I'm trying to make a GraphQL interface on top of the data coming from a very slow physical device (I have both a solar converter and a heatpump that both only speak modbus).

The root problem I have is that these devices have many fields available (my solarconverter has a few hundred fields) and there is a limit on modbus of a maximum of only 125 fields per call. On some of my devices such a call sometimes takes almost a second.

To allow getting the data as often as possible I want to only fetch the fields from the real device for which there is an actual need in either a query or subscription.

In a related project (not modbus, so I get the full record in all cases) I have used the Spring Graphql library and used a QueryMapping and a SubscriptionMapping to implement the query and subscription.

This works fine but I do not see a way to get the data I think I need.

For the modbus case I see a need to have

• A way to know which fields are requested in both the query and subscription scenarios.
• Have hooks/handlers for starting and stopping the subscription that can register/unregister the need for the queried fields towards the (slow) backend.

The only example code I have found so far that does something like this was this ~7 year old StockTickerWebSocket example.
This does however use something completely different than Spring-graphql.

What I'm looking for a some guidance going into the right direction.

• Can what I need be done with spring-graphql?
• If it cannot be done with spring-graphql (yet) then is the example I found still the best way forward?
• If this is not the best way; what is?

In all of the above: Any example/javadoc/code link that points me towards how to do this is highly appreciated.

Thanks.