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Load Simulator

Free, no AI, no credits

Build Twitter, level by level

Start with a warm-up, then each level breaks your design in a new way. Predict what breaks, run the load test, and see why with real numbers.

Start Level 1Start Level 1See all features0 of 8 levels complete
  1. 1One serverYou are here
  2. 2Scaling outOpen
  3. CachingLocked
  4. Read replicasLocked
  5. ShardingLocked
  6. Fan-outLocked
  7. CDNLocked
  8. Multi-regionLocked
  1. 1
    Level 1You are here

    One server, 100 users

    The warm-up. Client, server and database, what the charts mean (requests per second, p95 latency, CPU %, errors), how to guess and run. Nothing breaks at first. Then one small thing does.

    Breaks first The CPU the app and the database share, when many more users arrive

    You addClientApp serverDatabase

    Sets solved: 0 of 2

    Open Level 1Open Level 1
    Your design at this level
    1. Clientnew
    2. App servernewbreaks first
    3. Databasenew
  2. 2
    Level 2Open

    Your app went viral

    CPU is the bottleneck. Vertical and horizontal scaling, load balancers, crashes, spikes, slow queries, connection limits.

    You addLoad balancerApp servers

    Sets solved: 0 of 5

  3. Level 3Locked

    Reads are crushing the database

    Caching: cache-aside, hit rate, TTL, invalidation and stale reads, a cold cache after a restart, hot keys, and more keys than the cache can hold.

    You addCache

    Opens after Level 2 is complete. You can still try it.

  4. Level 4Locked

    Writes and reads outgrow one database

    Read replicas: the primary and its copies, read-your-writes, replication lag and stale reads, failover and lost writes, the write queue, and why replicas do not help writes.

    You addRead replicas

    Opens after Level 3 is complete. You can still try it.

  5. Level 5Locked

    One primary can't hold every write

    Sharding: the shard key, hash and range sharding, hot shards and skewed keys, cross-shard reads, and resharding with consistent hashing.

    You addShards

    Opens after Level 4 is complete. You can still try it.

  6. Level 6Locked

    Everyone follows the celebrity

    Home timeline fan-out: push or pull, the hybrid with a celebrity threshold, the fan-out queue and its priority lane, timeline memory, and skipping inactive users.

    You addTimeline service

    Opens after Level 5 is complete. You can still try it.

  7. Level 7Locked

    The photo went viral

    CDN and edge caching: edge locations near the users, hit ratio and the long tail, TTL and cache keys, purges against versioned URLs, the origin shield, signed URLs, and never caching a private response.

    You addCDN edge

    Opens after Level 6 is complete. You can still try it.

  8. Level 8Locked

    The region went dark

    A second region: far users and the road to Mumbai, replication lag across regions, read-your-writes again, failover, the recovery time and recovery point, and what two regions cost.

    You addSecond region

    Opens after Level 7 is complete. You can still try it.