How can I get GraphQL subscriptions working?

[vladimirlogachev]

There are 2 GraphQL examples, and there are 2 instances of RunDocument in graphql/src/Mu/GraphQL/Query/Run.hs which require (Proxy @('Just sub)).

• library example (the one with DB) uses one instance (let's call it instance A)

instance
  ( p ~ 'Package pname ss
  , KnownSymbol qr
  , RunQueryFindHandler m p hs chn ss (LookupService ss qr) hs
  , MappingRight chn qr ~ ()
  , KnownSymbol mut
  , RunQueryFindHandler m p hs chn ss (LookupService ss mut) hs
  , MappingRight chn mut ~ ()
  , KnownSymbol sub
  , RunQueryFindHandler m p hs chn ss (LookupService ss sub) hs
  , MappingRight chn sub ~ ()
  , Intro.Introspect p ('Just qr) ('Just mut) ('Just sub)
  ) => RunDocument p ('Just qr) ('Just mut) ('Just sub) m chn hs where
  runDocument f req svr d
    = let i = Intro.introspect (Proxy @p) (Proxy @('Just qr)) (Proxy @('Just mut)) (Proxy @('Just sub))
      in case d of
           QueryDoc q
             -> runQuery f req i svr [] () q
           MutationDoc q
             -> runQuery f req i svr [] () q
           SubscriptionDoc _
             -> pure $ singleErrValue "cannot execute subscriptions in this wire"
  runDocumentSubscription f req svr (SubscriptionDoc d)
    = runSubscription f req svr [] () d
  runDocumentSubscription f req svr d = yieldDocument f req svr d

• library-graphql example (without DB) uses another instance (let's call it instance B)

instance
  ( p ~ 'Package pname ss
  , KnownSymbol qr
  , RunQueryFindHandler m p hs chn ss (LookupService ss qr) hs
  , MappingRight chn qr ~ ()
  , KnownSymbol sub
  , RunQueryFindHandler m p hs chn ss (LookupService ss sub) hs
  , MappingRight chn sub ~ ()
  , Intro.Introspect p ('Just qr) 'Nothing ('Just sub)
  ) => RunDocument p ('Just qr) 'Nothing ('Just sub) m chn hs where
  runDocument f req svr d
    = let i = Intro.introspect (Proxy @p) (Proxy @('Just qr)) (Proxy @'Nothing) (Proxy @('Just sub))
      in case d of
           QueryDoc q
             -> runQuery f req i svr [] () q
           SubscriptionDoc _
             -> pure $ singleErrValue "cannot execute subscriptions in this wire"
  runDocumentSubscription f req svr (SubscriptionDoc d)
    = runSubscription f req svr [] () d
  runDocumentSubscription f req svr d = yieldDocument f req svr d

Both examples return cannot execute subscriptions in this wire.
I don't quite understand which instance should I aim to use (If I understand it right, there are only 2 for subscriptions defined).

Clients I tried to use:

1. graphqurl
2. graphiql

Queries:

• for library

subscription whatever {
  allBooks {
    id
    title
  }
}

• for library-graphql

subscription whatever {
  books {
    id
    title
  }
}

(Though I suppose it's a server-side issue, not a client-side).

Please help me understand.

[serras]

How does your server schema look like? The current instances only allow you yo have subscriptions also if you have queries, but there's nothing technical stopping us from creating one only for subscriptions.

[vladimirlogachev]

How does your server schema look like?

It's about 2 existing mu-haskell examples. I can't use subscriptions for some reason, which I don't understand.

But I can use graphql subscriptions "in general" - I mean, on the client-side.

[serras]

Usually that means you are asking for subscriptions via HTTP. You need to use a streaming transport, such as WebSockets, to make it work. In most cases, it's as easy as changing http:// for ws://.

[serras]

I am actually not sure how it works over HTTP, if you know of a good pointer to the spec. we could consider adding that support.

[serras]

For example, Apollo requires WebSockets, see the last line in the code block here

[vladimirlogachev]

There is a GraphQL over WebSocket Protocol. I would like to mention it here as long as it's related to the topic.

But I still can't figure out how to run subscriptions on existing examples (they are supposed to be working over ws://, right?).

[serras]

Something weird is indeed happening, and I don't know what it is. The code for accepting the connection works OK (I added a few Debug.Trace.traces around). However, whenever receiveJSON in Subscriptions.hs is run, there's always a ConnectionClosed exception trying to receive any message from the client. In theory, that exception is only thrown when the client unilaterally closes the connection (although there are bug reports stating that sometimes those are also raised when the connection is closed gracefully); but here it's always thrown.

Unfortunately, I think we really need to dig deeper, maybe writing a few examples using the websockets library directly; some change in network may have changed how messages are received. We also depend on wai-websockets to expose the server via Warp, this seems like a very thing layer and not so probable, but who knows...

[vladimirlogachev]

git checkout bfdf5f4

git clean -dxf
stack run library-graphql

Running the simplest example with runGraphQLApp 8000 libraryServer without any headers middleware, Prometheus, database or logger.

...and it still doesn't work.

Do I understand right that it's because dependencies are not specified by their exact versions?

[serras]

I don’t think so, there seems to be some problem on the way we use websockets, which somehow worked on previous versions. We would need to test the example against the LTS 14 and see whether that works on that environment.