This isn't hit in ongoing replication, this is when creating the resources for a new replica on the publisher. And even there the poll based thing is only hit due to a function being used in a context that wasn't possible at the time it was being written (waiting for transactions to complete in a hot standby).
> The Postgres team needs to go back to the drawing board and avoid polling altogether and more importantly have event-listener based approaches for primary and replicas separately
That's, gasp, actually how it already works.
Right?
Armchair software engineers are so incredibly annoying.
Postgres codebase is a shining example of good engineering. They walk a very fine line between many drawbacks. Code is battle tested (and surprisingly readable). Not many people (or groups) can do much better - and if they think they can, they are probably going to learn a lesson in humility.
To be fair, we have/PG has plenty crud-y code... Some of it even written by yours truly :(. Including some of the code involved here.
But just saying "go back to the drawing board" without really understanding the problem isn't particularly useful.
As mentioned, there should be a read-replica specific code that works using event listeners. Repurposing the primary's code is what causes the issue.
The following pseudocode would give a better picture on the next steps
// Read replica specific implementation
void StandbyWaitForTransactionCompletion(TransactionId xid) {
// Register to receive WAL notifications about this transaction RegisterForWALNotification(xid);
while (TransactionIdIsInProgress(xid)) {
// Wait for WAL record indicating transaction completion
WaitForWALEvent();
CHECK_FOR_INTERRUPTS();
// Process any incoming WAL records
ProcessIncomingWAL();
}
UnregisterForWALNotification(xid);
}
The Postgres team needs to go back to the drawing board and avoid polling altogether and more importantly have event-listener based approaches for primary and replicas separately
It's great to see ClickHouse contributing to Postgres though. Cross-pollination between two large database communities can have a multiplying effect.