  First come, first served: Critical choices between alternative actions are
often made based on events external to an organization, and reacting promptly
to their occurrence can be a major advantage over the competition. In Business
Process Management (BPM), such deferred choices can be expressed in process
models, and they are an important aspect of process engines. Blockchain-based
process execution approaches are no exception to this, but are severely limited
by the inherent properties of the platform: The closed-world environment
prevents direct access to external entities and data, and the passive runtime
based entirely on atomic transactions impedes continual monitoring and
detection of events. In this paper we provide an in-depth examination of the
semantics of deferred choice, and transfer them to environments such as the
blockchain. We introduce and compare several oracle architectures able to
satisfy certain requirements, and show that they can be implemented using
state-of-the-art blockchain technology.
