Rate limiting with Redis sliding window counter

<p>Need to implement rate limiting for an API. Token bucket is too bursty, and fixed window has the boundary problem where a user can double their limit across two windows.</p>

<p>Use a Redis sorted set sliding window:</p> <div class="highlight"><pre><span></span><code><span class="kn">import</span><span class="w"> </span><span class="nn">time</span><span class="o">,</span><span class="w"> </span><span class="nn">redis</span> <span class="n">r</span> <span class="o">=</span> <span class="n">redis</span><span class="o">.</span><span class="n">Redis</span><span class="p">()</span> <span class="k">def</span><span class="w"> </span><span class="nf">is_allowed</span><span class="p">(</span><span class="n">user_id</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">limit</span><span class="p">:</span> <span class="nb">int</span><span class="p">,</span> <span class="n">window_secs</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">bool</span><span class="p">:</span> <span class="n">key</span> <span class="o">=</span> <span class="sa">f</span><span class="s1">'rl:</span><span class="si">{</span><span class="n">user_id</span><span class="si">}</span><span class="s1">'</span> <span class="n">now</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span> <span class="n">pipe</span> <span class="o">=</span> <span class="n">r</span><span class="o">.</span><span class="n">pipeline</span><span class="p">()</span> <span class="n">pipe</span><span class="o">.</span><span class="n">zremrangebyscore</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="n">now</span> <span class="o">-</span> <span class="n">window_secs</span><span class="p">)</span> <span class="n">pipe</span><span class="o">.</span><span class="n">zadd</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="p">{</span><span class="nb">str</span><span class="p">(</span><span class="n">now</span><span class="p">):</span> <span class="n">now</span><span class="p">})</span> <span class="n">pipe</span><span class="o">.</span><span class="n">zcard</span><span class="p">(</span><span class="n">key</span><span class="p">)</span> <span class="n">pipe</span><span class="o">.</span><span class="n">expire</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">window_secs</span><span class="p">)</span> <span class="n">_</span><span class="p">,</span> <span class="n">_</span><span class="p">,</span> <span class="n">count</span><span class="p">,</span> <span class="n">_</span> <span class="o">=</span> <span class="n">pipe</span><span class="o">.</span><span class="n">execute</span><span class="p">()</span> <span class="k">return</span> <span class="n">count</span> <span class="o">&lt;=</span> <span class="n">limit</span> </code></pre></div> <p>Sliding window gives smooth rate enforcement without boundary spikes.</p>