PranaviAncha commented on code in PR #3085:
URL: https://github.com/apache/helix/pull/3085#discussion_r2592103127
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website/1.4.3/src/site/markdown/tutorial_messaging.md:
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@@ -25,6 +25,26 @@ under the License.
In this chapter, we\'ll learn about messaging, a convenient feature in Helix
for sending messages between nodes of a cluster. This is an interesting
feature that is quite useful in practice. It is common that nodes in a
distributed system require a mechanism to interact with each other.
+### Performance Considerations
+
+**IMPORTANT:** When using the messaging API with `Criteria`, be aware of the
following performance characteristics:
+
+- **ExternalView Scanning:** By default, the messaging service uses
`DataSource.EXTERNALVIEW` to resolve criteria. This can scan **all**
ExternalView znodes in the cluster, even when targeting specific instances. At
high resource cardinality (thousands of resources), this can cause severe
performance degradation.
+
+**Recommended Patterns:**
+
+- **Use `DataSource.LIVEINSTANCES`** when you only need to target live
instances and do not require resource/partition-level filtering. This is much
faster and more efficient.
+- **Specify exact resource names** instead of wildcards if you must use
ExternalView scanning.
+
+Example of efficient messaging:
+```java
+Criteria recipientCriteria = new Criteria();
+recipientCriteria.setInstanceName("instance123");
+recipientCriteria.setRecipientInstanceType(InstanceType.PARTICIPANT);
+recipientCriteria.setDataSource(DataSource.LIVEINSTANCES); // Efficient:
avoids EV scan
Review Comment:
```suggestion
recipientCriteria.setDataSource(DataSource.LIVEINSTANCES);
```
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