GraphQL subscription updates may broadcast data to multiple subscribers.

A broadcast is a subscription update which is executed once, then delivered to any number of subscribers. This reduces the time your server spends running GraphQL queries, since it doesn’t have to re-run the query for every subscriber.

But, take care: this approach risks leaking information to subscribers who shouldn’t receive it.


To enable broadcasts, add broadcast: true to your subscription setup:

class MyAppSchema < GraphQL::Schema
  # ...
  use SomeSubscriptionImplementation,
    broadcast: true # <----

Then, any broadcastable field can be configured with broadcastable: true:

field :name, String, null: false,
  broadcastable: true

When a subscription comes in where all of its fields are broadcastable: true, then it will be handled as a broadcast.

Additionally, you can set default_broadcastable: true:

class MyAppSchema < GraphQL::Schema
  # ...
  use SomeSubscriptionImplementation,
    broadcast: true,
    default_broadcastable: true # <----

With this setting, fields are broadcastable by default. Only a field with broadcastable: false in its configuration will cause a subscription to be handled on a subscriber-by-subscriber basis.

What fields are broadcastable?

GraphQL-Ruby can’t infer whether a field is broadcastable or not. You must configure it explicitly with broadcastable: true or broadcastable: false. (The subscription plugin also accepts default_broadcastable: true|false.)

A field is broadcastable if all clients who request the field will see the same value. For example:

For fields like this, you can add broadcastable: true.

A field is not broadcastable if its value is different for different clients. For example:

These fields can be tagged with broadcastable: false so that GraphQL-Ruby will handle them on a subscriber-by-subscriber basis.

If you want to use subscriptions but have a lot of non-broadcastable fields in your schema, consider building a new set of subscription fields with limited access to other schema objects. Instead, optimize those subscriptions for broacastability.

Under the Hood

GraphQL-Ruby determines which subscribers can receive a broadcast by inspecting:

So, take care to set subscription_scope whenever a subscription should be implicitly scoped!

(See GraphQL::Subscriptions::Event#fingerprint for the implementation of broadcast fingerprints.)

Checking for Broadcastable

For testing purposes, you can confirm that a GraphQL query string is broadcastable by using Subscriptions#broadcastable?:

subscription_string = "subscription { ... }"
# => true or false

Use this in your application’s tests to make sure that broadcastable fields aren’t accidentally made non-broadcastable.

Connections and Edges

You can configure your generated Connection and Edge types to be broadcastable by setting default_broadcastable(true) in their definition:

# app/types/base_connection.rb
class Types::BaseConnection < Types::BaseObject
  include GraphQL::Types::Relay::ConnectionBehaviors

# app/types/base_edge.rb
class Types::BaseEdge < Types::BaseObject
  include GraphQL::Types::Relay::EdgeBehaviors

(In your BaseObject, you should also have connection_type_class(Types::BaseConnection) and edge_type_class(Types::BaseEdge).)

PageInfo is broadcastable by default.