For Write requests, Unity does not wait for all responses from each Teradata Database system. Unity checks the first response for a successful operation or error code such as communication failures, memory errors, and disk space and database errors. If the first response is successful, notification is sent to the client application. If the first response is an error, all Active Teradata Database systems must return a response before the client application is notified.
The Active Sequencer determines which response is the most common successful response, and sends that response back to the client application. If no successful response is available, the most common error response is sent to Unity. If responses are at the same time, the first response that Unity receives returns to the client application. If the error code specifies the action, Exit, Unity returns the error and exits the session on all Teradata systems. If the response indicates a successful operation, Unity compares the results returned from all Teradata systems to determine consistency across the systems. All responses returned by the Teradata Database systems must match to verify data consistency.
Unity uses the activity count (number of rows) to indicate data consistency for all INSERT, DELETE, UPDATE, and MERGE statements. The activity count from all Teradata Database systems must match the activity count of the response sent back to the client. The activity count is not checked for any other types of Writes. For these type of statements, Unity only checks for statement success or failure. If the activity count or success/ failure status of the Write operation does not match, Unity automatically changes the table state to Interrupted or Unrecoverable depending on the type of statement.