https://scastie.scala-lang.org

• enhance communication and collaborative debugging by providing extensive insight in code snippets
• fast and easy to use, no ads, no registration

• reusing sbt instances for fast compilation
• distributes workload amongst multiple sbt instances (including remote)
• realtime update of snippet compilation/running progress
• timeout long-running snippets
• allow specifying scala version, target and external libraries

Scastie relies on existing scala infrastructure to provide close to real world experience inside a pastebin-like sandbox. This minimizes the effort to create meaningful snippet but might require some prior experience to fully unleash its (and scala) power.

The application consists of three modules:

• scastie - standard akka-http web application module
• sbt-runner - encapsulates snippets processing
• balancer - split the load across multiple sbt instances

For each snippet create/read request Application controller in scastie module creates a message and sends it to DispatchActor in renderer module. The DispatchActor routes messages to a remote SbtActor via akka router.

SbtActor(s) interacts with locally running sbt instance via process I\O streams, which is quite hacky, but works good enough. If the sbt instance terminates or otherwise encounters an error while processing a snippet, the actor will be restarted.

Currently the snippet are stored directly on file system on master node (the one which is running web application).