Ensuring an event is processed only once

[mrwillis]

Hi Richard,

I'm trying to listen to event X and make sure each instance of it is processed exactly once (even if multiple instances of the same event are fired within 1 tx) using contract.on("EventName", handleEvent). What would be a sane way to do this? Currently I'm using code like

async function handleEvent(
  data: any
  event: Event
) {
  try {
    const transaction = await event.getTransaction();
    const { from, hash } = transaction;
    const logIdentifier = `${hash}:${event.logIndex}`;
    const processed = await getTransactionProcessed(logIdentifier);
    if (processed) {
      return;
    } else {
      await setTransactionProcessed(logIdentifier);
    }

where getTransactionProcessed and setTransactionProcessed make sure that if the tx is sent again from another node, it won't be processed twice. I've tried to do this with logIndex but doesn't seem 100% reliable. How do I handle re-orgs? For example, I've had a situation where the logIndex is two different numbers when this handler is called twice (there was only one instance of the event in the tx):

tx-processed:0x919e9973a7655d8f36aff64d921586bbdbab1093b17e41c740a5c64a454345d5:72
tx-processed:0x919e9973a7655d8f36aff64d921586bbdbab1093b17e41c740a5c64a454345d5:124

Thanks!

[zemse]

To handle re-org, you may use the standard json rpc API, it contains a removed property as true when the log is reorged out (which is a bit low level).

const changes = await provider.send('eth_getfilterchanges', [filter]);
for(const log of changes) {
  if(log.removed) {
    // handle log removed
  }
}