News feed

POST /posts { text, media }   { id }
GET  /feed?cursor=...         { items, next }

posts(id, author_id, created_at, body), sharded by author
follows(follower_id, followee_id), indexed both ways
feed:{user}  Redis list of post ids, capped at about 800

Push (fan-out on write) copies the post id into each follower's list, so a read is one fast list read. Pull (fan-out on read) gathers posts from everyone you follow at read time: cheap writes, slow reads. Hybrid: push for normal authors, pull for huge accounts, merge at read time.

Post ids only. Bodies come from a post cache by id, so edits and deletes show everywhere at once.

Don't fan out to them. Rebuild their feed when they come back.

A cursor on (created_at, id), so new posts don't shift pages.

Copy// Hybrid fan-out. Normal authors: push the post into each follower's list on write (fast reads).
// Celebrities: skip the push (millions of writes); merge their posts in at read time.
function publish(feeds, followers, author, post, celebrities, cap = 800) {
  if (celebrities.has(author)) return 0;
  const fans = followers.get(author) ?? [];
  for (const f of fans) {
    const list = feeds.get(f) ?? [];
    list.unshift(post); // Redis: LPUSH feed:{f} id, then LTRIM feed:{f} 0 cap-1
    if (list.length > cap) list.length = cap;
    feeds.set(f, list);
  }
  return fans.length; // writes this post cost
}
function readFeed(feeds, following, user, postsBy, celebrities, k = 10) {
  const pulled = [...(following.get(user) ?? [])].filter(a => celebrities.has(a)).map(a => postsBy.get(a) ?? []);
  return mergeNewest([feeds.get(user) ?? [], ...pulled], k);
}
// k-way merge of lists already sorted newest first (a heap if there are many lists)
function mergeNewest(lists, k) {
  const idx = lists.map(() => 0), out = [];
  while (out.length < k) {
    let best = -1;
    for (let i = 0; i < lists.length; i++)
      if (idx[i] < lists[i].length && (best < 0 || lists[i][idx[i]].at > lists[best][idx[best]].at)) best = i;
    if (best < 0) break;
    out.push(lists[best][idx[best]++]);
  }
  return out;
}

• Push: fast reads, many writes. Pull: few writes, slow reads.
• Ranking adds a scoring service. Leave it out unless asked.