Reactome Pengine is a Prolog based API to the human reactome. It is available at https://apps.nms.kcl.ac.uk/reactome-pengine/. Users can send their own small Prolog programs alongside a query to Reactome Pengine, so that the computation takes place in the cloud. Users send their 'small' program to the 'large' data, rather than sending data to the program as is more typical for a webservice, saving both bandwidth and storage space.