Server Setup

Before using @defer in queries, you have to:

• Update graphql and graphql-pro gems
• Add @defer to your GraphQL schema
• Update your HTTP handlers (eg, Rails controllers) to send streaming responses
• Optionally, customize @defer to work with GraphQL-Batch

You can also see a full Rails & Apollo-Client demo.

Updating the gems

GraphQL-Ruby 1.9+ and GraphQL-Pro 1.10+ are required:

gem "graphql", "~>1.9"
gem "graphql-pro", "~>1.10"

And then install them:

$ bundle update graphql graphql-pro

Adding @defer to your schema

Then, add GraphQL::Pro::Defer to your schema as a plugin:

class MySchema < GraphQL::Schema
  use GraphQL::Pro::Defer
end

This will:

• Attach a custom directive called @defer
• Add instrumentation to queries to track deferred work and execute it later

Sending streaming responses

Many web frameworks have support for streaming responses, for example:

• Rails has ActionController::Live
• Sinatra has Sinatra::Streaming
• Hanami::Controller can stream responses

See below for how to integrate GraphQL’s deferred patches with a streaming response API.

To investigate support with a web framework, please open an issue or email support@graphql.pro.

Checking for deferrals

When a query has any @defered fields, you can check for context[:defer]:

if context[:defer]
  # some fields were `@defer`ed
else
  # normal GraphQL, no `@defer`
end

Working with deferrals

To handle deferrals, you can enumerate over context[:defer], for example:

context[:defer].each do |deferral|
  # do something with the `deferral`, eg
  # stream_to_client(deferral.to_h)
end

The initial result is also present in the deferrals, so you can treat it just like a patch.

Each deferred patch has a few methods for building a response:

• .to_h returns a hash with path:, data:, and/or errors:. (There is no path: for the root result.)
• .to_http_multipart(incremental: true) returns a string which works with Apollo client’s @defer support. (Use incremental: true to format patches for the forthcoming spec.)
• .path returns the path to this patch in the response
• .data returns successfully-resolved results of the patch
• .errors returns an array of errors, if there were any

Calling .data or .errors on a deferral will resume GraphQL execution until the patch is complete.

Example: Rails with Apollo Client

In this example, a Rails controller will stream HTTP Multipart patches to the client, in Apollo Client’s supported format.

class GraphqlController < ApplicationController
  # Support `response.stream` below:
  include ActionController::Live

  def execute
    # ...
    result = MySchema.execute(query, variables: variables, context: context, operation_name: operation_name)

    # Check if this is a deferred query:
    if (deferred = result.context[:defer])
      # Required for Rack 2.2+, see https://github.com/rack/rack/issues/1619
      response.headers['Last-Modified'] = Time.now.httpdate
      # Use built-in `stream_http_multipart` with Apollo-Client & ActionController::Live
      deferred.stream_http_multipart(response, incremental: true)
    else
      # Return a plain, non-deferred result
      render json: result
    end
  ensure
    # Always make sure to close the stream
    response.stream.close
  end
end

You can also investigate a full Rails & Apollo-Client demo

With GraphQL-Batch

GraphQL-Batch is a third-party data loading library that wraps GraphQL-Ruby execution. Deferred resolution happens outside the normal execution flow, so to work with GraphQL-Batch, you have to customize GraphQL::Pro::Defer a bit. Also, you’ll need GraphQL-Pro v1.24.6 or later. Here’s a custom Defer implementation:

# app/graphql/directives/defer.rb
module Directives
  # Modify the library's `@defer` implementation to work with GraphQL-Batch
  class Defer < GraphQL::Pro::Defer
    def self.resolve(obj, arguments, context, &block)
      # While the query is running, store the batch executor to re-use later
      context[:graphql_batch_executor] ||= GraphQL::Batch::Executor.current
      super
    end

    class Deferral < GraphQL::Pro::Defer::Deferral
      def resolve
        # Before calling the deferred execution,
        # set GraphQL-Batch back up:
        prev_executor = GraphQL::Batch::Executor.current
        GraphQL::Batch::Executor.current ||= @context[:graphql_batch_executor]
        super
      ensure
        # Clean up afterward:
        GraphQL::Batch::Executor.current = prev_executor
      end
    end
  end
end

And update your schema to use your custom defer implementation:

# Use our GraphQL-Batch-compatible defer:
use Directives::Defer

Next Steps

Read about client usage of @defer.