.rst

@@ -199,14 +199,25 @@ Dictionary. Current available keys are:
 
 -  connection_retry_backoff_time
 
-   Integer. Sets the back off time in seconds for reries of
+   Integer. Sets the back off time in seconds for retries of
    the database connection process. Default value is ``5``.
 
 -  query_timeout
 
    Integer. Sets the timeout in seconds for the database query.
    Default value is ``0`` which disables the timeout.
 
+-  failover_partner
+
+   String. Same as HOST but for failover partner.
+   Default is not specified which disable the failover partner.
+
+-  connection_retry_failover_backoff_time
+
+   Integer.  Sets the back off time in seconds before trying
+   to connect to failover partner.
+   Default value is ``0`` which disables the timeout.
+
 backend-specific settings
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 

sql_server/pyodbc/base.py

@@ -170,6 +170,12 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         '49919',
         '49920',
     )
+    _unrecoverable_error_numbers = (
+        '18486',  # account is locked
+        '18487',  # password expired
+        '18488',  # password should be changed
+        '18452',  # login from untrusted domain
+    )
 
     def __init__(self, *args, **kwargs):
         super(DatabaseWrapper, self).__init__(*args, **kwargs)
@@ -230,15 +236,14 @@ def get_connection_params(self):
             conn_params['NAME'] = 'master'
         return conn_params
 
-    def get_new_connection(self, conn_params):
+    def _get_connection_strings(self, conn_params, options):
         database = conn_params['NAME']
         host = conn_params.get('HOST', 'localhost')
         user = conn_params.get('USER', None)
         password = conn_params.get('PASSWORD', None)
         port = conn_params.get('PORT', None)
 
         default_driver = 'SQL Server' if os.name == 'nt' else 'FreeTDS'
-        options = conn_params.get('OPTIONS', {})
         driver = options.get('driver', default_driver)
         dsn = options.get('dsn', None)
 
@@ -298,24 +303,56 @@ def get_new_connection(self, conn_params):
         if options.get('extra_params', None):
             connstr += ';' + options['extra_params']
 
+        failover_host = options.get('failover_partner', None)
+        failover_connstr = None
+        if failover_host:
+            failover_connstr = connstr.replace(host, failover_host, 1)
+
+        return connstr, failover_connstr
+
+    def get_new_connection(self, conn_params):
+        options = conn_params.get('OPTIONS', {})
+
+        connstr, failover_connstr = self._get_connection_strings(conn_params, options)
+
         unicode_results = options.get('unicode_results', False)
         timeout = options.get('connection_timeout', 0)
         retries = options.get('connection_retries', 5)
         backoff_time = options.get('connection_retry_backoff_time', 5)
+        failover_backoff_time = options.get('connection_retry_failover_backoff_time', 0)
         query_timeout = options.get('query_timeout', 0)
 
         conn = None
         retry_count = 0
         need_to_retry = False
+        failover = False
+        error_numbers, failover_error_numbers = '', ''
         while conn is None:
             try:
                 conn = Database.connect(connstr,
                                         unicode_results=unicode_results,
                                         timeout=timeout)
             except Exception as e:
+                current_error_numbers = e.args[1]
+                for error_number in self._unrecoverable_error_numbers:  # never retry upon receiving unrecoverable code
+                    if error_number in current_error_numbers:
+                        raise
+
+                if not failover:
+                    error_numbers = current_error_numbers
+                else:
+                    failover_error_numbers = current_error_numbers
+
+                if failover_connstr:  # retry with failover if available
+                    connstr, failover_connstr = failover_connstr, connstr
+                    failover = not failover
+                    if failover:
+                        time.sleep(failover_backoff_time)
+                        continue
+
                 for error_number in self._transient_error_numbers:
-                    if error_number in e.args[1]:
-                        if error_number in e.args[1] and retry_count < retries:
+                    if error_number in error_numbers or error_number in failover_error_numbers:
+                        if retry_count < retries:
                             time.sleep(backoff_time)
                             need_to_retry = True
                             retry_count = retry_count + 1