Moquette and Greengrass NullPointerException

Question

When subscribing to the MQTT on a Greengrass V2 core device running Moquette version 2.2.0 and I'm trying to subscribe from a device client I see this error in the logs:

```
Caused by: java.lang.NullPointerException
	at io.moquette.broker.PostOffice.subscribeClientToTopics(PostOffice.java:236)
	at io.moquette.broker.MQTTConnection.lambda$processSubscribe$6(MQTTConnection.java:402)
	at io.moquette.broker.SessionCommand.execute(SessionCommand.java:23)
	at io.moquette.broker.PostOffice.lambda$routeCommand$5(PostOffice.java:635)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)

```

It seems there may be a bug with this implementation as shown [https://github.com/moquette-io/moquette/issues/473](here). That issue describes a setting named "cleanSession" that can be tried but it does not seem to be an option as documented [https://github.com/moquette-io/moquette/blob/main/distribution/src/main/resources/moquette.conf](here). Is there a workaround for this? I've tried switching over to the **aws.greengrass.clientdevices.mqtt.EMQX ** broker instead but I can't seem to get that one to deploy. During deployment I get the following error:

```
FAILED_NO_STATE_CHANGE: NoAvailableComponentVersionException: No local or cloud component version satisfies the requirements. Check whether the version constraints conflict and that the component exists in your AWS account with a version that matches the version constraints. If the version constraints conflict, revise deployments to resolve the conflict. Component aws.greengrass.clientdevices.mqtt.EMQX version constraints: thinggroup/GreengrassPIDevices requires =1.0.0.
```

According to the component it requires the following:

```
aws.greengrass.clientdevices.Auth	>=2.2.0 <2.3.0
```
and I have version 2.2.0 installed on my core device.

I'm looking for suggestions on how to get around this issue. Thanks.

Answer

Hi Al,

The Moquette bug you linked is very old and likely doesn't apply anymore. I'll ping the maintainer to close it.

Can you provide complete logs leading up to that NPE along with specific steps to reproduce? [This](https://github.com/aws-greengrass/aws-greengrass-moquette-mqtt/blob/3d10aa51170fbf5c681cc16deef9cff904a7ad2c/moquette-0.16/broker/src/main/java/io/moquette/broker/PostOffice.java#L236) is the line throwing the exception. It indicates that there is no MQTT session with the provided client ID, which would suggest your client is getting disconnected at some point after subscribing. We need more context to dig in further.

Consider opening an issue with us in [Github](https://github.com/aws-greengrass/aws-greengrass-moquette-mqtt/issues/new/choose).

Also, if you haven't already, take a look at our [developer guide](https://docs.aws.amazon.com/greengrass/v2/developerguide/connect-client-devices.html) for setting up client devices.

Thanks,
-joe

Answer

Hello,

First for Moquette, `cleanSession` is an option which you set in your MQTT client when connecting to the MQTT broker. Review your MQTT client's documentation for setting `cleanSession` to false or 0.

EMQX is available for Windows on x64 CPUs and on Linux for x64 and arm aarch64 CPUs. Given the error message that you received, it is most likely that you are using Linux on armv7 which is not one of the supported platforms for EMQX.

Moquette and Greengrass NullPointerException

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