This post will take a comprehensive overview of the various aspects of the Microsoft SQL Server CDC feature, the technology that drives it, its functioning, and finally, the types of SQL Server CDC.
However, we will first understand what the Change Data Capture (CDC) as a standalone entity is all about.
What is Change Data Capture (CDC)
The Change Data Capture (CDC) feature is especially critical in the modern data-driven business environment. Hence, the focus of organizations around the world is having fool-proof data security and safety norms in place that guarantee data durability and integrity. This is where the Change Data Capture feature comes into the picture.
CDC makes sure that any changes made to a database are stored and handled in a way that does not compromise its structure, history, and values. It ensures that stringent data security norms are followed without any deviation. At various stages, solutions had cropped up to follow these norms. These included intricate queries, data auditing, timestamps, and triggers placed in databases that warned of any changes made to data.
None of these solutions met with any success until Microsoft launched their SQL Server CDC and got great success.
The Development of Microsoft SQL Server CDC
SQL Server CDC was first launched by Microsoft in 2005 with “after update” “after insert” and “after delete” capabilities. It was then at the test stage and did not find much favor with DBAs, who found it rather unwieldy and intrusive to work with.
Depending on this feedback, Microsoft made some critical changes to the form of CDC, and a new and better version of the SQL Server CDC was introduced in 2008. It was very user-friendly and allowed DBAs to capture and store changes made to the source database directly without the need for any configuration or set up processes. This version was very well-received and is still in use today.
The Technology That Drives SQL Server CDC Feature
The focus of the SQL Server CDC feature is to present users with changed data like Delete, Update, and Insert, in a simple relational format. The inputs that are needed to capture these changes made in the source database to the target, such as metadata and column information, are present in the changed and modified rows.
After the changes are registered and recorded in the tables at source, they are replicated in the column information of the tables in the target location. However, this is not an open-end data repository and access to the changes made in the target tables is strictly controlled by table-valued firewalls.
If we compare SQL CDC with others in this niche, we will see that it is way ahead of the competition because of its advanced and cutting-edge technology. In other modes, users have to refresh the source tables at fixed intervals to replicate the changes made to the target database. This becomes a very time-consuming activity, with the process being a long-drawn-out one.
On the other hand, SQL Server CDC provides a continuous flow of changed data that can be inputted to any table or application immediately when the need arises. A very relevant example of the technology behind the SQL Server CDC is the ETL (Extract, Load, Transform) application. In this instance, the changed data in the SQL Server source tables is moved to a data warehouse or other data storage repository by the ETL application.
How Does The SQL Server CDC Feature Work
The main purpose of the Change Data Capture software pattern is to track and monitor all changes made to tables that are stored in relational tables to be accessed and retrieved later with T-SQL. Whenever the CDC technology is applied to a database table, it creates a replicated image of the tracked table.
Also, the structure of the changes made in the database row is checked by additional columns of metadata that are present in the format of the replicated tables. This is the only deviation in features of the source and the replicated tables, with all other characteristics of the two being similar. DBAs working on the SQL Server have access to the current audit tables after taking up SQL Server CDC activity for tracking logged tables.
The source of the changes made in CDC is shown in the transaction log of the SQL Server CDC. All changes are recorded in the log whenever a modification takes place in the tracked source tables. This log, together with the details of the changes made, is linked to the change table portion of the source table.
Forms of SQL Server CDC
There are two forms of SQL Server CDC – log-based and trigger-based – and it is advisable to go through the first before starting with the second.
Log-based CDC
In this form of SQL Server CDC, changes made to a database are present in the transaction log and file, which are then replicated to the target database. It is a very reliable method, and no changes are left unaccounted for when being replicated to the target database. New tables need not be added, and there is no requirement to change the schemas of the production database.
Trigger-based CDC
In this form of SQL Server CDC, triggers are set off automatically whenever any changes take place in the source database, thereby significantly lowering the data extraction costs. However, there is an increase in operating costs of the database as the source system has to be refreshed every time a change occurs.
There are several benefits of the trigger-based SQL Server CDC. Among them are detailed logs of transactions provided by shadow tables, seamless and faster implementation, and direct support from SQL API for specific types of databases.