Forum Discussion

Brass Contributor

Nov 21, 2019

Solved

Need Heartbeat Query

Hi Team, I am trying to write a KQL query to catch if any single heartbeat missed. Like we could see in my below screenshot, this server is sending heartbeat after every minute interval. And no...

Nov 22, 2019

personally I prefer the example query of

// Availability rate
// Calculate the availability rate of each connected computer
Heartbeat
// bin_at is used to set the time grain to 1 hour, starting exactly 24 hours ago
| summarize heartbeatPerHour = count() by bin_at(TimeGenerated, 1h, ago(24h)), Computer
| extend availablePerHour = iff(heartbeatPerHour > 0, true, false)
| summarize totalAvailableHours = countif(availablePerHour == true) by Computer 
| extend availabilityRate = totalAvailableHours*100.0/24

Heartbeats are expected to be missed (pauses, glitches, load etc...) and the data will catch-up - so you may get false positives.

You can use a date_diff to compare
Go to Log Analytics and Run Query

Heartbeat
| where TimeGenerated >= ago(1h)
| where Computer == "hardening-demo"
| project Computer, TimeGenerated
| order by TimeGenerated desc
| project n = TimeGenerated, nminus = prev(TimeGenerated), TimeGenerated, Computer
| where isnotempty(nminus)
// show time NOW vs time  n -1 row
| extend second = datetime_diff('second',nminus, n)
| where second >= 60

Results for seconds below 60 (mainly 9 and 51 for the demo data) - just remove the last line of the above query to see this

n	nminus	TimeGenerated	Computer	second
2019-11-22T17:42:37.88Z	2019-11-22T17:42:46.523Z	2019-11-22T17:42:37.88Z	hardening-demo	9
2019-11-22T17:41:46.52Z	2019-11-22T17:42:37.88Z	2019-11-22T17:41:46.52Z	hardening-demo	51
2019-11-22T17:41:37.877Z	2019-11-22T17:41:46.52Z	2019-11-22T17:41:37.877Z	hardening-demo	9
2019-11-22T17:40:46.52Z	2019-11-22T17:41:37.877Z	2019-11-22T17:40:46.52Z	hardening-demo	51
2019-11-22T17:40:37.873Z	2019-11-22T17:40:46.52Z	2019-11-22T17:40:37.873Z	hardening-demo	9

ScottAllison

Iron Contributor

Jan 28, 2020

CliveWatson Just to add to this conversation, I've come up with a slightly different way of doing this--would love feedback:

let current = now();
let ostype = 'Windows';
let computername = '';
let environment = 'Non-Azure';
let threshold = 600;
Heartbeat
| where TimeGenerated >= ago(1h)
// --for a specific computer:
| where Computer contains computername
// --for a specific computer group:
//| where Computer in (group)
// --for a specific OS type:
| where OSType contains ostype
// --for on-prem or Azure VMs:
| where ComputerEnvironment contains environment
| project Computer, TimeGenerated, current
| order by TimeGenerated desc
| project nminus = prev(TimeGenerated), current, Computer
| where isnotempty(nminus)
| extend ['LastHeartbeat (in seconds)'] = datetime_diff('second', current, nminus)
| summarize arg_max(nminus, *) by Computer
| where ['LastHeartbeat (in seconds)'] >= threshold
| project Computer, QueryTime = current, LastTimeStamp = nminus, ['LastHeartbeat (in seconds)']

CliveWatson

Former Employee

Jan 29, 2020

ScottAllison

Looks good ScottAllison , I would just swap contains to has as per best practise https://docs.microsoft.com/en-us/azure/kusto/query/best-practices

ScottAllison
Iron Contributor
Jan 29, 2020
CliveWatson Thanks! I've seen weirdness with has versus contains. I haven't noted what that weirdness is, but if I run across it again, I'll be sure to share.
- ScottAllison
  Iron Contributor
  Feb 03, 2020
  CliveWatson - here's a perfect example of why the HAS operator isn't useful for many operations:
  
  This query returns the expected results every time:
  Heartbeat | where Computer contains 'abc' | distinct Computer
  For example, this would return:
  SERVERABC1
  SERVERABC2
  COMPUTERABC24
  
  When I replace CONTAINS with HAS, I get 0 results. So in 99% of my use cases, HAS doesn't work at all.
  - CliveWatson
    Former Employee
    Feb 03, 2020
    ScottAllison
    
    That is the behavior I'd expect
    
    From the docs:
    Prefer has operator over contains when looking for full tokens. has is more performant as it doesn't have to look-up for substrings.
    
    What does that mean in practice:
    
    1. This query example will fail (as its not a substring). Computers named: aks-nodepool1.nnnnnnnnn
    
    Go to Log Analytics and run query
    
    Heartbeat | where Computer has 'pool' | distinct Computer
    
    Note: if you used "nodepool1" it would work
    
    Where as this works ("aks" is a full string match(full token)
    
    Go to Log Analytics and run query
    
    Computer
    
    aks-nodepool1-25494468-4
    
    aks-nodepool1-25494468-1
    
    So on a small dataset it wont matter if you use contains vs. has - however on a large one it could improve perf.
    
    When I create a query I will often start with a "contains" but will (If I remember! Sorry if you find a query from me that isn't optomised) then check and swap to a "has" if that works - but you need to evaluate on a case by case.
    
    Essentially KQL only scans relevant data using indices with has, rather than have to read ALL the data (imagine if the string was really really long, rather than a simple computer name).
    
    Its sort of like a book, and the index, would you want to find out in what chapters an occurrence of "Scott" was by checking the index, or have to read each line, word and words within a word?
    
    Make sense?

Computer
aks-nodepool1-25494468-4
aks-nodepool1-25494468-1