OP’s approach has fast cache updates, but it’s still eventually consistent. Like, the whole flow of PG trigger, handled by Ruby server, then writing to a file,

OP’s approach has fast cache updates, but it’s still eventually consistent. Like, the whole flow of PG trigger, handled by Ruby server, then writing to a file, that happens outside the flow of POST comments, and could sometimes finish after POST comments returns to the caller. Also, with my suggestions, you can cache invalidate too, if a short TTL isn’t good enough. Based on writing files to local disk, it seems like a “single server” scenario, so invalidating the in-memory cache on POST comments is dead simple, and can be truly immediately consistent. Likewise, for full page caching, if using Varnish as a cache, Varnish lets you invalidate easily too. NGINX and CDNs don’t (generally), but if a short TTL isn’t enough, and you need instant invalidation on new comments, then just go in-memory cache or Varnish.