SEPURI-SAI-KRISHNA commented on issue #12405:
URL: https://github.com/apache/seatunnel/issues/12405#issuecomment-5748103671
Thanks for the report, @zixiao144. This reproduces from the code alone, with
no broker needed, so I have opened #12406 with a fix. If you would rather
submit it yourself, say so and I will close mine.
**Root cause.** `MqttSource` is a `SeaTunnelSource`, and `SeaTunnelSource`
extends `Serializable` (`SeaTunnelSource.java:39`). It holds its configuration
as a field:
```java
public class MqttSource extends AbstractSingleSplitSource<SeaTunnelRow> {
private final MqttSourceConfig sourceConfig; // MqttSource.java:38
```
but the configuration class does not implement `Serializable`:
```java
public class MqttSourceConfig { //
MqttSourceConfig.java:24
```
So Zeta cannot serialize the logical DAG, and Hazelcast reports that as the
`LogicalVertex` failure you saw. Serializing an `MqttSource` on current `dev`
(`5f7f5c1d`) fails with:
```
java.io.NotSerializableException:
org.apache.seatunnel.connectors.seatunnel.mqtt.source.MqttSourceConfig
```
Every field of that class is a `String`, `int` or `boolean`, so nothing
about its contents is the problem. Only the class declaration is missing, and
adding `implements Serializable` is enough.
**The JDBC sink is not involved.** I checked the MQTT sink as well: it holds
`ReadonlyConfig`, `SeaTunnelRowType` and `CatalogTable`, all already
serializable, and round-trips fine. Only the source path is broken, so the same
failure should occur with any sink, not just Jdbc.
**Why it went unnoticed.** The connector has an E2E module, but it contains
only `MqttSinkIT`. There is no source IT, so the only end-to-end coverage
exercises the side that works. The source appears to have been in this state
since it was added in #10935 on 2026-06-08.
The PR adds a regression test to the existing `MqttSourceTest` that writes
an `MqttSource` through an `ObjectOutputStream`. It fails with the exception
above on unmodified `dev` and passes with the change.
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