ETL Incremental Loading 101: A Comprehensive Guide

on Data Loading, ETL • April 5th, 2022 • Write for Hevo

etl incremental - Featured Image

The loading stage of the ETL(Extract, Transform & Load) process is an essential part of your Data Pipeline. While consolidating data from multiple sources and loading it to your desired destination such as a Data Warehouse, you need to take notice of the data volume, structure, target, and load type. On a broader level, you can use the ETL Full Loading or the ETL Incremental Loading method based on your specific use case.

ETL Incremental Loading is often advantageous when dealing with data sources of relatively larger sizes. Compared to Full loading, ETL Incremental Loading only uploads the data that is either newly added or changed instead of fully dumping the entire dataset.

In this article, you will learn about ETL Incremental Loading and how to effectively implement it. 

Table of Contents

What is ETL Incremental Loading?

ETL(Extract, Transform & Load) is a popular process for consolidating data from several sources into a central repository. Many organizations often use the ETL Incremental Loading for their load stage of the ETL depending on their use case. An Incremental Data Load can be referred to as a selective transfer of data from one system to another. This process attempts to search for any of the newly created or modified data compared to the last run made for the data transfer process. 

The ETL Incremental Loading is more efficient in contrast to the traditional full data load that completely copies the full dataset from a particular source. Reducing the overhead in the ETL process, the ETL Incremental Loading is often designed time-based i.e. when was the data created or modified. To correctly identify any change(new data, updates, or deleted data), ETL Incremental Loading compares the data present in the target system with the source. 

Why do you need ETL Incremental Loading?

The ETL Incremental Loading is often the choice for many data pipelines due to the following advantages:

  • Faster Processing: It usually runs much faster because there is lesser data to interact with. Considering there are no bottlenecks, the time it takes to transfer and transform data is directly related to the amount of data involved. In many cases, interacting with half the data will reduce the execution time by the same amount. 
  • Better Risk Handling: Lesser amount of data to touch reduces any potential surface risk associated with a particular load. There are times when a given loading process may fail or malfunction, thereby leaving the target data in an inconsistent state. The ETL Incremental Loading technique is a fractional loading method. It reduces the amount of data that you add or change and that may need to be rectified in the event of any irregularity. Because less data is reviewed, it also takes less time to validate the data and review changes.
  • Consistent Performance: With ETL Incremental Loading, you get a constant performance despite any fluctuating workloads. On a general note, today’s load always contains more data than yesterday. Therefore, running a full load can be a time-consuming method as the time required for processing increases monotonously. ETL Incremental Loading only transfers data in case of any data modifications, thereby increasing the likelihood of more consistent performance.
  • Recording Historical Data: You will notice that many of the source systems regularly delete the old data. This can be problematic as often you may require to report this data to your downstream systems. Using the ETL Incremental Loading process, you will need to load only the new and modified data. This allows you to keep all the source data(including the data deleted from the upstream sources) in your target destination system.

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Difference between ETL Full Loading and ETL Incremental Loading

In ETL Full Loading, the entire dataset is replaced with a newly updated dataset. Hence, it doesn’t require you to maintain any extra information such as timestamps. For instance, you can consider a supermarket that daily uploads the sales data to a Data Warehouse at the end of the day. So, if on Monday there were 10 sales, then the 10 sales records will be uploaded at the night. On Tuesday, 7 sales were made. Now, on Tuesday night, a complete data set of 10 Sales records from Monday and 7 sales from Tuesday will be uploaded to the Data Warehouse. Though setting up this system is fairly simple & easy to maintain, performance-wise this is an inefficient system.

In ETL Incremental Loading, you only need to upload the source data that is different(newly created or updated) from the target system. Based on the amount of data you are loading, Incremental loading can be classified into the following 2 categories:

  • Stream Incremental Load: For loading small data volumes.
  • Batch Incremental Load: For loading large data volumes.

Considering the same example discussed above, ETL Incremental Loading would only require you to load the changes on Tuesday. Instead of uploading data for both Monday & Tuesday, you only have to update the 7 newly created sales records. This is a popular method to save time and resources, although it adds complexity. Though Incremental loading is faster than the full load, it requires high maintainability. In comparison to the full load, the incremental load does not allow the entire load to be redone if an error occurs. In addition, the files need to be loaded in sequence, so if other data is pending, the error aggravates the problem. 

When should you use ETL Incremental Loading?

ETL Incremental Loading is a great alternative to ETL Full Loading in the following cases:

  • Dealing with a comparatively bigger data source
  • Facing slower performance issues while querying data due to data size and technical limitations.
  • Tracking the data changes is possible.
  • In several data sources, old data gets deleted. You might want to retain that deleted data in the target system such as a Data Warehouse.  

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Incremental Load Method for Loading Data Warehouse Example 

To understand the working of the ETL Incremental Loading, you can consider a data source having 2 tables i.e sales and customer

Customer Table

CustomerID  CustomerName  Type         Entry Date
1           Jack          Individual   11-Apr-2021
2           Roland        Individual   11-Apr-2021
3           Bayes          Corporate   12-Apr-2021

Sales Table

ID         CustomerID    ProductDescription   Qty   Revenue  Sales Date
1          1             Chart Paper          100    4.00    11-Apr-2021
2          1             Board Pin (Box)      1      2.50    11-Apr-2021
3          2             Permanent Marker     1      2.00    11-Apr-2021
4          3             Eraser       	      200   75.00    12-Apr-2021
5          1             Pencil (HB)          12     4.00    12-Apr-2021

Using ETL Incremental Loading, 2 records from Customer Table and 3 records from the sales table will be uploaded to your target data warehouse on 11 April 2021. Now on the next day (12 April 2021), only 1 record from the customer table and 2 records from the sales table are uploaded to your destination system. This is because ETL Incremental Loading only uploads the records that are newly created or modified instead of copying the entire tables.

To know which records are already present in the target data warehouse, you can use the Entry data column of the customer table and the Sales Data column of the sales table. You can now record the date till which the data is present after the data is loaded on that day and then only upload data with the date greater than the previously recorded date. You can create a table “batchdate” to store these dates.

Batch_ID  Loaded_Until  Status
1         11-Apr-2021   Success
2         12-Apr-2021   Success

You can now write you respective SQL Queries to extract data from the: 

Customer Table 

SELECT c.* 
FROM Customer c
WHERE c.entry_date > (select nvl(
    max(bd.loaded_until), 
    to_date('01-01-1900', 'MM-DD-YYYY')
  )
from batchdate bd
where bd.status = 'Success'
);

Sales Table

SELECT s.* 
FROM Sales s
WHERE s.sales_date > ( select nvl(
    max(bd.loaded_until), 
    to_date('01-01-1900', 'MM-DD-YYYY')
  )
from batchdate bd
where bd.status = 'Success'
);

Initially, no data has been loaded, so the batchdate table will be empty. Hence, when you execute the above queries, the nvl function will give the date 01-01-1900 as the max(bd.loaded_until) will return a NULL value. This means everything will be extracted. After loading the data for April 11, the first entry can be made into the batchdate table. For the next day(April 12), the max(bd.loaded_until) will now return April 11, 2021. Hence, the data only after the April 11 will now be uploaded to the data warehouse.

Implementing ETL Incremental Loading

To effectively implement the ETL Incremental Loading for your use case, you can through the following two methods:

Method 1: Destination Change Comparison

This method requires a row-wise analysis to compare the unchanged data and the data that has been newly added or modified. Hence, this process might provide a slower performance compared to the Source change Identification method. This technique requires you to bring all the data you need to monitor for changes into the ETL Data Pipeline. Compared to the Source Change method, it has fairly fewer assumptions. Owing to its flexibility, it can work for almost any structured data source, including text files, XML, API result sets, and other non-relational structures.

You can use this method via the following approaches: 

1. Brute Force

This is the most direct way & with the least requirements to go about using the Destination change comparison method by analyzing the data row-by-row. If no other change comparison method is available, the Brute force comparison works. If you can not use other methods, use it as the last resort. 

2. Row Hash

etl incremental - hash rows
Image Source

For Row hashes, you will be using a single column that stores a kind of calculated binary version of the column used for comparison. This hashed binary value is a calculated aggregate of all the specified columns. Unlike the traditional method of manually comparing each column, Row hashes are usually more efficient. However,  the hash value needs to be calculated, which adds a bit of complexity and overhead but is usually worth improving performance.

3. Upsert Operations

ETL Incremental - Update Insert operations
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UPSERT i.e. update & insert will effectively process both newly added and modified data in one step. This is done by matching the unique key columns and comparing the columns that confirm the change. You can also bring in the row hash method within this merge operation to make the comparison. Popular RDMS systems such as SQL Server, Oracle, Postgres, and MySQL work with the UPSERT logic. On most systems, UPSERT operations can also handle data deleted at the source. You can also decide whether to delete the data on the target or softly delete it i.e. leave it as it is and mark it as deleted.

Method 2: Source Change Identification

This method opts for a selective approach by extracting only the new and modified data from the source system since the last run of the loading process. This essentially puts a limit on the amount of data that is being brought to the ETL Data Pipeline. It is done by only extracting the data that actually needs to be moved and excluding the unchanged data from the load cycle. The lesser the data you need to interact with, the faster is the data processing. To apply this technique, you can employ the following approaches:

1. Change Tracking

etl incremental - Change Tracking
Image Source

For a simple, reliable & easy-to-use method, you can try out the change tracking technique present in most of the RDBMSs today. Change tracking makes it easy to see which rows have been newly created, modified, or deleted since the last load. However, this is not always an option because you need to modify the source database to use change tracking for incremental loads. 

2. Update Dates or Row-level Insert

This is the easiest way to detect changes in the source. With this method, for each table, you will have a column that records the date the data was loaded and the date the data was modified. This allows you to effectively identify each row when it changes. Although, it turns out that this method can be unreliable.  Especially on third-party systems that you can’t control. 

3. Change Data Capture

ETL Incremental - Change Data Capture
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Change data capture is another brilliant source-side change detection method available in SQL Server and other RDBMS systems. Though change data capture allows you to perform several tasks compared to simple change detection, it can still be used to identify which data has been newly created, modified, or deleted.

Key Challenges of ETL Incremental Loading

While implementing and working with ETL Incremental Loading, you may encounter the following obstacles:

  • Monitoring: While extracting and consolidating data from different sources, you will observe errors once in a while. These can occur because your API credentials have expired or you have difficulty interacting with your API.  To identify and correct these errors as quickly as possible, you need constant monitoring of your processes. 
  • Incompatibilities: You can add new records that invalidate existing data. For example, providing an integer to a column that is expecting a text. This is especially problematic when adding real-time data, which creates a bottleneck because end users can query that data to get inaccurate or incomplete results and not be able to add new datasets. 
  • Sequencing: Data pipelines are often distributed systems to maximize availability. This can cause the data to be processed in a different order than when it was received, especially when the data is modified or deleted. 
  • Dependencies: When it comes to ETL management, it’s important to understand the dependencies between processes or subprocesses. For example,  if process 1 fails, do you want to run process 2? This gets more complicated as more processes and subprocesses grow. 
  • Tuning: The tuning process is required to ensure that the data in the ETL data warehouse is accurate and consistent. This requires you to perform ETL testing regularly, however, data warehouse tuning is an ongoing process.

Managing the Challenges of ETL Incremental Loading

To effectively manage & maintain an efficient ETL Incremental Loading, you need to find the right balance between parallel and serial processing. Serial processing occurs when one task is completed at a time and the processor executes the tasks in a prescribed order. Therefore, you start a new task as soon as the previous task is finished. Compared to Serial Processing, Parallel processing executes multiple tasks at the same time. 

If the processes are not interdependent when loading the data, the processes can be processed in parallel, which can save a lot of time. However, if there are dependencies, this can be complicated and the data must be processed serially. You can also use Parallel processing by dividing your data into smaller data files for parallel access. You can easily create a pipeline by running multiple components on the same data stream at the same time, and the components running multiple processes on the same data stream. 

Conclusion

In this article, you have learned in detail about the ETL Incremental Loading process. Compared to the Full Loading process, the incremental loading is fast, can perform consistently with fluctuating workloads, and allows you to store historical data. You can implement either using the source or destination-sided change detection methods.  

As you collect and manage your data across several applications and databases in your business, it is important to consolidate it for complete performance analysis of your business. However, it is a time-consuming and resource-intensive task to continuously monitor the Data Connectors. To achieve this efficiently, you need to assign a portion of your engineering bandwidth to Integrate data from all sources, Clean & Transform it, and finally, Incrementally Load it to a Cloud Data Warehouse or a destination of your choice for further Business Analytics. All of these challenges can be comfortably solved by a Cloud-based ETL tool such as Hevo Data.   

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