Flush expired transactions from mempool

[mpguerra]

Motivation

Specifications

Designs

Related Work

[mpguerra]

Hey team! Please add your planning poker estimate with ZenHub @conradoplg @dconnolly @jvff @oxarbitrage @teor2345 @upbqdn

[oxarbitrage]

Need #2749

[oxarbitrage]

I have two questions in order to do this.

1. Where we should do it? We need a place that is executed often so we can remove transactions that are expired. One option is in the crawler. We can add a mempool service argument in https://github.com/ZcashFoundation/zebra/blob/main/zebrad/src/commands/start.rs#L104 . This will mean we need to develop a mempool service request to remove transactions that will use the function added at Add mempool::Storage::remove() #2749 in the background. Any other ideas on where to do expired tx removal ?

2. At some point we will have a transaction timestamp. Should we just compare this with the system clock ?

Thanks.

[oxarbitrage]

https://zips.z.cash/zip-0203

[oxarbitrage]

1. Where we should do it? We need a place that is executed often so we can remove transactions that are expired. One option is in the crawler. We can add a mempool service argument in https://github.com/ZcashFoundation/zebra/blob/main/zebrad/src/commands/start.rs#L104 . This will mean we need to develop a mempool service request to remove transactions that will use the function added at Add mempool::Storage::remove() #2749 in the background. Any other ideas on where to do expired tx removal ?

One idea that was suggested by @conradoplg in discord was to add this in the mempool service poll_next():

"I think we can do this in the mempool poll_ready since it will get called periodically. At least the current code is relying on that to periodically check if transactions have been downloaded and adding them to the mempool if they were."

The implementation at #2774 uses the above.

There was another suggestion from @jvff however i was not able to code up this one:

"Idea: It could be a separate task spawned when the mempool is created, that:

loop {
    select! {
        _ = sleep_until(self.next_expiration()) => {
            self.remove_expired_transactions();
        }
        _ = next_expiration_rescheduled.next() => {}
    }
}

where next_expiration_rescheduled is an mpsc channel receiver whose sender is owned by the mempool::Storage and messages are sent when new transactions are inserted"

2. At some point we will have a transaction timestamp. Should we just compare this with the system clock ?

After more research this question is not valid anymore, there is no timestamp involved in the mempool transaction expirations but just block heights.