Without backpressure, queues only delay the inevitable. At best. Queuing delays will increase latency. Extra memory pressure can even make things worse by decreasing throughput. If latency gets high enough, clients will start to error out or misbehave in all sorts of unpredictable ways.
> In the situations where it doesn’t, add some more capacity.
That obviously has its limits too. Some people actually care about cost. Also, added capacity might be totally wasted if there's any sort of affinity between servers and data. Maybe it works if you're in a stateless middle tier, but not e.g. in a storage system. You can add capacity to those, sure, but rebalancing the data to take advantage of the new capacity can be a lengthy process and compete with user traffic (making user-perceived latency even worse) if you're not careful. I've also seen too many cases where rebalancing undid the careful work done during initial data placement to maximize data safety. Oops.
> Back pressure is something I’ve never had to deal with
> In the situations where it doesn’t, add some more capacity.
That obviously has its limits too. Some people actually care about cost. Also, added capacity might be totally wasted if there's any sort of affinity between servers and data. Maybe it works if you're in a stateless middle tier, but not e.g. in a storage system. You can add capacity to those, sure, but rebalancing the data to take advantage of the new capacity can be a lengthy process and compete with user traffic (making user-perceived latency even worse) if you're not careful. I've also seen too many cases where rebalancing undid the careful work done during initial data placement to maximize data safety. Oops.
> Back pressure is something I’ve never had to deal with
Count your blessings.