Design a News Feed (Twitter or Instagram)

// Write path with hybrid fan-out
const CELEBRITY_THRESHOLD = 100_000;

export async function publishPost(post) {                         // { id, authorId, createdAt, … }
  await db.posts.insert(post);
  await redis.zadd(`posts:${post.authorId}`, post.createdAt, post.id);   // author timeline (used for pull)
  await redis.zremrangebyrank(`posts:${post.authorId}`, 0, -801);
  if ((await graph.followerCount(post.authorId)) > CELEBRITY_THRESHOLD) return;   // merged at read time instead
  for await (const batch of graph.followersInBatches(post.authorId, 1_000)) {
    await queue.publish('fanout', { postId: post.id, ts: post.createdAt, to: batch });   // 1 message per 1,000 followers
  }
}

// Fan-out worker. With ioredis enableAutoPipelining, these calls are batched per node (works with Redis Cluster).
export async function fanoutWorker({ postId, ts, to }) {
  const active = await activeSince(to, 30);                       // skip users inactive for 30 days
  await Promise.all(active.flatMap(userId => [
    redis.zadd(`feed:${userId}`, ts, postId),
    redis.zremrangebyrank(`feed:${userId}`, 0, -801),             // keep the newest 800
  ]));
}

// Optimistic like with rollback
import { useMutation, useQueryClient } from '@tanstack/react-query';
import { likeInFeed } from './optimistic-like.js';

export function useLike() {
  const qc = useQueryClient();
  return useMutation({
    mutationFn: postId => fetch(`/api/posts/${postId}/like`, { method: 'POST' }).then(r => { if (!r.ok) throw new Error('like failed'); }),
    onMutate: async postId => {
      await qc.cancelQueries({ queryKey: ['feed'] });               // an in-flight refetch must not overwrite the optimistic state
      const previous = qc.getQueryData(['feed']);
      qc.setQueryData(['feed'], data => likeInFeed(data, postId));
      return { previous };
    },
    onError: (_err, _postId, ctx) => qc.setQueryData(['feed'], ctx?.previous),   // roll back
    // No blanket invalidate: refetching an infinite feed reloads every page. Reconcile the one post if needed.
  });
}