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26 lines
1.5 KiB
ReStructuredText
26 lines
1.5 KiB
ReStructuredText
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Oracles
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=======
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.. topic:: Summary
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* *A fact can be included in a transaction as part of a command*
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* *An oracle is a service that will only sign the transaction if the included fact is true*
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In many cases, a transaction's contractual validity depends on some external piece of data, such as the current
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exchange rate. However, if we were to let each participant evaluate the transaction's validity based on their own
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view of the current exchange rate, the contract's execution would be non-deterministic: some signers would consider the
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transaction valid, while others would consider it invalid. As a result, disagreements would arise over the true state
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of the ledger.
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Corda addresses this issue using *oracles*. Oracles are network services that, upon request, provide commands
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that encapsulate a specific fact (e.g. the exchange rate at time x) and list the oracle as a required signer.
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If a node wishes to use a given fact in a transaction, they request a command asserting this fact from the oracle. If
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the oracle considers the fact to be true, they send back the required command. The node then includes the command in
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their transaction, and the oracle will sign the transaction to assert that the fact is true.
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If they wish to monetize their services, oracles can choose to only sign a transaction and attest to the validity of
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the fact it contains for a fee.
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Transaction tear-offs are used to prevent the oracle from seeing information about the transaction that is not
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relevant to them. See :doc:`merkle-trees` for further details.
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