Business Source License (BSL) was created (…) to provide a mutually beneficial balance between the user benefits of true Open Source software that is free of cost and provides open access to all of the product code for modification, distribution, etc., and the sustainability needs of software developers to continue delivering product innovation and maintenance.

The BSL is structured to allow free and open usage for the majority of use cases, and only requires a commercial license by those who use the software above a certain threshold, which is typically indicative of an environment that is delivering significant value to a business.

BSL gives users complete access to the source code so users can modify, distribute and enhance it. It also guarantees a path for the software to become Open Source over time so that users will never be locked into a single vendor. These features help preserve the critical freedom aspects of Open Source while enabling a viable business model for professional software developers.

https://mariadb.com/bsl-faq-adopting/