Kafka Heartbeat Request Wire Format v4 — Binary Protocol Layout

What is Heartbeat?

Classic-protocol consumers send this on a fixed interval to prove liveness to the group coordinator. Missing a heartbeat within session.timeout.ms kicks the member and triggers a rebalance, usually because consumers spend too long in poll() processing, starving the heartbeat thread.

Schema
{
  "GroupId": string,
  "GenerationId": int32,
  "MemberId": string,
  "GroupInstanceId": string?
}

Example
{
  "GroupId": "order-processors",
  "GenerationId": 5,
  "MemberId": "consumer-1-abc123",
  "GroupInstanceId": null
}