Database sharding

<div class="demo"><div class="head"><b>user_id % 4</b><span id="user">user 42</span></div><div class="stage shard"><div class="router">ID →</div><div class="shards"><div class="cell">shard 0</div><div class="cell">shard 1</div><div class="cell">shard 2</div><div class="cell">shard 3</div></div></div><div class="foot"><span id="route">42 → shard 2</span><span>one key, one route</span></div></div>

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let n=0;const ids=[42,17,28,39];function tick(){document.getElementById('user').textContent='user '+ids[n];document.querySelectorAll('.shards .cell').forEach((e,i)=>e.classList.toggle('on',i===ids[n]%4));document.getElementById('route').textContent=ids[n]+' → shard '+ids[n]%4;n=(n+1)%4}tick();const t=setInterval(tick,850);document.querySelector('.demo').onclick=()=>{clearInterval(t);tick()}