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Fix sending out of order POSITION over replication
#16639
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clokep
reviewed
Nov 15, 2023
synapse/replication/tcp/resource.py
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| # We need to send out POSITIONs for all streams, usually | ||
| # because a worker has reconnected. | ||
| self.command_handler.will_should_announce_positions() |
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will_should_announce_positions sets _should_announce_positions to false, so we're essentially saying that we've now announced positions, don't do it again next time around the loop?
Is it possible for another REPLICATE to come in after this? I guess that's fine, we'll just send them again next loop?
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Yup, pretty much.
so we're essentially saying that we've now announced positions, don't do it again next time around the loop?
We say we are about to send positions (to handle the race where something asks us to send out positions after we've just sent them out but before we clear the flag)
clokep
approved these changes
Nov 16, 2023
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If a worker reconnects to Redis we send out the current positions of all our streams. However, if we're also trying to send out a backlog of RDATA at the same time then we can end up sending a
POSITIONwith the current token before we've sent all the RDATA before the current token.This doesn't cause actual bugs as the receiving servers see the POSITION, fetch the relevant rows from the DB, and then ignore the old RDATA as they come in. However, this is inefficient so it'd be better if we didn't send out-of-order positions