For the last couple of decades ETL (extract, transform, load) has been the traditional approach for data warehousing and analytics. The ELT (extract, load, transform) approach changes the old paradigm. But, what’s actually happening when the “T” and “L” are switched?
ETL and ELT solve the same need:
Billions of data and events need to be collected, processed and analyzed by businesses. The data needs to be be clean, manageable and ready to analyze. It needs to be enriched, molded and transformed. To make it meaningful.
But, the “how” is what’s different and leads to new possibilities in many modern data projects. There are differences in how raw data is managed, when processing is done and how analysis is performed.
In this article, we’ll demonstrate the ETL and ELT technological differences showing data engineering and analysis examples of the two approaches and summarizing 10 pros and cons of ETL and ELT.
Understand how ETL and ELT fit into the broader trends in data warehousing:
The Technological Differences: Lets first align on the 3 stages - E, T, L:
ETL requires management of the raw data, including the extraction of the required information and running the right transformations to ultimately serve the business needs. Each stage - extraction, transformation and loading - requires interaction by data engineers and developers, and dealing with capacity limitations of traditional data warehouses. Using ETL, analysts and other BI users have become accustomed to waiting, since simple access to the information is not available until the whole ETL process has been completed.
In the ELT approach, after you’ve extracted your data, you immediately start the loading phase - moving all the data sources into a single, centralized data repository. With today’s infrastructure technologies using the cloud, systems can now support large storage and scalable compute. Therefore, a large, expanding data pool and fast processing is virtually endless for maintaining all the extracted raw data.
Here’s an example to illustrate the technological differences between ETL and ELT, and drill down into the details.
Our demonstration will use two data tables: one for purchases and another for currencies, as below:
To understand the fundamentals, we’ll look at how this sample is processed in ETL and ELT. For each, we’ll show how to calculate a single summary table using these two tables - including the average purchase per country (based on the IP address provided).
In the ETL process, the transform stage applies to a series of rules or functions on the extracted data to create the table that will be loaded.
Here’s some code to demonstrate the preliminary data transformation process for ETL:
Using this script, we are mapping the IP addresses to their related country. We are deriving a new calculated value ‘amount’ by multiplying the values of both source tables group by currency attribute. Then we are sorting data by the country column, joining the data from the purchases and currencies tables, and summing up the average values per country.
This data transformation results in a new table with the average amount per country:
AVG AMOUNT PER COUNTRY
In contrast to ETL, with ELT all data is already loaded and can be used at any point in time.
Therefore, the transformation is done at query runtime:
In the query, we are selecting the IP address by country, multiplying amount from the purchases table and rate from the currencies table to calculate the average amount. Then, joining both tables based on the common columns of both tables and grouping by country.
This will result the same exact output table as in the ETL process above. However, in this case, since all raw data has been loaded, we can more easily continue running other queries in the same environment to test and identify the best possible data transformations that match the business requirements.
ELT is more efficient than ETL for development code. In addition, ELT is much more flexible than ETL. With ELT, users can run new transformations, test and enhance queries, directly on the raw data as it is required - without the time and complexity that we’ve become used to with ETL.
According to Gartner, the data management and data integration needs of businesses today require both small and big, unstructured and structured data. Here’s what they suggest about what needs to change in the way of work:
“The traditional BI team needs to continue developing clear best practices, with well understood business objectives… there is a second mode of BI which is more fluid and … highly iterative with unforeseen data discovery and is allowed to fail fast.”
This type of conversation has created a lot of talk in the industry about data warehouses vs. data lakes. The data lake concept is a new way of thinking about big data for unstructured data made for infinite scaling - using tools like Hadoop for implementing the second mode of BI work described by Gartner. However, although enterprise still use data warehouses to support a traditional paradigm such as ETL, scalable modern data warehouses such as Redshift and Bigquery can be used to implement the ELT modern paradigm with all its inherent benefits mentioned above.
IBM talks about 5 things that modern big data projects require - showing the need for new data concepts like the data lake. It’s the 5 V’s:
ETL continues to be a good match when dealing with legacy data warehouses - looking at smaller subsets and moving them into the data warehouse. But it’s hard to provide a solution with ETL for the 5 V’s as you go down the list - how to deal with the volumes? The unstructured data? Speed? etc.
The ELT approach opens opportunity for working in a more fluid, iterative BI environment due to its efficiency and flexibility. ELT supports both the data warehouse and extends to the data lake concept - enabling the incorporation of unstructured data into its BI solution.
To summarize the two approaches, we’ve grouped the differences into 10 criteria:
1. Time - Load
ETL: Uses staging area and system, extra time to load data
ELT: All in one system, load only once
2. Time - Transformation
ETL: Need to wait, especially for big data sizes - as data grows, transformation time increases
ELT: All in one system, speed is not dependant on data size
3. Time - Maintenance
ETL: High maintenance - choice of data to load and transform and must do it again if deleted or want to enhance the main data repository
ELT: Low maintenance - all data is always available
4. Implementation complexity
ETL: At early stage, requires less space and result is clean
ELT: Requires in-depth knowledge of tools and expert design of the main large repository
5. Analysis & processing style
ETL: Based on multiple scripts to create the views - deleting view means deleting data
ELT: Creating adhoc views - low cost for building and maintaining
6. Data limitation or restriction in supply
ETL: By presuming and choosing data a priori
ELT: By HW (none) and data retention policy
7. Data warehouse support
ETL: Prevalent legacy model used for on-premises and relational, structured data
ELT: Tailored to using in scalable cloud infrastructure to support structured, unstructured such big data sources
8. Data lake support
ETL: Not part of approach
ELT: Enables use of lake with unstructured data supported
ETL: Fixed tables, Fixed timeline, Used mainly by IT
ELT: Ad Hoc, Agility, Flexibility, Usable by everyone from developer to citizen integrator
ETL: Not cost-effective for small and medium businesses
ELT: Scalable and available to all business sizes using online SaaS solutions
ETL is outdated. It helped to cope with the limitation of the traditional rigid and data center infrastructures which with the cloud are no longer a barrier today. In organizations with large data sets of even only a few terabytes, load time can take hours, depending on the complexity of the transformation rules.
ELT is an important part of the future of data warehousing. With ELT, businesses of any size can capitalize on the current technologies. By analyzing larger pools of data with more agility and less maintenance, businesses gain key insights to create a real competitive advantages and excel in their business.
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