Blockchain has evolved into a platform for decentralized applications, withbeneficial properties like high integrity, transparency, and resilience against censorshipand tampering. However, blockchains are closed-world systems which do not haveaccess to external state. To overcome this limitation, oracles have been introduced invarious forms and for different purposes. However so far common oracle best practiceshave not been dissected, classified, and studied in their fundamental aspects. In thispaper, we address this gap by studying foundational blockchain oracle patterns in twofoundational dimensions characterising the oracles: (i) the data flow direction, i.e.,inbound and outbound data flow, from the viewpoint of the blockchain; and (ii) the ini-tiator of the data flow, i.e., whether it is push or pull-based communication. We providea structured description of the four patterns in detail, and discuss an implementationof these patterns based on use cases. On this basis we conduct a quantitative analysis,which results in the insight that the four different patterns are characterized by distinctperformance and costs profiles.

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