ETL Process

At its most basic, the ETL process encompasses data extraction, transformation, and loading. While the abbreviation implies a neat, three-step process – extract, transform, load – this simple definition doesn’t capture: 

  • The transportation of data 
  • The overlap between each of these stages
  • How new technologies are changing this flow

It’s best to think of ETL in the context of the business value it provides – ETL is the process by which data is extracted from data sources that are not optimized for analytics, moved to a central host, and optimized for analytics. The exact steps in that process might differ among ETL tools, but the end result is the same.

Traditional ETL process

Historically, the process has looked like this: 

Data is extracted from online transaction processing (OLTP) databases, today more commonly known just as transactional databases, and other data sources. Data is then transformed in a staging area. These transformations cover both data cleansing and optimizing the data for analysis. The transformed data is then loaded into an online analytical processing (OLAP) database, today more commonly known as just an analytics database. 

Business intelligence (BI) teams then run queries on that data, which are eventually presented to end users, or to individuals responsible for making business decisions, or used as input for machine learning algorithms. One common problem encountered here is if the OLAP summaries can’t support the type of analysis the BI team wants to do, then the whole process needs to run again, this time with different transformations.

Modern ETL process

Modern technology has changed most organizations’ approach to ETL, for several reasons. The biggest is the advent of powerful analytics warehouses like Amazon Redshift and Google BigQuery. These newer cloud-based analytics databases have the horsepower to perform transformations in place rather than requiring a special staging area.

Also, data today is frequently analyzed in raw form rather than from preloaded OLAP summaries. This has led to the development of lightweight, flexible, and transparent ETL systems with processes that look something like this:

The biggest advantage to this setup is that transformations and data modeling happen in the analytics database, in SQL. This gives the BI team, data scientists, and analysts greater control over how they work with it, in a common language they all understand.

Critical ETL components

Regardless of the exact ETL process you choose, there are some critical components you’ll want to consider: 

  • Support for change data capture (CDC) (a.k.a. binlog replication): Incremental loading allows you to update your analytics warehouse with new data without doing a full reload of the entire data set. We say more about this in the ETL Load section.
  • Auditing and logging: You need detailed logging within the ETL pipeline to ensure that data can be audited after it’s loaded and that errors can be debugged.
  • Handling of multiple source formats: To pull in data from diverse sources such as Salesforce’s API, your back-end financials application, and databases such as MySQL and MongoDB, your process needs to be compatible with data in a variety of formats.
  • Fault tolerance: In any system, problems inevitably occur. ETL systems need to be able to recover gracefully, making sure that data can make it from one end of the pipeline to the other even when the first run encounters problems.
  • Notification support: If you want your organization to trust its analyses, you have to build in notification systems to alert you when data isn’t accurate. These might include:
    • Proactive notification directly to end users when API credentials expire
    • Passing along an error from a third-party API with a description that can help developers debug and fix an issue
    • If there’s an unexpected error in a connector, automatically creating a ticket to have an engineer look into it
    • Utilizing systems-level monitoring for things like errors in networking or databases
  • Low latency: Some decisions need to be made in real time, so data freshness is critical. While there will be latency constraints imposed by particular source data integrations, data should flow through your ETL process with as little latency as possible.
  • Scalability: As your company grows, so will your data volume. All components of an ETL process should scale to support arbitrarily large throughput.
  • Accuracy: Data cannot be dropped or changed in a way that corrupts its meaning. Every data point should be auditable at every stage in your process.

Click any of the buttons below for more detail about each step in the ETL process:

Keep Learning about the ETL Process

  • Engineers Shouldn’t Write ETL – “In case you did not realize it, nobody enjoys writing and maintaining data pipelines or ETL. It’s the industry’s ultimate hot potato,” writes Jeff Magnusson, director of data platform at Stitch Fix, in an excellent writeup on how to structure data science teams.
  • ELT vs. ETL – This white paper explains the benefits of performing transformations after the data is loaded into a data warehouse. 
  • Overview of Extraction, Transformation, and Loading – This explanation from Oracle explains why the term ETL is too simplistic, because it suggests that each phase is distinct. Depending on the data source’s system capabilities, the chosen way of transportation, and other factors, transformations can happen many times throughout the ETL process.
  • The Data Infrastructure Meta-Analysis – An analysis of how today’s top tech companies are building their ETL workflows, from data extraction to insights.