After data is extracted, cleaned, and shaped into the desired format during processing, the next step involves moving that processed data into its final destination. A straightforward method for this is the Full Load strategy.
Imagine you have a whiteboard (your target table) and a set of notes (your transformed data). A full load is like erasing the entire whiteboard completely and then writing all your current notes onto it. Every time you run the process, you wipe the slate clean and rewrite everything based on the latest transformed data.
How Full Load Works
In technical terms, a full load (Lfull) typically involves these steps:
- Preparation: The ETL process completes the extraction and transformation steps, resulting in a dataset ready for loading.
- Target Preparation (Optional but Common): The existing data in the target table is often removed entirely. This is frequently done using a
TRUNCATE TABLE command in SQL databases, which is usually faster than deleting rows one by one (DELETE FROM table). Sometimes, the table might even be dropped and recreated, especially if schema changes are involved.
- Data Loading: The entire transformed dataset is inserted into the now-empty target table.
This process ensures that after the load operation finishes, the target table contains an exact copy of the dataset produced by the transformation stage for that specific run.
A diagram illustrating the full load process. Data is extracted and transformed. The target table is emptied before the transformed data is loaded, completely replacing the previous contents.
When to Use a Full Load
While simple, a full load isn't always the best choice, especially for very large datasets. However, it's often suitable in several scenarios:
- Initial Data Load: When populating a data warehouse or target table for the very first time, a full load is the natural approach.
- Small Datasets: If the volume of data is relatively small, erasing and reloading everything might be faster and easier than figuring out what changed. "Small" is relative and depends on your system's capabilities and the acceptable loading time window.
- Non-Critical History: If the target system doesn't need to maintain historical state (e.g., it only needs the current status), a full load simplifies things considerably.
- Lack of Change Tracking: When the source system doesn't provide reliable ways to identify what data has changed since the last extraction (like timestamps or version numbers), a full load might be the only practical way to ensure the target reflects the current source state.
- Development and Testing: Full loads are often used in development or testing environments because they are easy to reset and ensure a known state.
- Dimension Tables: Sometimes, smaller dimension tables in a data warehouse (tables that describe business entities like products, customers, or time) are fully reloaded because they don't grow excessively large and simplicity is preferred.
Advantages of Full Load
- Simplicity: The logic is straightforward. You don't need complex mechanisms to track changes, handle updates, or manage deletions. Erase and replace.
- Guaranteed Consistency (Post-Load): After a successful full load, the target table perfectly mirrors the source data (as processed by the transformation stage) at that point in time.
- Easier Error Recovery: If a full load fails midway, you can often just truncate the target again and restart the load process without worrying about partial updates.
Disadvantages of Full Load
- Inefficiency for Large Data: Reloading millions or billions of rows every time can consume significant time, network bandwidth, and computational resources. Load times can become prohibitively long.
- Loss of History: Because you erase the target data, you lose any historical record within that specific table unless you design your system to archive data beforehand.
- Potential Downtime/Locking: Truncating and loading large amounts of data can lock the target table, making it unavailable for reading or writing by other applications during the load process.
- Resource Intensive: Both the source and target systems experience heavy load during the full load operation.
The full load strategy provides a fundamental way to populate target systems. Its simplicity makes it attractive, especially for smaller datasets or initial loads. However, its limitations with large data volumes often lead data engineers to consider alternative approaches, such as incremental loading, which we'll discuss next.
