graphql-engine

This note is in Hasura.GraphQL.Execute.Action.Subscription. It is referenced at:

Async action subscriptions

We have two kinds of async action query execution based on the defined relationships on output object type. See Note [Resolving async action query]. Actions with table relationships on output object type have to a generate SQL select query with the action webhook response embedded in so as to execute on the source database that emits client response with joined relationship rows (see Note [Resolving async action query]). We can multiplex this queries and run a live subscription just like our usual database queries. But the action log from the metadata storage is subjected to change after calling action webhook in two ways,

  1. When the webhook returns successful response, ‘response_payload’ column value isn’t null
  2. When any exception occurs in the process, ‘errors’ column value isn’t null

So, we have a background thread (@’asyncActionSubscriptionsProcessor’) which constantly fetches the action log response from the metadata storage and restarts the live query to reflect those changes in subscriptions.

In case of action queries without relationships, there are no SQL queries associated with. We can’t multiplex them. The thread, @’asyncActionSubscriptionsProcessor’ also handles these action subscriptions by sending the responses to the websocket client after fetching the action log response.

We can’t support multiple async query fields of different kinds in a single subscription.

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