Earlier quoted context omitted.
I cannot even begin to express how many times I've seen engineers working super hard to optimize happy-paths so that we turn 3 nines of availability into 4 nines but introduce unintended emergent behaviors in unhappy-paths that turn 1 nine into zero nines via thundering herds, retry storms, etc.
You have my empathy for this kind of sentiment. Personally this seems somewhat rare in practice. That being said I'm curious if anyone has anecdotes they can share about these kinds of things?
1. People add retries w/o backoff. Now we have a retry storm.
2. People don't add jitter so we get huge waves of highly correlated retries that cause self-perpetuating overload cycles and failure.
3. People add retries (even w/ backoff) at more than one layer (e.g. one in process, one in envoy), so now we have a deeply confusing multi-level of retries with super weird n^2 patterns.
4. People find ways to fetch from multiple data sources to make a composite object but don't cache/reuse data they fetched, so one data source being down causes DOS on _other_ data sources because of retries.
5. People add failover mechanisms where all failures happily pick the same failover target because, ya know, it has the most free capacity.
6. People underestimate connection setup cost so "failover" causes huge increase in overall load due to connection setup (often tls setup) causing huge influx of "new" cpu work in a loaded system. CPU spike causes unhealthy destinations, causing more failovers, causing metastable failure.
7. People realize purely local decisions aren't optimal so add a layer of global health metrics (e.g. global retry budgets) but these systems add a time delay to the metric (ie its the view of the world 5ms ago). This delay makes their naive control algorithm go into oscillations or divergence spraying stuff everywhere.
All of these things have a similar pattern that when things are almost all good, they will make the system more robust. You'll get an extra 9 of reliability on good days. But they make everything far worse when things are bad.