Is contract.on(event, listener) expected to skip events?

[claasahl]

Hi 👋

I have a small NodeJS application (similar to the example below) that is subscribing to Sync events from Uniswap-like pairs on BSC. At a first glance, things work just fine and Sync events are received from all subscribed addresses. At some point the application starts to skip Sync events for no apparent reason (i.e. there arn't any obvious network issues or similar).

import { ethers } from "ethers";

async function main() {
    const url = "https://bsc-dataseed.binance.org/";
    const provider = new ethers.providers.JsonRpcProvider(url);
    const contract = new ethers.Contract("0x8fa59693458289914db0097f5f366d771b7a7c3f", [
        "event Sync(uint112 reserve0, uint112 reserve1)"
    ], provider)

    // eventually, this will skip "Sync" events
    contract.on("Sync", (reserve0, reserve1) => console.log(">>> on", reserve0.toString(), reserve1.toString()));

    // poll "Sync" events for reference
    provider.on("block", blockNumber => {
        setImmediate(async () => {
            const filter = contract.filters["Sync"]();
            const events = await contract.queryFilter(filter, blockNumber);
            const data = events.map(e => [
                e.args?.reserve0.toString(),
                e.args?.reserve1.toString()
            ]);
            console.log(">>> poll", blockNumber, data);
        })
    })
}
main();

When running the above example, I would expect to see matching >>> on messages for every event in >>> poll messages. In practice, this is not the case. Every now and then >>> on messages are skipped.

Is it expected behavior for contract.on("Sync", ...); to occassionally skip Sync events? If not, am I missing something terribly obvious? Do I need to somehow handle removed Sync events?

---

That said, I am fairly certain that I can get things working with one of these approaches:

• Listen to same event across multiple contracts. #1768
• Missed event when listening to Smart Contract #1228

[zemse]

Hi @claasahl, I just tried to reproduce. I am able to see >>> on as well as >>> poll as expected

At some point the application starts to skip Sync events for no apparent reason

Do you know any approx time it takes to reproduce on your end?

[claasahl]

Hi @zemse

Thanks for taking a look :) You are absolutely right, I do see both >>> on and >>> poll for most events as well. Only occasionally is the >>> on-part skipped.

I created a repository (https://github.com/claasahl/ethers.js-discussions-2167/tree/main) based on the above example. Though, I tried to made it easier to see when an >>> on-event is skipped. Based on my latest tests, it usually takes less than 15min. for the first >>> on-event to get skipped.

• 7:28 min
• 11:04 min
• 5:57 min
• 5:08 min
• 8:32 min
• 16:28 min
• 11:28 min
• 3:08 min

The logs for these tests are also in the repositry (https://github.com/claasahl/ethers.js-discussions-2167/tree/main/logs).

[claasahl]

For future visitors, the above example has been refined to show the same behaviour with just provider.getLogs(...) calls.

[atropos0902]

Hi @claasahl
I have an issue on bsc while listening events.
Here's a link for the issue.
#2652
Can you please take a look and let me know your thought?
Thanks!