heartbeat fail // cannot connect

[me2017]

I'm not finding much detail as to what this means exactly, or if it requires remediation...

Upon stop/start/restart of agent (eg, systemctl restart zabbix-agent2) I see this in the logs:

2023/12/11 08:08:38.885244 Plugin communication protocol version is 6.2.7
2023/12/11 08:08:38.885266 Zabbix Agent2 hostname: [ssh02]
2023/12/11 08:08:42.887683 [101] cannot connect to [florax.somewhere.io:10051]: dial tcp :0->192.168.1.25:10051: i/o timeout
2023/12/11 08:08:42.887708 [101] active check configuration update from host [ssh02] started to fail
2023/12/11 08:08:45.888858 [101] cannot connect to [florax.somewhere.io:10051]: dial tcp :0->192.168.1.25:10051: i/o timeout
2023/12/11 08:08:45.889099 [101] sending of heartbeat message for [ssh02] started to fail

It isn't clear to me if there's a silent recovery from this because my troubleshooting and other observations show that everything is fine:

- I'm able to connect from the agent to the zabbix server on the specified port
- I've verified the specified port is listening on the server
- tcpdump on the server shows connections being made to the specified port
- status and availability on all hosts, including the one noted here is enabled, green, and happy
- no problems are reported on the zabbix server

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I'd like to know the following:

- what is a hearbeat message?
- is there some other means I can make this connection to determine the message is not currently valid?
- is the recovery silent (not written to log)?