Heartbeat on SSE connections

[jerem1e]

Component(s)

router

Is your feature request related to a problem? Please describe.

When using WunderGraph SSE subscriptions, connections often pass through intermediate gateways, proxies, or load balancers. These components typically have idle timeout configurations that automatically close seemingly inactive connections after a certain period (e.g., 60-120 seconds).

Describe the solution you'd like

Implement a configurable feature within the WunderGraph Gateway/Router to periodically send lightweight SSE comment heartbeats (:\n\n) on active subscription connections. E.g. like this:

subscriptions:
  sse:
    enableHeartbeats: true
    heartbeatInterval: 30s # send a heartbeat every 30 seconds

Describe alternatives you've considered

No response

Additional context

Here is a mozilla guide about SSE where this is mentioned: https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#event_stream_format

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[lghinet]

Hello, you can enable heartbeats but looking at the code, only on accept content-type : "multipart/mixed" and not on "text/event-stream"

https://github.com/wundergraph/graphql-go-tools/blob/master/v2/pkg/engine/resolve/context.go#L37

https://github.com/wundergraph/cosmo/blob/main/router/core/flushwriter.go#L162

also your client needs to be able to handle heartbeats (empty data {} ) https://www.apollographql.com/docs/graphos/routing/operations/subscriptions/multipart-protocol#heartbeats

[endigma]

Hi @jerem1e, thanks for the issue, this is a great suggestion. We'll take a look at implementing this internally.