Akka is a toolkit and runtime that simplifies the building of concurrent and distributed applications on the JVM. The project announced a license change, from Apache 2.0 to BSL v1.1 (Business Source License). BSL is a “source code available” license, and future developers who want to use Akka (version 2.7+) in production will need to obtain a commercial license from Lightbend. But Akka’s commercial license will be available for free to companies with less than $25 million in annual revenue, and any new code under the BSL license will revert to the Apache 2.0 license after three years.

Apache Flink is an open source stream processing framework suitable for distributed, high-performance data streaming applications; Akka is used internally.Akka is used in Flink’s coordination layer for

• exchange status messages between processes/components (e.g. JobManager and TaskManager),
• Enforces certain guarantees against multithreading (ie, only one thread can change the component’s internal state)
• Watch for unexpected crashes of components (ie, watch out for and handle TaskManager thread crashes).

Therefore, after Akka announced the license change, some Flink users expressed their concerns. In response, Flink officially released a blog to clarify its position on the matter, saying:

There is no immediate danger to Flink and we will ensure that users are not affected by this change.

Flink’s license will not change; it will remain the Apache license and only include dependencies that are compatible with it.

We will not use Akka releases with new licenses.

Going forward, Flink will also continue to use Akka 2.6, the current latest version still available under the original Apache 2.0 license. Akka 2.6 will continue to receive critical security updates and critical bug fixes under the current Apache 2 license until September 2023.

Flink believes that “Historically, Akka has been very stable, and coupled with our limited use of features, we do not expect this to be a problem.” At the same time, they will:

• Watch how the situation develops (especially in terms of community branches)
• Look for alternatives to Akka.

If a community fork is created (which currently seems possible), Flink plans to switch to that fork in all possible ranges for 1.15+. And the announcement pointed out that Flink can replace Akka with something else without major changes to Flink.