This article was written by Seth Tisue, Software Engineer at Lightbend.
We on the Scala team would like to call some attention to a lesser-known but crucial component of the development effort behind Scala. It’s called the Scala community build.
What is it?
It’s a collection of open-source Scala code that includes many of the most-used libraries in the Scala ecosystem.
But it’s more than just a big pile of code; we actually compile all of these codebases, run their test suites, and rewire their builds to depend on each other, so only freshly built code, built by freshly built Scala, is involved.
Why do we do this?
The goal is to enable Scala the language and Scala the ecosystem of libraries to evolve in tandem with each other.
Having the community build as a backstop and testbed means we can confidently assess the impact of proposed changes to the Scala language, compiler, and standard library.
Our Jenkins cluster runs the community build every day against the latest Scala nightly build.
We often also run the community build against individual pull requests in the scala/scala repo to assess the impact of the PR and detect regressions before the PR is merged.
Has it helped?
Definitely. Over the past few years, the build has often caught regressions and brought unanticipated compatibility issues to light.
During the Scala 2.12 cycle, feedback from the community build was key for guiding the work on SAMs and the new trait encoding. And, seeing what went wrong in downstream projects as the 2.12 changes went in was a major source for developing the migration guidelines in the 2.12 release notes.
In the Scala 2.13 cycle, we expect the community build to play a similar role in transitioning first our own code, then the entire open-source ecosystem, to the new collections library. We’ve also begun using it to gauge our progress on Java 9 support.
Library authors have also benefited. The community build has often provided early warning of the effects of changes to Scala or changes to other libraries. It has helped maintainers standardize their builds, keep their dependencies up-to-date, identify flaky tests, and shake out assorted other issues.
How big is it?
The community build has been growing steadily since 2013. These days it includes:
2.8 million lines of code
That’s a lot!
The build uses a custom compiler plugin to make sure that only code that is actually compiled is counted. cloc, the standard tool for counting lines of code, takes care of filtering out blank lines and comments.
There are now 141 projects in the community build, as listed in the config file. They are: acyclic, akka, akka-contrib-extra, akka-http, akka-http-cors, akka-http-session, akka-persistence-cassandra, algebra, ammonite, argonaut, atto, autowire, base64, better-files, blaze, breeze, cachecontrol, case-app, catalysts, cats, cats-effect, circe, circe-config, conductr-lib, coursier, discipline, dispatch, doodle, elastic4s, fansi, fastparse, fs2, genjavadoc, geny, gigahorse, github4s, http4s, http4s-websocket, jackson-module-scala, jawn-fs2, jawn, json4s, kind-projector, kxbmap-configs, lagom, lift-json, lightbend-emoji, log4s, machinist, macro-compat, macro-paradise, meta-paradise, metaconfig, mima, minitest, monix, monocle, multibot, nscala-time, nyaya, paiges, paradox, parboiled, parboiled2, pcplod, play, play-doc, play-json, play-webgoat, play-ws, pprint, pureconfig, sbinary, sbt-io, sbt-librarymanagement, sbt-testng, sbt, sbt-util, scala-async, scala-collections-laws, scala-continuations, scala-debugger, scala-gopher, scala-java8-compat, scala-js, scala-json-ast, scala-logging, scala-parser-combinators, scala-partest-interface, scala-partest, scala-records, pscala-refactoring, scala-ssh, scala-stm, scala-swing, scala-xml-quote, scalacheck, scalacheck-shapeless, scalafix, scalafmt, scalaj-http, scalachess, scaladex, scalalib, scalameta, scalameter, scalamock, scalapb-lenses, scalapb, scalaprops, scalariform, scalastyle, scalatags, scalatest, scalatex, scalaz, scalikejdbc, scallop, scodec-bits, scodec, scopt, scoverage, semanticdb-sbt, shapeless, simulacrum, sjson-new, sksamuel-exts, slick, sourcecode, specs2, spire, spray-json, ssl-config, tut, twirl, twitter-util, twotails, unfiltered, upickle, utest, zinc.
Want to add your project to the community build? See our eligibility guidelines.
Learning more, getting involved
The community build is documented in a wiki. Many of the questions you might have are already answered there.
The main contributors have been myself (Seth Tisue), Adriaan Moors, Grzegorz Kossakowski, Jason Zaugg, and Toni Cunei, all of whom are current or former employees of Lightbend. Toni is also the primary author of dbuild, the meta-build tool that makes the community build possible.
The community build also couldn’t exist without continual help and advice from the maintainers of the included projects. You are marvelous!