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# troubleshootings
Slackbot
05/24/2023, 11:25 AMThis message was deleted.
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Didip Kerabat
05/24/2023, 9:55 PMThere is a heartbeat http call that checks for historical’s liveliness.
Didip Kerabat
05/24/2023, 9:56 PMif using ZK, the ZK client provides the liveliness check.
Didip Kerabat
05/24/2023, 9:58 PMwhen it’s down temporarily, then broker will ask coordinator where is the other replica.
when it’s down for longer time, I believe there’s a flagging system that mark the node as down. Then coordinator’s background thread will pick another historical and starts a new replication process.
This is just in the nutshell. Others know much more than me.
p
Pavel Belenkovich
05/29/2023, 6:30 AMthanx @Didip Kerabat!
Do you know how is this
then broker will ask coordinator where is the other replicag
Gian Merlino
05/31/2023, 12:20 AMslight correction— Broker will ask either ZK, or the historicals directly, depending on whether you use ZK or HTTP based segment announcing
Gian Merlino
05/31/2023, 12:21 AMin particular— Coordinator tells Historicals which segments to load, and Broker gets that info from ZK or HTTP calls to Historicals, depending on what kind of segment announcing you use. Recently the default changed to HTTP, so if you use the current version of Druid it'll be HTTP
Gian Merlino
05/31/2023, 12:21 AMhope this answers your question
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Pavel Belenkovich
05/31/2023, 1:38 PMthanx @Gian Merlino!
I'm still a bit not sure about the timings.
we use ZK.
So lets say some historical goes down at 19:00, and after 10 seconds it's records disappear from ZK.
When will the broker get to know it? I guess it doesn't fetch the segments info from ZK for each query - but rather periodically. So how frequent is it? Can it be controlled?
g
Gian Merlino
05/31/2023, 3:09 PMthe Broker uses ZK watches to stay in sync, so it will know about the update almost immediately after ZK does
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