# The Low-level Oracle Query Contract

Zoe v0.18.1. Last updated 2021-09-13.
# View the code on Github (opens new window) (Last updated: 5-NOV-2020)
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NOTE: You almost certainly do not want to use this contract directly. Instead, please read the Chainlink integration documentation

This contract lets other contracts or users make single free or fee-based queries to a generic oracle node (a single instance). This provides a very low-level API to issue single queries that an individual off-chain oracle node answers.

CAUTION: The security of oracle networks (such as Chainlink) depends upon having higher-level contracts to aggregate the results of the individual nodes (this low-level contract). This protects against misbehaviour from an individual node.

Relying on just a single node can be both expensive and risky. Instead, use the higher-level APIs described in the Chainlink integration documentation.

# Making a Free Query

To make a free query, obtain the publicFacet for the oracle contract instance.

const response = await E(publicFacet).query(
  'What is the answer to the Ultimate Question of Life, the Universe, and Everything?',
);
// response = 42

The query that is passed in could be in any format that the oracle accepts. The response can be in any format, as the oracle determines.

# Making a Paid Query

To make a query that requires payment, obtain the publicFacet as before, but this time, make a queryInvitation. Use the queryInvitation to make an offer and escrow the required payments in the Fee brand. The response will be the result of your offer.

const queryInvitation = E(publicFacet).makeQueryInvitation(
  'What is *really* the answer?',
);

const proposal = harden({
  give: { Fee: link },
});

const payments = harden({
  Fee: linkPayment,
});

const querySeat = E(zoe).offer(queryInvitation, proposal, payments);

const offerResult = await E(querySeat).getOfferResult();
// offerResult = 42

# Instantiating a New Oracle Contract

If you want to create your own oracle contract instance, first bundle and install the code if it is not already installed.

const contractBundle = await bundleSource(
  require.resolve('@agoric/zoe/src/contracts/oracle'),
);
const installation = await E(zoe).install(contractBundle);

Then start the contract instance. You will receive a publicFacet and a creatorFacet.

const { creatorFacet, publicFacet } = await E(zoe).startInstance(
  installation,
  { Fee: linkIssuer },
);

You will need to use the creatorFacet to initialize an oracleHandler. The oracleHandler is what will actually be queried, so we do not want to put it in the contract terms, which are publicly accessible.

const initializedCreatorFacet = await E(creatorFacet).initialize({
  oracleHandler,
});

# The Oracle Handler API

The contract expects all oracleHandlers to offer the following API:

const oracleHandlerAPI = Far('oracleHandlerAPI', {
  onQuery: async (_query, _fee) => {},
  onError: async (_query, _reason) => {},
  onReply: async (_query, _reply, _fee) => {},
});