yes, if script returns false instance will be deleted. Currently there is no way to stop instance discovery by aborting filter script.

There is feature request for adding "grace period" for instances being deleted: https://dev.raden.solutions/issues/976. I think implementing it should solve your problem.

Best regards,
Victor

My understanding now is when the instance discovery filtering script returns False that particular instance is skipped or removed if already available. Is that true?
If yes, does that mean that if there's a single SNMP error (eg Timeout) in a regular Instance discovery and the script returns False for that particular instance it will be deleted from the node DCI table? As it happens now if i really delete the interface from the router?

This will be problematic, because now NetXMS immediately deletes a DCI when it detects its absense during instance discovery resulting in permanent history data loss, which is not acceptable for the bussiness. This is a feature that i have already requested in "FEature Requests" - to have the option to leave DCI's present (for example in UNSUPPORTED state) after they dissapear from instance discovery.

I was thinking of somehow "breaking" the script in order to avoid any changes instead of returing False which will eventually result in deleting DCIs with their history data. Am i on the right track?

just add appropriate error checking:

snmp = CreateSNMPTransport($node);
if (snmp == null)
   return false; // node does not support SNMP or transport cannot be created
ifName = SNMPGetValue(snmp, ".1.3.6.1.2.1.2.2.1.2." . $1);
if (ifName == null)
   return false; // cannot read from node
ifXName = SNMPGetValue(snmp, ".1.3.6.1.2.1.31.1.1.1.18." . $1);
if (ifXName != null)
{
   ifName .= " " . ifXName;
}
if (ifName ~= "Loopback.*") {
	return %(false, $1, ifName);
} else {
	return %(true, $1, ifName);
}

Best regards,
Victor