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• One optimization possibility is to materialize the data provided to the original node at Step 6. This would allow for changes to an event in This KTable which does not change the foreign key to shortcut sending an event to the Other node. The tradeoff is the need to maintain a materialized state store.

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Joiner Propagation of Stale Data

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While it is possible for a stale event to be propagated (ie: matches the foreign key, but is stale), the up-to-date event will be propagated when it eventually arrives. Entity changes do not cause the same race conditions that event-changes do, and so are not of a concern.

Original Value Hash

A hash value is computed for each message propagated from This KTable. It is passed through the entire process and returns to the co-located KTable in the final step. Before joining, a lookup is done to validate that the hash value is identical. If the hash is not identical, this indicates that the event has since changed, even if the primary and foreign key remain the same. This indicates that the value should be discarded, lest a duplicate be printed.

Example:

```

Start with two empty tables:

LHS is This KTable
RHS is Other KTable

1- RHS is updated to Y|bar
2- LHS is updated to A|Y,1
   -> sends Y|A+ subscription message to RHS
3- LHS is updated to A|Y,2
   -> sends Y|A- unsubscribe message to RHS
   -> sends Y|A+ subscription to RHS
4- RHS processes first Y|A+ message
   -> sends A|Y,bar back
5- LHS processes A|Y,bar and produces result record A|Y,2,bar.
6- RHS processes Y|A- unsubscribe message (update store only)
7- RHS processes second Y|A+ subscribe message
   -> sends A|Y,bar back
8- LHS processes A|Y,bar and produces result record A|Y,2,bar

Thus, the first result record, that should have been `A|Y,1,bar`, is now
`A|Y,2,bar`. Furthermore, we update result table from `A|Y,2,bar` to
`A|Y,2,bar`.

By using a hash function to compare in step 5, we can instead see that the message is stale and should be discarded, lest we produce double output (or more, in the case of a rapidly changing set of events).
```

Compatibility, Deprecation, and Migration Plan

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