SSIS Interview Questions on Transactions, Event Handling and Validation
When preparing for your next SSIS interview be sure to understand what could be questions asked in interview. In this tip series, I will try to cover as much as I can to help you prepare better for SSIS interview. This tip covers transactions, event handling and validations SQL Server Integration Services interview questions.
What is the transaction support feature in SSIS?
- When you execute a package, every task of the package executes in its own transaction. What if you want to execute two or more tasks in a single transaction? This is where the transaction support feature helps. You can group all your logically related tasks in single group. Next you can set the transaction property appropriately to enable a transaction so that all the tasks of the package run in a single transaction. This way you can ensure either all of the tasks complete successfully or if any of them fails, the transaction gets roll-backed too.
What properties do you need to configure in order to use the transaction feature in SSIS?
- Suppose you want to execute 5 tasks in a single transaction, in this case you can place all 5 tasks in a Sequence Container and set the TransactionOption and IsolationLevel properties appropriately.
- The TransactionOption property expects one of these three values:
- Supported - The container/task does not create a separate transaction, but if the parent object has already initiated a transaction then participate in it
- Required - The container/task creates a new transaction irrespective of any transaction initiated by the parent object
- NotSupported - The container/task neither creates a transaction nor participates in any transaction initiated by the parent object
- The TransactionOption property expects one of these three values:
- Isolation level dictates how two more transaction maintains consistency and concurrency when they are running in parallel. To learn more about Transaction and Isolation Level, refer to this tip.
When I enabled transactions in an SSIS package, it failed with this exception: "The Transaction Manager is not available. The DTC transaction failed to start." What caused this exception and how can it be fixed?
- SSIS uses the MS DTC (Microsoft Distributed Transaction Coordinator) Windows Service for transaction support. As such, you need to ensure this service is running on the machine where you are actually executing the SSIS packages or the package execution will fail with the exception message as indicated in this question.
What is event handling in SSIS?
- Like many other programming languages, SSIS and its components raise different events during the execution of the code. You can write an even handler to capture the event and handle it in a few different ways. For example consider you have a data flow task and before execution of this data flow task you want to make some environmental changes such as creating a table to write data into, deleting/truncating a table you want to write, etc. Along the same lines, after execution of the data flow task you want to cleanup some staging tables. In this circumstance you can write an event handler for the OnPreExcute event of the data flow task which gets executed before the actual execution of the data flow. Similar to that you can also write an event handler for OnPostExecute event of the data flow task which gets executed after the execution of the actual data flow task. Please note, not all the tasks raise the same events as others. There might be some specific events related to a specific task that you can use with one object and not with others.
How do you write an event handler?
- First, open your SSIS package in Business Intelligence Development Studio (BIDS) and click on the Event Handlers tab. Next, select the executable/task from the left side combo-box and then select the event you want to write the handler in the right side combo box. Finally, click on the hyperlink to create the event handler. So far you have only created the event handler, you have not specified any sort of action. For that simply drag the required task from the toolbox on the event handler designer surface and configure it appropriately. To learn more about event handling, click here.
What is the DisableEventHandlers property used for?
- Consider you have a task or package with several event handlers, but for some reason you do not want event handlers to be called. One simple solution is to delete all of the event handlers, but that would not be viable if you want to use them in the future. This is where you can use the DisableEventHandlers property. You can set this property to TRUE and all event handlers will be disabled. Please note with this property you simply disable the event handlers and you are not actually removing them. This means you can set this value to FALSE and the event handlers will once again be executed.
What is SSIS validation?
- SSIS validates the package and all of it's tasks to ensure it has been configured correctly. With a given set of configurations and values, all the tasks and package will execute successfully. In other words, during the validation process, SSIS checks if the source and destination locations are accessible and the meta data about the source and destination tables are stored with the package are correct, so that the task will not fail if executed. The validation process reports warnings and errors depending on the validation failure detected. For example, if the source/destination tables/columns get changed/dropped it will show as error. Whereas if you are accessing more columns than used to write to the destination object this will be flagged as a warning. To learn about validation click here.
Define design time validation versus run time validation.
- Design time validation is performed when you are opening your package in BIDS whereas run time validation is performed when you are actually executing the package.
Define early validation (package level validation) versus late validation (component level validation).
- When a package is executed, the package goes through the validation process. All of the components/tasks of package are validated before actually starting the package execution. This is called early validation or package level validation. During execution of a package, SSIS validates the component/task again before executing that particular component/task. This is called late validation or component level validation.
What is DelayValidation and what is the significance?
- As I said before, during early validation all of the components of the package are validated along with the package itself. If any of the component/task fails to validate, SSIS will not start the package execution. In most cases this is fine, but what if the second task is dependent on the first task? For example, say you are creating a table in the first task and referring to the same table in the second task? When early validation starts, it will not be able to validate the second task as the dependent table has not been created yet. Keep in mind that early validation is performed before the package execution starts. So what should we do in this case? How can we ensure the package is executed successfully and the logically flow of the package is correct? This is where you can use the DelayValidation property. In the above scenario you should set the DelayValidation property of the second task to TRUE in which case early validation i.e. package level validation is skipped for that task and that task would only be validated during late validation i.e. component level validation. Please note using the DelayValidation property you can only skip early validation for that specific task, there is no way to skip late or component level validation.
- Review the following resources in preparation for your next SSIS interview:
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Article Last Updated: 2011-12-12