equanz commented on PR #20179:
URL: https://github.com/apache/pulsar/pull/20179#issuecomment-1539314184
@poorbarcode
* [Q-1] Yes.
* [Q-2] When I created this PR, I introduced Map structure because it was
necessary. For more details, please see the description("2. Keep the last sent
position per key.").
I just noticed we could avoid introducing a Map structure if we
implemented the last sent position feature, like the mark delete position and
the individually deleted messages feature. In other words,
- The position is already scheduled to be sent.
- All positions less than or equal to it are already scheduled to be sent.
- Manage individually sent positions to update the position as expected.
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 `1:1` and the individually
sent positions is `[[1:3, 1:6]]` (list of closed intervals. not list of list).
- `c1` never acknowledge `msg-1`.
**scenario A**
<ol start="3">
<li>Add new consumer <code class="notranslate">c3</code>, and the
selector will return <code class="notranslate">c3</code> if <code
class="notranslate">key-a</code>.</li>
<li>Can't send <code class="notranslate">msg-2</code> - <code
class="notranslate">msg-3</code> to <code class="notranslate">c3</code> because
<code class="notranslate">1:2</code>, and <code class="notranslate">1:3</code>
are greater than the last sent position, <code
class="notranslate">1:1</code>.</li>
<li>Disconnect <code class="notranslate">c1</code>.</li>
<li>Send <code class="notranslate">msg-1</code> - <code
class="notranslate">msg-3</code> to <code
class="notranslate">c3</code>.<br>Now, <code class="notranslate">c3</code>
receives messages with expected order about <code
class="notranslate">key-a</code>.</li>
</ol>
**scenario B**
<ol start="3">
<li><code class="notranslate">c1</code> <code
class="notranslate">messagesForC</code> is back to 999.</li>
<li>Send <code class="notranslate">msg-2</code> - <code
class="notranslate">msg-3</code> to <code class="notranslate">c1</code></li>
<ul dir="auto">
<li>So, the current last sent position is <code
class="notranslate">1:6</code>, and the individually sent positions is <code
class="notranslate">[]</code>.</li>
</ul>
</ol>
My motivation is not to improve Key_Shared but fix Key_Shared.
Therefore, if the current approach is not accepted as a fix, I introduce
it and remove the feature by Map per key.
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