Foundational Oracle Patterns: Connecting Blockchain to the Off-Chain World

Blockchain has evolved into a platform for decentralizedapplications, with beneficial properties like high integrity, transparency,and resilience against censorship and tampering. However, blockchainsare closed-world systems which do not have access to external state. Toovercome this limitation, oracles have been introduced in various formsand for different purposes. However so far common oracle best prac-tices have not been dissected, classified, and studied in their fundamen-tal aspects. In this paper, we address this gap by studying foundationalblockchain oracle patterns in two foundational dimensions characteris-ing the oracles: (i) the data flow direction, i.e., inbound and outbounddata flow, from the viewpoint of the blockchain; and (ii) the initiatorof the data flow, i.e., whether it is push or pull-based communication.We provide a structured description of the four patterns in detail, anddiscuss an implementation of these patterns based on use cases. On thisbasis we conduct a quantitative analysis, which results in the insight thatthe four different patterns are characterized by distinct performance andcosts profiles.