Added back exactly-once delivery for Postgres #28
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| func (a DefaultPostgreSQLOffsetsAdapter) BeforeSubscribingQueries(topic string, consumerGroup string) []Query { |
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Hey @roblaszczak, what does this do? Sorry, I'm clearly an outsider to this codebase. :)
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Hey @jcmfernandes!
It's required for exactly-once-delivery guarantee which I enabled in this version (see https://github.com/ThreeDotsLabs/watermill-sql/pull/28/files#diff-9c7c1342e6c9993e31c519e27f897bea0ffb721cb39fd7cf9bb70cfc4a56b6ebR530)
It adds "zero offsets" to the table with offsets. Without that, this lock
Query: `
SELECT
offset_acked,
last_processed_transaction_id
FROM ` + a.MessagesOffsetsTable(topic) + `
WHERE consumer_group=$1
FOR UPDATE
`,
won't work, because there is nothing to lock. If "zero offsets" won't be present and multiple concurrent subscribers will try to consume them it will lead to multiple delivery (because offsets are not locked).
I hope that makes sense! :)
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Thank you, I believe I now understand. With that said, isn't "exactly-once-delivery" a misnomer here? You guarantee that multiple consumers don't concurrently deliver the message, but isn't the message delivered at least once? I.e., don't you attempt to redeliver if the message isn't acked?
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