Understanding the Difference Between CDC and SCD
When working with data pipelines and warehouses, two important concepts often come up: Change Data Capture (CDC) and Slowly Changing Dimensions (SCD). While they both deal with handling data changes, they serve different purposes. In this article, we will explore their differences and how they complement each other.
What is Change Data Capture (CDC)?
Change Data Capture (CDC) is a technique for identifying and capturing changes in a source database, so they can be replicated elsewhere, such as a data warehouse. CDC is crucial for real-time and incremental data processing, reducing the need to reload entire datasets.
Key Features of CDC
- Tracks all changes (INSERT, UPDATE, DELETE) in the source system.
- Works across all types of tables (fact, dimension, etc.).
- Used for real-time or batch ETL processes.
- Commonly implemented using:
- Log-based CDC (reads from database logs).
- Trigger-based CDC (uses database triggers).
- Timestamp-based CDC (compares timestamps).
- Versioning-based CDC (stores historical records in the source table).
What is Slowly Changing Dimensions (SCD)?
Slowly Changing Dimensions (SCD) refers to techniques used in data warehouses to handle changes in dimension tables while preserving historical accuracy. Unlike CDC, which captures every change, SCD focuses on tracking changes in descriptive attributes that evolve over time.
Types of SCD
There are several types of SCD, but the most common ones are:
- SCD Type 1: Overwrites old data (no history maintained).
- SCD Type 2: Retains history by adding a new record for each change (e.g., a new row with an effective date or version number).
- SCD Type 3: Stores only a limited history (e.g., keeping the current and previous value in separate columns).
CDC vs. SCD: A Side-by-Side Comparison
| Feature | Change Data Capture (CDC) | Slowly Changing Dimensions (SCD) |
|---|---|---|
| Purpose | Captures all changes in a source system. | Manages historical changes in dimension tables. |
| Scope | Applies to all tables (fact and dimension). | Applies only to dimension tables in a warehouse. |
| Types of Changes | Detects INSERT, UPDATE, DELETE. | Tracks changes in descriptive attributes (e.g., customer address). |
| Data Retention | Used for real-time replication or incremental loads. | Can overwrite, store limited history, or keep full history. |
| Common Uses | Real-time ETL, replication, streaming, auditing. | Data warehouse history tracking, reporting, analytics. |
| Implementation Methods | Log-based, trigger-based, timestamp-based, versioning. | SCD Type 1 (overwrite), Type 2 (new row), Type 3 (limited history). |
| Example Scenario | Detecting changes in an employee’s shift records. | Keeping track of an employee’s job title changes over time. |
How CDC and SCD Work Together
Although CDC and SCD serve different functions, they often work together in data pipelines. For example:
- CDC detects changes in a transactional database (OLTP) and sends them to the data warehouse.
- The data warehouse applies SCD techniques to store and manage historical data in dimension tables.
Example Use Case: Cleaning Company Workforce Monitoring System
- CDC is used to track changes in an OLTP PostgreSQL database, capturing modifications in shift records, payroll, and employee details.
- SCD Type 2 is implemented in the warehouse to preserve the history of employee records, ensuring past job titles, pay grades, and locations remain accessible.
Conclusion
Both Change Data Capture (CDC) and Slowly Changing Dimensions (SCD) are essential for handling data changes but serve distinct purposes:
- CDC ensures that all changes are captured efficiently for real-time processing.
- SCD ensures that historical data is preserved for analytical and reporting needs.
By using CDC to extract changes from OLTP systems and SCD to structure historical data in a warehouse, organisations can build robust data architectures that support both real-time and analytical requirements.
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