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+# 9. js-idle-heartbeat
+
+Date: 2021-06-30
+
+## Status
+
+Accepted
+
+## Context
+
+The JetStream ConsumerConfig option `idle_heartbeat` enables server-side
+heartbeats to be sent to the client. To enable the option on the consumer simply
+specify it with a value representing the number of nanoseconds that the server
+should use as notification interval.
+
+The server will only notify after the specified interval has elapsed and no new
+messages have been delivered to the consumer. Delivering a message to the
+consumer resets the interval.
+
+The idle heartbeats notifications are sent to the consumer's subscription as a
+regular NATS message. The message will have a `code` of `100` with a
+`description` of `Idle Heartbeat`. The message will contain additional headers
+that the client can use to re-affirm that it has not lost any messages:
+
+- `Nats-Last-Consumer` indicates the last consumer sequence delivered to the
+  client. If `0`, no messages have been delivered.
+- `Nats-Last-Stream` indicates the sequence number of the newest message in the
+  stream.
+
+Here's an example of a client creating a consumer with an idle_heartbeat of 10
+seconds, followed by a server notification.
+
+```
+$JS.API.CONSUMER.CREATE.FRHZZ447RL7NR8TAICHCZ6 _INBOX.FRHZZ447RL7NR8TAICHCQ8.FRHZZ447RL7NR8TAICHDQ0 136␍␊
+{"config":{"ack_policy":"explicit","deliver_subject":"my.messages","idle_heartbeat":10000000000},
+"stream_name":"FRHZZ447RL7NR8TAICHCZ6"}␍␊
+...
+
+> HMSG my.messages 2  75 75␍␊NATS/1.0 100 Idle Heartbeat␍␊Nats-Last-Consumer: 0␍␊Nats-Last-Stream: 0␍␊␍␊␍␊
+alive - last stream seq: 0 - last consumer seq: 0
+```
+
+This feature is intended as an aid to clients to detect when they have been
+disconnected. Without it the consumer's subscription may sit idly waiting for
+messages, without knowing that the server might have simply gone away and
+recovered elsewhere.
+
+## Consequences
+
+Clients can use this information to set client-side timers that track how many
+heartbeats have been missed and perhaps take some action such as re-create a
+subscription to resume messages.