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{"@context":"https://schema.org","@type":"QAPage","mainEntity":{"@type":"Question","name":"[Spring Graphql] Knowing which fields are requested by a query/subscription?","text":"
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).
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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.
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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.
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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.
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This works fine but I do not see a way to get the data I think I need.
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For the modbus case I see a need to have
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A way to know which fields are requested in both the query and subscription scenarios.
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Have hooks/handlers for starting and stopping the subscription that can register/unregister the need for the queried fields towards the (slow) backend.
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The only example code I have found so far that does something like this was this ~7 year old  StockTickerWebSocket example.
\nThis does however use something completely different than Spring-graphql.
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What I'm looking for a some guidance going into the right direction.
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Can what I need be done with spring-graphql?
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If it cannot be done with spring-graphql (yet) then is the example I found still the best way forward?
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If this is not the best way; what is?
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In all of the above: Any example/javadoc/code link that points me towards how to do this is highly appreciated.
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Thanks.
","upvoteCount":1,"answerCount":1,"acceptedAnswer":{"@type":"Answer","text":"
The bottom of this article is about you need I believe. I tried it a few weeks ago in a Spring 3 app and the context includes the fields got queried.
","upvoteCount":1,"url":"https://github.com/graphql-java/graphql-java/discussions/3444#discussioncomment-8476584"}}}