merlimat opened a new pull request #7106:
URL: https://github.com/apache/pulsar/pull/7106
Note: this is based on top of #6791, #7104 and #7105. Once these are merged,
I'll rebase here. For the sake of this review, check commit ad2e39be36
### Motivation
Fixes: #6554
Ordering is broken in KeyShared dispatcher if a new consumer `c2` comes in
and an existing consumer `c1` goes out.
This is because messages with keys previously assigned to `c1` may now route
to `c2`.
The solution here is to have new consumers joining in a "paused" state.
For example, consider this sequence:
1. Subscriptions has `c1` and `c2` consumers
2. `c3` joins. Some of the keys are now supposed to go to `c3`.
3. Instead of starting delivering to `c3`. We mark the current readPosition
(let's call it `rp0_c3`) of the cursor for `c3`.
4. Any message that now hashes to `c3` and that has `messageId >= rp0_c3`
will be deferred for later re-delivery
5. Any message that might get re-delivered (eg: originally sent to `c1`,
but `c1` has failed) to `c3` and that has `messageId < rp0_c3` will be sent to
`c3`
6. When the markDelete position of the cursor will move past `rp0_c3` the
restriction on `c3` will be lifted.
Essentially, `c3` joins but can only receive old messages, until everything
that was read before joining gets acked.
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