Latency is important. Money makers > Latency sensitive > Optional requests > Background requests > Batch traffic.
Bootstrapping is important. If A depends on B, you might to drain A first, or A and B together.
Hopefully one never has to face that scenario, but its much easier to pick up the pieces when you know where the priorities are whether you're having to power down servers or thin a herd.
Some cattle we can measure. Milk cows are carefully managed as to output - the farmer knows how much the milk each one is worth and so they can cull the low producers. However milk is so much more valuable than meat that they never cull based on drought - milk can always outbid meat for feed. If milk demand goes down the farmer might cull come - but often the farmer is under contract for X amount of milk and so they cannot manage prices.
A DNS service is not NTP is not mail gateway is not application load balancer is not database etc etc etc
At best, multiple replicas of those are cattle.
And while you can treat the servers underlying them as interchangeable, that doesn't change the fact the services you run on them are not.
Of course the pigs, cows, and chickens are not interchangeable. Nor are corn, hay, soybeans.
Depends on who's doin' the milkin'.
The regular setups are easier to understand, nicer to work on. The irregular ones are a trip hazard, they need careful setup, more careful maintenance, more detailed documentation, more aware monitoring. But there's probably ways they could be made regular, if the unique connectivity was moved out to a separate 'module' e.g. at the switch layer, or if the storage had been planned differently, sometimes with more cost, sometimes just with different design up-front.
Along these lines, yes DNS is not NTP but you could have a 'cattle' template Linux server which can run your DNS or NTP or SMTP relay which can be script deployed, and then standard DNS/NTP/SMTP containers deployed on top. Or you could build a new Linux server by hand and deploy a new service layer by hand, every time, each one slightly different depending how rushed you are and what verison of installers are conveniently available and whether the same person does the work following the latest runbook or an outdated one or going from memory. You could deploy a template OpnSense VM which can front DNS or NTP or SMTP instead of having to manually login to a GUI firewall interface and add rules for the new service by hand.
'Cattle not pets' is a call to standardise, regularise, modularize, template, script, automate; to move towards those ways of doing things. Servers are now software which can be copypasted in a way they weren't 10-30 years ago, at least in my non-FAANG world. To me it doesn't mean every server has to mean nothing to you, or every server is interchangeable, it means consider if thinking that wasy can help.
What grinds my gears is that over years I found it a thought limiting meme - it effectively swings a metaphor too hard into one direction, and some early responses under original article IMO present quite well the issue. It's not like people are stupid - but metaphors like this exist to make shortcuts for thinking and discussion, and for last few years I've seen that it short-circuits the discussion too hard, making people either stop thinking about certain interdependencies, or stopping noticing that there are still systems they treat like "pets", just named differently and in different scope, but now mentally pushing out how fragile they can be.