ApiListener#RelayMessageOne(): log🪵 to which Endpoint messages are relayed

[Al2Klimov]

if they're for our parent Zone.

ref/NC/820479

Requested in #10389, but useful independent of it.

Tests

Config

object Endpoint "akzl1" {
  host = "10.27.1.87"
}

object Endpoint "akzl2" {
  host = "10.27.1.90"
}

object Endpoint "akzl3" {
  host = "10.27.3.6"
}

object Endpoint "akzl4" {
  host = "10.27.0.129"
}

object Zone "master" {
  endpoints = [ "akzl1", "akzl2" ]
}

object Zone "sat" {
  parent = "master"
  endpoints = [ "akzl3", "akzl4" ]
}

include <itl>

object CheckerComponent "checker" { }

object ApiListener "api" {
  accept_config = true
  accept_commands = true
}

object ApiUser "root" {
  password = "123456"
  permissions = [ "*" ]
}

object Host "master" {
  check_command = "dummy"
  zone = "master"
}

object Host "sat" {
  check_command = "dummy"
  zone = "sat"
}

for (i in range(1000)) {
  apply Service i {
    check_command = "dummy"
    assign where true
  }
}

Starting point

sat's member akzl4 relays stuff to its zone-master akzl3 which relays it to the parent akzl1.

Let's break something... 😈

If we stop akzl1...

... akzl3 switches its parent from akzl1 to akzl2.

Let's break something... 😈 II

If we also stop akzl3...

... akzl4 switches from zone-master akzl3 to parent akzl2.

OK, the downtime is over... 👿

Likewise, whichever preferred nodes come back online...

... they're used again! 👍

[jschmidt-icinga]

I can understand @yhabteab's concern that the log message "Relaying messages for parent Zone 'x' to Endpoint 'y'" could be misunderstood as "Relaying a message to 'y'". Technically, the wording is correct, but maybe it could be improved by changing it to something like "Messages to parent Zone 'x' will (now) be relayed via Endpoint 'y'".

I would prefer it if evaluating this (and all the other routing checks) could be done whenever the routing graph updates, i.e. when nodes go up or down, instead of once per message. However that's past the scope of this PR, and this does the best it can with the infrastructure we have. And the message itself is useful, no doubt.

So, unless @yhabteab is adamant about the message being unclear, I think this should be merged.

[Al2Klimov]

Indeed. Those two messages are a bit too similar, given their different meaning..