Scala 2 community build reaches goals

Thursday 20 February 2020

Seth Tisue

The Scala 2 community build is a large collection of open-source Scala code that we build and run every day in order to validate changes to the Scala compiler and standard library.

If you’re not familiar with the community build at all, this introduction, from January 2018, still applies.

A year ago, we updated you on improvements made up through January 2019. The post you are reading highlights the progress the community build has made since then.

In short, we believe the Scala 2 community build is now essentially complete. Still, we will continue to run and maintain it throughout Scala 2’s lifetime.

We describe the build as complete because:

  • It has successfully supported the release of Scala 2.13 and the migration of the ecosystem to 2.13.
  • It is large and diverse enough to support future 2.13.x releases.
  • It is now entirely green on JDK 8 and 11 and nearly green on JDK 14.

Details below.

Overall growth

The 2.13 community build is now the main one. (The 2.11 and 2.12 builds still exist, but they have required only light maintenance for some time now.)

The 2.13 build currently has 209 repos and a total of 3.4 million lines of code. (A year ago the main build was the 2.12 one, with 185 repos and 3.2 million lines. 2.13 was still catching up, at only 79 repos.)

It takes about nine hours to run.

Repos added in the last year include: advxml, akka-http-webgoat, akka-management, akka-streams-tcp-chat, alpakka-kafka, avro4s, cats-effect-testing, cats-mtl, cats-retry, cats-testkit-scalatest, cats-time, circe-generic-extras, circe-jackson, ciris, claimant, context-applied, discipline-scalatest, discipline-specs2, doobie, droste, expression-evaluator, fastparse-scalameta, finagle, http4s-jwt-auth, implicitbox, jwt-scala, kits, libra, log4cats, mdoc, meow-mtl, mockito-scala, munit, natchez, pfps-shopping-cart, play-file-watch, playframework, prog-scala-examples, quicklens, redis4cats, scala-collection-contrib, scala-collection-laws, scala-hedgehog, scala-parallel-collections, scala-pet-store, scala-typed-holes, scalatestplus-junit, scalatestplus-mockito, scalatestplus-scalacheck, scalatestplus-testng, scodec-cats, sconfig, skunk, splain, squants, sttp-model, treehugger, tsec, unique, vault, verify.

See also the full list of all included repos.

Success of the Scala 2.13 series

The Scala open-source ecosystem has almost universally adopted Scala 2.13. Libraries and tools unavailable for 2.13 are few and far between, as this short list shows.

Most projects needed only minor changes around the edges to cross-compile against 2.11, 2.12, and 2.13. (Most maintainers have already dropped 2.10 from their cross-builds. Some are dropping 2.11, now, too.)

Not many new bugs were reported in Scala 2.13 itself, so without urgent fixes to ship, we’ve been able to do followup releases on a fairly leisurely schedule: 2.13.1 after three months, and 2.13.2 (coming soon) after a further six months.

We take the relative scarcity of new defects as evidence that the community build is providing a high level of quality assurance. The build can’t catch everything, but it has prevented many regressions from ever being released.

JDK 11 and 14

The 2.13 build is now 100% green on JDK 11. That’s because:

  • Support for JDK 11 in Scala and sbt has improved.
  • Across the Scala and Java ecosystems, maintainers have increasingly added JDK 11 to their own CI matrices.
  • We report JDK 11 related issues to upstream maintainers.

We also now run the build on an early-access build of JDK 14. Only two repos currently fail (for minor reasons not having to do with regressions in Scala).

All variants of the community build now run on AdoptOpenJDK. This seems to be an increasingly standard choice, within Lightbend and in the community at large.

Maintenance and process changes

The way the community build is configured and maintained has been overhauled. It is now easier for us to maintain, and it is easier for the maintainers of included repos to participate. If interested, see this post for details.

The future: Scala 2

We believe that for Scala 2, the community build is now essentially complete. It is sufficiently large, and sufficiently up-to-date with the ecosystem, to serve the needs of ongoing development on the Scala 2.13.x series. We will keep maintaining it indefinitely, but we don’t expect any further really notable changes or expansions.

(What about Scala 2.14? We don’t expect any 2.14 release to be needed. If you missed that news, see this blog post from December.)

The future: Scala 3

And what about Scala 3? Dotty has its own community build, and it continues to grow. It now contains 25 repos, compared to 5 a year ago.

Here is the list of included repos. If you want to add something, there are instructions.


I (Seth Tisue) am the primary maintainer of the Scala community build, as a member of the Scala team at Lightbend. Toni Cunei, also at Lightbend, is the primary author of dbuild, the meta-build tool that makes the community build possible.

The community build couldn’t exist without continual help and advice from the maintainers of the included projects.