Distributed research infrastructures embrace resources, services, and users. From those, services display the highest churn rate. The reasons are twofold. First, new versions or new services emerge, gain popularity, and have to be provisioned by infrastructure operators. Secondly, the demand to make research reproducible inclines the users to share the software they used to obtain their results. Coming to grips with these high churn rates is not easy, but has the potential to speed-up scientific discovery. In this paper we will describe an extensible Software-as-a-Service (SaaS) solution conceived to facilitate seamless exchange, evolution, and deployment of software services in distributed environments. In opposite to ordinary SaaS solutions, the portfolio of the software services in our approach is extensible. We implemented means for users and developers to make their services readily available in the distributed infrastructure with minimal overhead. In this process, not only the actual software is made available but also the knowledge of how to install and configure it is conveyed, thus running instances of the new service can be provided right away. We will share the experience gained during the implementation of the extensible SaaS solution for DARIAH-DE research infrastructure. It was used to provision on-demand instances of software used in digital humanities. Subsequently, the same solution was reused in a completely different context to provide on-demand instances of UNICORE Grid middleware for training and testing purposes. The operation of our extensible SaaS, yielded additional requirements and extensions. In particular, the need for monitoring and measuring the resource usage were identified and addressed. The presented insights can help to address the problem of exchange and provision of scientific software. Other distributed infrastructures can incorporate them to improve the scalability, maintainability, user-friendliness, and sustainability of their platforms.
Special Issue Papers