Hi,
We noticed that sometimes querying contract events in the latest block returns empty list even though the block does contain the relevant events. And if I wait for sometime and query the same block again the events will be there. Is this due to the inconsistent state between the nodes behind the load balancer?
I wrote a simple script to demonstrate the problem
const { ethers } = require("ethers")
const infuraId = “…”
function sleep(ms) {
return new Promise((resolve) => {
setTimeout(resolve, ms)
})
}
const events = {}
function addEvents(block, lookback, numEvents) {
events[block] = events[block] || {}
events[block][lookback] = numEvents
}
async function queryEvents(block, lookback, contract, filter) {
block = block - lookback
const events = await contract.queryFilter(filter, block, block)
addEvents(block, lookback, events.length)
}
async function main() {
const provider = new ethers.providers.InfuraProvider(1, infuraId)
provider.pollingInterval = 10 * 1000
const abi = [
"event Transfer(address indexed from, address indexed to, uint256 value)"
]
// https://etherscan.io/address/0xdac17f958d2ee523a2206206994597c13d831ec7
const address = "0xdAC17F958D2ee523a2206206994597C13D831ec7"
const contract = new ethers.Contract(address, abi, provider)
const filter = contract.filters.Transfer()
const lookbacks = [0, 3, 5]
const maxLookback = lookbacks[lookbacks.length-1]
provider.on('block', async (blockNumber) => {
for (const lookback of lookbacks) {
await queryEvents(blockNumber, lookback, contract, filter)
}
console.log(`${blockNumber}:`, Object.values(events[blockNumber - maxLookback] || {}))
})
while (true) {
await sleep(100)
}
}
main()
The output of the script is like this
15159969: [ 0, 22, 22 ]
15159970: [ 0, 0, 0 ]
15159971: [ 11, 11, 11 ]
15159972: [ 17, 17, 17 ]
15159974: [ 14, 14, 14 ]
Each line means how many events are found when querying after 0, 3 and 5 blocks.
Most of the time they’re pretty consistent but you can see on block 15159969 it didn’t find any when queried immediately but there’re acutally 22 events in it.
Thanks !