onFHIR is an implementation of HL7 FHIR standard which can be used as a central data service for FHIR compliant healthcare applications. onFHIR is open-source, the source code is available on GitHub, and it is being actively maintained and improved. onFHIR repository is being actively used and improved by many research projects in health informatics. Moreover, there are many commercial deployments; it is a part of the central chronic disease management software within the national health information system of Turkey, handling billions of records from an increasing number of 25 million patients, processing 100 millions of FHIR interactions daily.
onFHIR includes many supporting tools around the FHIR repository implementation which makes it a software ecosystem. I am a major contributor to onFHIR ecosystem. It is implemented with Scala, based on Akka and MongoDB. The full tech stack can be found on stackshare.