What I am trying to do is publish a message from a device in one registry to a device in another registry.
What I found out is that you can only publish to topics in the registry your device is in and if you try to publish to a topic in another registry you get a EOFException and you get disconnected from the MQTT server.
The flow I was trying to go for is I have a registry of mobile devices (phones) that have their own topics to do things but they also need to communicate to a hub registry that communicates with IoT devices (gets/updates device information) which also have their own IoT registry
As I said I can publish messages to topics in my registry without error but as soon as I try to do it outside my registry I get an error.
Is there no way to have inter-registry communication?
What is a "standard" way to communicate between devices through different topics if you cant do cross registry?
You can achieve your goal using Cloud Functions that handle the data received thought Cloud IoT Core and PubSub in a registry A then sending this data as a configuration message do a device on the registry B.
The flow will be something like: Device 1 > MQTT Server Registry A > Pub/Sub Topic Registry A > Cloud Functions > HTTP Config message to > Registry B > MQTT Server Registry B > Devices 2
You can see in this tutorial that I wrote about Cloud IoT Core and I show how to send configuration messages to the devices. In your case, the only change that you need is that the config message will go to a device in a different registry.
Link to the tutorial: https://medium.com/google-cloud/gps-cellular-asset-tracking-using-google-cloud-iot-core-firestore-and-mongooseos-4dd74921f582
Code part that sends config messages, but in my code, the registry is a constant variable: https://github.com/alvarowolfx/asset-tracker-gcp-mongoose-os/blob/master/functions/index.js#L22
Hope that this can help you.