"being in the corner at scale doesn't force you to rewrite everything. Just the piece that put you there. You can leave Rails app to handle most things, and extract parts of specialized infra as needed."
You can't easily, because this now puts a network boundary between your Rails app and the part you extracted. A network boundary that requires you to do IO, which will (by default) cause you Rails app to blockingly wait for a response (vs not doing so in other runtimes, ie Node).
"Would be curious to look at specifics — what kind of pains your company is experiencing with Rails today."
Some things I've seen in the last 3 month:
* Rails timing out after 30s while allocating 500MB of memory (mostly) in ActiveRecord to compute 5MB of JSON to return to an API caller
* 90% of request latency of ~10s spent waiting for downstream services to respond to requests. Most of these could be fired off concurrently (ie `Promise.all` in node). 9s/10s this Rails worker is sitting around doing nothing and eating up ~300MB of memory.
* trying to extract out Authorization to a centralized service (so that other extracted services don't have to call into the monolith in order to make authorization decisions) is a major pain as the monolith now has to make calls out to the centralized auth system to in order to make authz decisions.
