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4. Put everything together and fine-tune the Witnet request

This article is part of the beginner tutorial on creating a totally decentralized Bitcoin price feed on Ethereum with Solidity and Witnet.

Put everything together

Time to put everything together and create the Witnet.Request object that you will need to export:

// This is the Witnet.Request object that needs to be exported
const request = new Witnet.Request()
  .addSource(bitstamp)       // Use source 1
  .addSource(coindesk)       // Use source 2
  .setAggregator(aggregator) // Set the aggregation script
  .setTally(tally)           // Set the tally script
  .setQuorum(4, 2)           // Set witness count
  .setFees(10, 1, 1, 1)      // Set economic incentives
  .schedule(0)               // Make this request immediately solvable

// Do not forget to export the request object
export { request as default }

The .setQuorum, .setFees and .schedule methods are explained below.

Fine-tune fees, quorum and schedule

Witnet requests are highly parameterizable. You can fine-tune almost every single aspect of their life cycle:

  • .setQuorum() sets how many Witnet nodes will be "hired" for resolving the request.
  • .setFees() specifies how much you want to pay for rewarding each of the Witnet nodes implicated in resolving the request.
  • .schedule() schedules the request to be resolved in a particular date and time in the future.

Set the quorum

.setQuorum(witnesses, backup_witnesses)

The witnesses is the minimum number of Witnet nodes that will be resolving each specific request.

In general, the higher the number of witnesses, the safer the request. However, fees should be proportional to this number.

The actual number of nodes that will resolve each request is guaranteed to be equal than the specified number. If for some reason the network fails to assign the request to enough nodes, it will be reassigned in every subsequent epoch to a different randomly-selected set of nodes until the required number is reached.

The backup_witnesses is the number of Witnet nodes that will be used as a backup in case some of the originally assigned nodes fail to fulfill their commitments.

A higher backup_witnesses number implies more fees but also guarantees that the request will be timely resolved. In the other hand, if you use small backup_witnesses values, the risk is that your request will need to be retried many times and therefore the result may be potentially inaccurate (in case the queried data point changes very fast).

Optional arguments for .setQuorum

The .setQuorum method accepts three more optional arguments:

.setQuorum(
  witnesses,
  backup_witnesses,
  extra_commit_rounds,
  extra_reveal_rounds,
  minimum_consensus
)

The extra_commit_rounds number is how many extra epochs will Witnet nodes be given for commiting their partial results. A number of rounds greater than 0 strengthens the chances of a Witnet request being resolved to a value instead of timing out. This parameter is actually an upper threshold, i.e. the request will progress into reveal stage as soon as the number of commitments equals the number of required witnesses. If not set, this parameter defaults to 1. This parameter has no impact on the price of the request.

The extra_reveal_rounds number is how many extra epochs will Witnet nodes be given for revealing their partial results. A number of rounds greater than 0 strengthens the security of a Witnet request by preventing miners from withholding reveal transactions—as the subsequent miners can include any reveal transactions withheld by a former miner. This parameter is actually an upper threshold, i.e. the request will get tallied and finalized as soon as the number of reveals equals the number of commitments. If not set, this parameter defaults to 1. This parameter has no impact on the price of the request.

The minimum_consensus percentage allows to define a threshold for aborting resolution of a request if the witnessing nodes did not arrive to broad consensus. That is, aggregator and tally functions will not be applied if the ratio of valid values vs. errors is below this threshold. E.g. a minimum_consensus threshold of 70 requires a 70% of the witnesses to report a valid value, otherwise the result of the request will be an error stating "insufficient consensus".

Set the fees

.setFees(reward, commit_fee, reveal_fee, tally_fee)

Witnet allows parametrization of many of the economic incentives that affect the life cycle of your requests. Namely, those incentives are:

  • request_fee: the amount of wit tokens that will be earned by the Witnet miner that publishes your request in a block.
  • reward: the amount of wit tokens that every each of the Witnet nodes assigned to your request will earn if they honestly fulfill their commitments and reveals.
  • commit_fee: the amount of wit tokens that will be earned by Witnet miners for each each valid commitment transaction they include in a block.
  • reveal_fee: the amount of wit tokens that will be earned by Witnet miners for each valid reveal transaction they include in a block.
  • tally_fee: the amount of wit tokens that will be earned by the Witnet miner that publishes in a block the tally of all the reveal transactions related to your request.

How can I compute the total cost of a request?

The total cost of a Witnet request equals:

request_fee + witnesses * (reward + commit_fee + reveal_fee) + tally_fee

There are two special cases in which some fees are automatically refunded to the requester upon an eventual tally:

  • For every valid reveal that later does not pass the filters in the tally stage (aka outliers), you get reward back.
  • For every missing reveal after the extra_reveal_rounds threshold is reached, you get reward + reveal_fee back.

Set the schedule

.schedule(timestamp)

Witnet requests can be scheduled for resolution in a particular date and time in the future.

Timestamps need to be provided as POSIX timestamps, i.e. seconds elapsed from 00:00:00 UTC on 1 January 1970 until the desired date.

Double check

Time to double check everything is fine. Your BitcoinPrice.js file should look more or less like this:

import * as Witnet from "witnet-requests"

// Retrieves USD price of a bitcoin from the BitStamp API
const bitstamp = new Witnet.Source("https://www.bitstamp.net/api/ticker/")
  .parseMapJSON()   // Parse a `Map` from the retrieved `String`
  .getFloat("last") // Get the `Float` value associated to the `last` key

// Retrieves USD price of a bitcoin from CoinDesk's "bitcoin price index" API
// The JSON here is a bit more complex, thus more operators are needed
const coindesk = new Witnet.Source("https://api.coindesk.com/v1/bpi/currentprice.json")
  .parseMapJSON()         // Parse a `Map` from the retrieved `String`
  .getMap("bpi")          // Get the `Map` value associated to the `bpi` key
  .getMap("USD")          // Get the `Map` value associated to the `USD` key
  .getFloat("rate_float") // Get the `Float` value associated to the `rate_float` key

// Filters out any value that is more than 1.5 times the standard
// deviationaway from the average, then computes the average mean of the
// values that pass the filter.
const aggregator = new Witnet.Aggregator({
  filters: [
   [ Witnet.Types.FILTERS.deviationStandard, 1.5 ]
  ],
  reducer: Witnet.Types.REDUCERS.averageMean
})

// Filters out any value that is more than 1.5 times the standard
// deviationaway from the average, then computes the average mean of the
// values that pass the filter.
const tally = new Witnet.Tally({
  filters: [
   [ Witnet.Types.FILTERS.deviationStandard, 1.0 ]
  ],
  reducer: Witnet.Types.REDUCERS.averageMean
})

// This is the Witnet.Request object that needs to be exported
const request = new Witnet.Request()
  .addSource(bitstamp)       // Use source 1
  .addSource(coindesk)       // Use source 2
  .setAggregator(aggregator) // Set the aggregator function
  .setTally(tally)           // Set the tally function
  .setQuorum(4, 2,)          // Set witness count
  .setFees(10, 1, 1, 1)      // Set economic incentives
  .schedule(0)               // Make this request immediately solvable

// Do not forget to export the request object
export { request as default }

Time to go ahead and compile the request.

Remember: You are not alone!

You are invited to join the Witnet Community Discord. Members of the Witnet community will be happy to answer your questions and doubts, as well as assisting you through this tutorial.