The Technology and the Functioning of SQL Server Change Data Capture


In this post, you will get a detailed overview of Microsoft SQL Server CDC (Change Data Capture), the technology behind this feature, and its functioning.    

The Change Data Feature has a lot of importance in the modern business environment where most enterprises are data-driven and have to focus greatly on data safety and security to prevent data breaches and hacking. However, apart from this aspect CDC plays a great role in ensuring that changed data and their values are stored in a manner that preempts their history from being compromised in any way. A lot of solutions had been tried in the past to save changed data securely such as complex queries, triggers, timestamps, and even data auditing but none guaranteed data safety until the launch of the SQL Server CDC.

Introduction of SQL Server CDC

In 2005, Microsoft launched the SQL Server CDC product which had several cutting-edge and technologically advanced features. These included “after update”, “after insert”, and “after delete” abilities. This technology though was not very well received by DBAs who found it invasive and very complex. This forced Microsoft to release a revised and more advanced version of the SQL Server CDC in 2008 which became very popular as it enabled developers and DBAs to capture and document historical data with a minimum of human intervention. 

The Concept of SQL Server CDC

The SQL Server CDC product is fine-tuned for making and recording changes made in the source database such as “Insert”, “Update”, or “Delete”. These changes can be accessed by any user seamlessly with all details available in an easy-to-understand relational format. All information required for capturing the changes to an intended target like column information and metadata for modified and changed rows can be had in CDC. The changes made are stored in tables and these are reflected in the structure of the tracked source tables. Importantly, table-valued functions strictly control access to the changed data thereby ensuring its safety. 

The ETL (Extract, Transform, Load) application is one of the best examples of a consumer who has benefitted from this SQL Server CDC technology. In this instance, all data that has changed in the source tables of SQL is moved incrementally to a data warehouse or data mart by an ETL application. 

To understand why this SQL Server CDC technology is ahead of the others in this field it is necessary to know how traditional systems have worked. In the past, the source tables in a data warehouse reflected changes made to them but only after they were refreshed continually. This process was not only tedious but very time-consuming too. On the other hand, the SQL Server CDC technology is structured in a manner that allows a smooth flow of the changed data, thereby helping you to apply it to various target platforms. This is why SQL Server CDC is considered to be a cut above the others. 

The Operational Aspects of SQL Server CDC

All changes that are made in tables by users are tracked and monitored by Change Data Capture. These changes can be accessed quickly and seamlessly retrieved with T-SQL from the relational tables where they are stored. A mirror image is created of the tracked table in all instances where CDC has been applied to a database table. The changes that are made in the database rows are recognized by the columns of metadata that are present in the structure of the columns of the replicated tables. 

The advantage of SQL Server CDC is that all aspects of the source and the target tables are the same. Once a SQL Server CDC task has been completed all the work that has taken place and the logged tables can be tracked and monitored through the new audit tables.  The source of the changes in CDC is reflected in the transaction logs of the SQL Server.  

The changes made in the SQL Server become an integral part of the CDC after the details of these entries are added to the log as soon as changes like insert, update, or delete are seen in the tracked tables at the source. The log that includes a comprehensive description of the changes is then read once it is added to the change table section of the original table. 

Forms of SQL Server CDC

There are two types of CDC.

Log-based CDC                   

In this form of SQL Server CDC, the system analyzes the log and the file of a database to verify the changes made at the source before they are replicated to the target database. The main benefit here is that this form of CDC is very accurate without any possibility of errors. Moreover, since the schemas of the production tables need not be changed or new tables added, the effect of CDC is negligible on the production database system. The only drawback of this method is that it works only where the databases have the support of the log-based CDC. 

Trigger-based CDC

In this form of SQL Server CDC, triggers placed in the database respond instantly and automatically when any change or event occurs. This significantly reduces the costs of data extraction. However, the cost of operating the source systems increases substantially as the database has to be continually refreshed leading to more runtime.    

Trigger-based CDC is easy to implement and all details of the logs of transactions can be found in the shadow tables. Changes also take place faster in this method. On the flip side, the performance of the databases might take a hit as many writes to a database are required whenever changes are made to the rows.   

Summing up, SQL Server CDC is a big boon today for data-driven businesses. 

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