equanz opened a new pull request, #20179:
URL: https://github.com/apache/pulsar/pull/20179
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### Motivation
The Key_Shared subscription type has the following issues.
1. Key_Shared subscription has out-of-order cases because of the race
condition of [the recently joined consumers
feature](https://github.com/apache/pulsar/blob/35e9897742b7db4bd29349940075a819b2ad6999/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentStickyKeyDispatcherMultipleConsumers.java#L400-L408).
Consider the following flow.
1. Assume that the current read position is `1:6` and the recently joined
consumers is empty.
2. Called
[OpReadEntry#internalReadEntriesComplete](https://github.com/apache/pulsar/blob/35e9897742b7db4bd29349940075a819b2ad6999/managed-ledger/src/main/java/org/apache/bookkeeper/mledger/impl/OpReadEntry.java#L92-L95)
from thread-1.
Then, the current read position is updated to `1:11` (Messages from
`1:6` to `1:10` have yet to be dispatched to consumers).
3. Called
[PersistentStickyKeyDispatcherMultipleConsumers#addConsumer](https://github.com/apache/pulsar/blob/35e9897742b7db4bd29349940075a819b2ad6999/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentStickyKeyDispatcherMultipleConsumers.java#L130-L131)
from thread-2.
Then, the new consumer is stored to recently joined consumers with
read position `1:11`.
4. Called
[PersistentDispatcherMultipleConsumers#trySendMessagesToConsumers](https://github.com/apache/pulsar/blob/35e9897742b7db4bd29349940075a819b2ad6999/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentStickyKeyDispatcherMultipleConsumers.java#L174)
from thread-5.
Then, messages from `1:6` to `1:10` are dispatched to consumers. From
the recently joined consumers feature, the new consumer can receive messages
from `1:6` to `1:10`. **However, it is not expected.**
For example, if existing consumers have some unacked messages and
disconnect, it causes out of order in some cases.
2. Key_Shared subscription has a redundant process.
[The stuckConsumers
feature](https://github.com/apache/pulsar/blob/35e9897742b7db4bd29349940075a819b2ad6999/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentStickyKeyDispatcherMultipleConsumers.java#L369-L375)
was introduced from https://github.com/apache/pulsar/pull/7553 .
However, it can't fix the issue entirely because it doesn't consider the
range changes.
After this commit, https://github.com/apache/pulsar/pull/10762 was
introduced. It fixes the issue.
### Modifications
1. Store last sent position instead of read position to recently joined
consumers.
Updating read position, then dispatching messages, and adding new
consumer are not exclusive.
I used the last send position to get a position without any new exclusion
features.
2. Keep the last sent position per key.
When I introduced the last sent position, I noticed a new concern.
Consider the following flow.
1. Assume that the
[entries](https://github.com/apache/pulsar/blob/35e9897742b7db4bd29349940075a819b2ad6999/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentStickyKeyDispatcherMultipleConsumers.java#L174)
has the following messages,
- `msg-1`, key: `key-a`, position: `1:1`
- `msg-2`, key: `key-a`, position: `1:2`
- `msg-3`, key: `key-a`, position: `1:3`
- `msg-4`, key: `key-b`, position: `1:4`
- `msg-5`, key: `key-b`, position: `1:5`
- `msg-6`, key: `key-b`, position: `1:6`
the dispatcher has two consumers (`c1` `messagesForC` is 1, `c2`
`messageForC` is 1000), and the selector will return `c1` if `key-a` and `c2`
if `key-b`.
2. Send `msg-1` to `c1` and `msg-4` - `msg-6` to `c2`.
- So, the current last sent position is `1:6`.
- `c1` never acknowledge `msg-1`.
3. Add new consumer `c3`, and the selector will return `c3` if `key-a`.
4. Send `msg-2` - `msg-3` to `c3` because `1:2` and `1:3` are less than
the last sent position, `1:6`.
5. Disconnect `c1`.
6. Send `msg-1` to `c3`.
Now, `c3` receives messages without expected order about `key-a`.
To avoid this issue, I introduce the last sent positions feature.
3. Remove redundant features due to the addition of the last sent positions
feature.
1. Remove this condition because the last sent positions feature
restricts part of messages.
New consumers can receive any new messages with the message key that
is not in the last sent position.
https://github.com/apache/pulsar/blob/35e9897742b7db4bd29349940075a819b2ad6999/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentStickyKeyDispatcherMultipleConsumers.java#L323-L336
2. Remove this behavior because the last sent positions has positions per
key.
If some key is stuck at a certain point in time, from that point, new
consumers store the same information about the key to the
`recentlyJoinedConsumers`.
https://github.com/apache/pulsar/blob/35e9897742b7db4bd29349940075a819b2ad6999/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentStickyKeyDispatcherMultipleConsumers.java#L365-L399
3. Remove this behavior because this calculation is moved to
LastSentPositions#compareToLastSentPosition.
https://github.com/apache/pulsar/blob/35e9897742b7db4bd29349940075a819b2ad6999/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentStickyKeyDispatcherMultipleConsumers.java#L400-L408
4. Remove the stuckConsumers feature.
5. Reconstruct some consumer stats fields related to Key_Shared.
**This is a breaking change.** However, if this PR is merged, the
existing field which is removed in this PR is no longer needed.
### Verifying this change
- [ ] Make sure that the change passes the CI checks.
This change added tests and can be verified as follows:
- Added unit test to ensure the behavior of last sent positions feature
works properly
### Does this pull request potentially affect one of the following parts:
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- [ ] Dependencies (add or upgrade a dependency)
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- [ ] The schema
- [ ] The default values of configurations
- [ ] The threading model
- [ ] The binary protocol
- [ ] The REST endpoints
- [ ] The admin CLI options
- [x] The metrics
- [ ] Anything that affects deployment
### Documentation
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- [x] `doc-required` <!-- Your PR changes impact docs and you will update
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- [ ] `doc-not-needed` <!-- Your PR changes do not impact docs -->
- [ ] `doc-complete` <!-- Docs have been already added -->
### Matching PR in forked repository
PR in forked repository:
https://github.com/equanz/pulsar/tree/change_definition_of_recently_joined_consumers_position
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