Fun with Generator Functions. First backfill XRPL Ledgers, then keep up with current Ledgers closing, fall back to backfilling if needed to sync gaps.

index.mjs

import { XrplClient } from 'xrpl-client'

const config = { backfillLedgers: 20, lastLedger: null }

const client = new XrplClient(['wss://xrplcluster.com'])
await client.ready()
config.lastLedger = client.getState().ledger.last - config.backfillLedgers

function getLedgerTransactions(ledger_index) {
  return client.send({ command: 'ledger', transactions: true, expand: true, ledger_index })
}

async function backfill () {
  console.log('Strategy: backfill')
  return getLedgerTransactions(config.lastLedger)
}

async function stayUpToDate () {
  return new Promise(resolve => {
    console.log('Strategy: stay up to date')
    client.once('ledger', l => resolve(getLedgerTransactions(l.ledger_index)))
  })
}

// Notice the *. Yay, this is a generator.
async function* ledgerToProcess () {
  /**
   * This will backfill if needed, and live watch the ledger for newly closed ledgers. If live
   * watch skips ledgers, the backfill will take care of it again. Ledgers will come in ordered.
   */
  while (true) {
    const newLedger = config.lastLedger <= client.getState().ledger.last
      ? await backfill()
      : await stayUpToDate()

    /**
     * Check for a gap
     */
    yield newLedger?.ledger_index > config.lastLedger + 1
      ? backfill()
      : newLedger

    config.lastLedger++
  }
}

for await (const l of ledgerToProcess()) {
  const index = Number(l.ledger.seqNum)
  console.log(index)
}