Apache OpenWhisk

Apache OpenWhisk is the elder statesman of open source serverless platforms. It powered IBM Cloud Functions for years, runs at Apache governance, and supports a wide range of runtimes: Node, Python, Go, Swift, PHP, Ruby, dotnet, Rust, Scala, Java, and Docker images. Version 2.0.0 shipped last spring, and the project recently migrated to Apache Pekko after Akka changed licenses. Still alive, still active.

Deployment is the real question. You can run a Java standalone for local dev, or stand up a Kubernetes-native cluster for production. Most platform teams pick OpenFaaS or Knative for simpler operational stories; OpenWhisk is heavier, with more components (controller, invoker, Kafka, CouchDB) to manage. The reward is a mature, battle-tested platform.

Solo developers: probably skip. The setup overhead doesn't pay off until you're running real production workloads. Small teams: evaluate OpenFaaS or Knative first; come here if those don't fit. Large teams running enterprise workloads who want the Apache Software Foundation backing and broad runtime support: OpenWhisk is worth the ops investment.

The catch: more moving parts than the alternatives. Operating a production OpenWhisk cluster requires managing Kafka, CouchDB, and the controller/invoker split. The platform is excellent; the day-two operations are not trivial.