Chapter 11. Oracles
In this chapter we discuss oracles, which are systems that can provide external data sources to Ethereum smart contracts. The term “oracle” comes from Greek mythology, where it referred to a person in communication with the gods who could see visions of the future. In the context of blockchains, an oracle is a system that can answer questions that are external to Ethereum. Ideally oracles are systems that are trustless, meaning that they do not need to be trusted because they operate on decentralized principles.
Why Oracles Are Needed
A key component of the Ethereum platform is the Ethereum Virtual Machine, with its ability to execute programs and update the state of Ethereum, constrained by consensus rules, on any node in the decentralized network. In order to maintain consensus, EVM execution must be totally deterministic and based only on the shared context of the Ethereum state and signed transactions. This has two particularly important consequences: the first is that there can be no intrinsic source of randomness for the EVM and smart contracts to work with; the second is that extrinsic data can only be introduced as the data payload of a transaction.
Let’s unpack those two consequences further. To understand the prohibition of a true random function in the EVM to provide randomness for smart contracts, consider the effect on attempts to achieve consensus after the execution of such a function: node A would execute the command and store 3 on behalf of ...
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