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<h1 id="design-a-key-value-cache-to-save-the-results-of-the-most-recent-web-server-queries">Design a key-value cache to save the results of the most recent web server queries<a class="headerlink" href="#design-a-key-value-cache-to-save-the-results-of-the-most-recent-web-server-queries" title="Permanent link">&para;</a></h1>
<p><em>Note: This document links directly to relevant areas found in the <a href="https://github.com/donnemartin/system-design-primer#index-of-system-design-topics">system design topics</a> to avoid duplication. Refer to the linked content for general talking points, tradeoffs, and alternatives.</em></p>
<h2 id="step-1-outline-use-cases-and-constraints">Step 1: Outline use cases and constraints<a class="headerlink" href="#step-1-outline-use-cases-and-constraints" title="Permanent link">&para;</a></h2>
<blockquote>
<p>Gather requirements and scope the problem.
Ask questions to clarify use cases and constraints.
Discuss assumptions.</p>
</blockquote>
<p>Without an interviewer to address clarifying questions, we'll define some use cases and constraints.</p>
<h3 id="use-cases">Use cases<a class="headerlink" href="#use-cases" title="Permanent link">&para;</a></h3>
<h4 id="well-scope-the-problem-to-handle-only-the-following-use-cases">We'll scope the problem to handle only the following use cases<a class="headerlink" href="#well-scope-the-problem-to-handle-only-the-following-use-cases" title="Permanent link">&para;</a></h4>
<ul>
<li><strong>User</strong> sends a search request resulting in a cache hit</li>
<li><strong>User</strong> sends a search request resulting in a cache miss</li>
<li><strong>Service</strong> has high availability</li>
</ul>
<h3 id="constraints-and-assumptions">Constraints and assumptions<a class="headerlink" href="#constraints-and-assumptions" title="Permanent link">&para;</a></h3>
<h4 id="state-assumptions">State assumptions<a class="headerlink" href="#state-assumptions" title="Permanent link">&para;</a></h4>
<ul>
<li>Traffic is not evenly distributed<ul>
<li>Popular queries should almost always be in the cache</li>
<li>Need to determine how to expire/refresh</li>
</ul>
</li>
<li>Serving from cache requires fast lookups</li>
<li>Low latency between machines</li>
<li>Limited memory in cache<ul>
<li>Need to determine what to keep/remove</li>
<li>Need to cache millions of queries</li>
</ul>
</li>
<li>10 million users</li>
<li>10 billion queries per month</li>
</ul>
<h4 id="calculate-usage">Calculate usage<a class="headerlink" href="#calculate-usage" title="Permanent link">&para;</a></h4>
<p><strong>Clarify with your interviewer if you should run back-of-the-envelope usage calculations.</strong></p>
<ul>
<li>Cache stores ordered list of key: query, value: results<ul>
<li><code>query</code> - 50 bytes</li>
<li><code>title</code> - 20 bytes</li>
<li><code>snippet</code> - 200 bytes</li>
<li>Total: 270 bytes</li>
</ul>
</li>
<li>2.7 TB of cache data per month if all 10 billion queries are unique and all are stored<ul>
<li>270 bytes per search * 10 billion searches per month</li>
<li>Assumptions state limited memory, need to determine how to expire contents</li>
</ul>
</li>
<li>4,000 requests per second</li>
</ul>
<p>Handy conversion guide:</p>
<ul>
<li>2.5 million seconds per month</li>
<li>1 request per second = 2.5 million requests per month</li>
<li>40 requests per second = 100 million requests per month</li>
<li>400 requests per second = 1 billion requests per month</li>
</ul>
<h2 id="step-2-create-a-high-level-design">Step 2: Create a high level design<a class="headerlink" href="#step-2-create-a-high-level-design" title="Permanent link">&para;</a></h2>
<blockquote>
<p>Outline a high level design with all important components.</p>
</blockquote>
<p><a class="glightbox" href="http://i.imgur.com/KqZ3dSx.png" data-type="image" data-width="auto" data-height="auto" data-desc-position="bottom"><img alt="Imgur" src="http://i.imgur.com/KqZ3dSx.png" /></a></p>
<h2 id="step-3-design-core-components">Step 3: Design core components<a class="headerlink" href="#step-3-design-core-components" title="Permanent link">&para;</a></h2>
<blockquote>
<p>Dive into details for each core component.</p>
</blockquote>
<h3 id="use-case-user-sends-a-request-resulting-in-a-cache-hit">Use case: User sends a request resulting in a cache hit<a class="headerlink" href="#use-case-user-sends-a-request-resulting-in-a-cache-hit" title="Permanent link">&para;</a></h3>
<p>Popular queries can be served from a <strong>Memory Cache</strong> such as Redis or Memcached to reduce read latency and to avoid overloading the <strong>Reverse Index Service</strong> and <strong>Document Service</strong>. Reading 1 MB sequentially from memory takes about 250 microseconds, while reading from SSD takes 4x and from disk takes 80x longer.<sup><a href=https://github.com/donnemartin/system-design-primer#latency-numbers-every-programmer-should-know>1</a></sup></p>
<p>Since the cache has limited capacity, we'll use a least recently used (LRU) approach to expire older entries.</p>
<ul>
<li>The <strong>Client</strong> sends a request to the <strong>Web Server</strong>, running as a <a href="https://github.com/donnemartin/system-design-primer#reverse-proxy-web-server">reverse proxy</a></li>
<li>The <strong>Web Server</strong> forwards the request to the <strong>Query API</strong> server</li>
<li>The <strong>Query API</strong> server does the following:<ul>
<li>Parses the query<ul>
<li>Removes markup</li>
<li>Breaks up the text into terms</li>
<li>Fixes typos</li>
<li>Normalizes capitalization</li>
<li>Converts the query to use boolean operations</li>
</ul>
</li>
<li>Checks the <strong>Memory Cache</strong> for the content matching the query<ul>
<li>If there's a hit in the <strong>Memory Cache</strong>, the <strong>Memory Cache</strong> does the following:<ul>
<li>Updates the cached entry's position to the front of the LRU list</li>
<li>Returns the cached contents</li>
</ul>
</li>
<li>Else, the <strong>Query API</strong> does the following:<ul>
<li>Uses the <strong>Reverse Index Service</strong> to find documents matching the query<ul>
<li>The <strong>Reverse Index Service</strong> ranks the matching results and returns the top ones</li>
</ul>
</li>
<li>Uses the <strong>Document Service</strong> to return titles and snippets</li>
<li>Updates the <strong>Memory Cache</strong> with the contents, placing the entry at the front of the LRU list</li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>
<h4 id="cache-implementation">Cache implementation<a class="headerlink" href="#cache-implementation" title="Permanent link">&para;</a></h4>
<p>The cache can use a doubly-linked list: new items will be added to the head while items to expire will be removed from the tail. We'll use a hash table for fast lookups to each linked list node.</p>
<p><strong>Clarify with your interviewer how much code you are expected to write</strong>.</p>
<p><strong>Query API Server</strong> implementation:</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-0-1"><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="k">class</span> <span class="nc">QueryApi</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
</span><span id="__span-0-2"><a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a>
</span><span id="__span-0-3"><a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a> <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">memory_cache</span><span class="p">,</span> <span class="n">reverse_index_service</span><span class="p">):</span>
</span><span id="__span-0-4"><a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a> <span class="bp">self</span><span class="o">.</span><span class="n">memory_cache</span> <span class="o">=</span> <span class="n">memory_cache</span>
</span><span id="__span-0-5"><a id="__codelineno-0-5" name="__codelineno-0-5" href="#__codelineno-0-5"></a> <span class="bp">self</span><span class="o">.</span><span class="n">reverse_index_service</span> <span class="o">=</span> <span class="n">reverse_index_service</span>
</span><span id="__span-0-6"><a id="__codelineno-0-6" name="__codelineno-0-6" href="#__codelineno-0-6"></a>
</span><span id="__span-0-7"><a id="__codelineno-0-7" name="__codelineno-0-7" href="#__codelineno-0-7"></a> <span class="k">def</span> <span class="nf">parse_query</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">query</span><span class="p">):</span>
</span><span id="__span-0-8"><a id="__codelineno-0-8" name="__codelineno-0-8" href="#__codelineno-0-8"></a><span class="w"> </span><span class="sd">&quot;&quot;&quot;Remove markup, break text into terms, deal with typos,</span>
</span><span id="__span-0-9"><a id="__codelineno-0-9" name="__codelineno-0-9" href="#__codelineno-0-9"></a><span class="sd"> normalize capitalization, convert to use boolean operations.</span>
</span><span id="__span-0-10"><a id="__codelineno-0-10" name="__codelineno-0-10" href="#__codelineno-0-10"></a><span class="sd"> &quot;&quot;&quot;</span>
</span><span id="__span-0-11"><a id="__codelineno-0-11" name="__codelineno-0-11" href="#__codelineno-0-11"></a> <span class="o">...</span>
</span><span id="__span-0-12"><a id="__codelineno-0-12" name="__codelineno-0-12" href="#__codelineno-0-12"></a>
</span><span id="__span-0-13"><a id="__codelineno-0-13" name="__codelineno-0-13" href="#__codelineno-0-13"></a> <span class="k">def</span> <span class="nf">process_query</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">query</span><span class="p">):</span>
</span><span id="__span-0-14"><a id="__codelineno-0-14" name="__codelineno-0-14" href="#__codelineno-0-14"></a> <span class="n">query</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">parse_query</span><span class="p">(</span><span class="n">query</span><span class="p">)</span>
</span><span id="__span-0-15"><a id="__codelineno-0-15" name="__codelineno-0-15" href="#__codelineno-0-15"></a> <span class="n">results</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">memory_cache</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">query</span><span class="p">)</span>
</span><span id="__span-0-16"><a id="__codelineno-0-16" name="__codelineno-0-16" href="#__codelineno-0-16"></a> <span class="k">if</span> <span class="n">results</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
</span><span id="__span-0-17"><a id="__codelineno-0-17" name="__codelineno-0-17" href="#__codelineno-0-17"></a> <span class="n">results</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">reverse_index_service</span><span class="o">.</span><span class="n">process_search</span><span class="p">(</span><span class="n">query</span><span class="p">)</span>
</span><span id="__span-0-18"><a id="__codelineno-0-18" name="__codelineno-0-18" href="#__codelineno-0-18"></a> <span class="bp">self</span><span class="o">.</span><span class="n">memory_cache</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="n">query</span><span class="p">,</span> <span class="n">results</span><span class="p">)</span>
</span><span id="__span-0-19"><a id="__codelineno-0-19" name="__codelineno-0-19" href="#__codelineno-0-19"></a> <span class="k">return</span> <span class="n">results</span>
</span></code></pre></div>
<p><strong>Node</strong> implementation:</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-1-1"><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a><span class="k">class</span> <span class="nc">Node</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
</span><span id="__span-1-2"><a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></a>
</span><span id="__span-1-3"><a id="__codelineno-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a> <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">query</span><span class="p">,</span> <span class="n">results</span><span class="p">):</span>
</span><span id="__span-1-4"><a id="__codelineno-1-4" name="__codelineno-1-4" href="#__codelineno-1-4"></a> <span class="bp">self</span><span class="o">.</span><span class="n">query</span> <span class="o">=</span> <span class="n">query</span>
</span><span id="__span-1-5"><a id="__codelineno-1-5" name="__codelineno-1-5" href="#__codelineno-1-5"></a> <span class="bp">self</span><span class="o">.</span><span class="n">results</span> <span class="o">=</span> <span class="n">results</span>
</span></code></pre></div>
<p><strong>LinkedList</strong> implementation:</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-2-1"><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a><span class="k">class</span> <span class="nc">LinkedList</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
</span><span id="__span-2-2"><a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a>
</span><span id="__span-2-3"><a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a> <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
</span><span id="__span-2-4"><a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a> <span class="bp">self</span><span class="o">.</span><span class="n">head</span> <span class="o">=</span> <span class="kc">None</span>
</span><span id="__span-2-5"><a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a> <span class="bp">self</span><span class="o">.</span><span class="n">tail</span> <span class="o">=</span> <span class="kc">None</span>
</span><span id="__span-2-6"><a id="__codelineno-2-6" name="__codelineno-2-6" href="#__codelineno-2-6"></a>
</span><span id="__span-2-7"><a id="__codelineno-2-7" name="__codelineno-2-7" href="#__codelineno-2-7"></a> <span class="k">def</span> <span class="nf">move_to_front</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">node</span><span class="p">):</span>
</span><span id="__span-2-8"><a id="__codelineno-2-8" name="__codelineno-2-8" href="#__codelineno-2-8"></a> <span class="o">...</span>
</span><span id="__span-2-9"><a id="__codelineno-2-9" name="__codelineno-2-9" href="#__codelineno-2-9"></a>
</span><span id="__span-2-10"><a id="__codelineno-2-10" name="__codelineno-2-10" href="#__codelineno-2-10"></a> <span class="k">def</span> <span class="nf">append_to_front</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">node</span><span class="p">):</span>
</span><span id="__span-2-11"><a id="__codelineno-2-11" name="__codelineno-2-11" href="#__codelineno-2-11"></a> <span class="o">...</span>
</span><span id="__span-2-12"><a id="__codelineno-2-12" name="__codelineno-2-12" href="#__codelineno-2-12"></a>
</span><span id="__span-2-13"><a id="__codelineno-2-13" name="__codelineno-2-13" href="#__codelineno-2-13"></a> <span class="k">def</span> <span class="nf">remove_from_tail</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
</span><span id="__span-2-14"><a id="__codelineno-2-14" name="__codelineno-2-14" href="#__codelineno-2-14"></a> <span class="o">...</span>
</span></code></pre></div>
<p><strong>Cache</strong> implementation:</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-3-1"><a id="__codelineno-3-1" name="__codelineno-3-1" href="#__codelineno-3-1"></a><span class="k">class</span> <span class="nc">Cache</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
</span><span id="__span-3-2"><a id="__codelineno-3-2" name="__codelineno-3-2" href="#__codelineno-3-2"></a>
</span><span id="__span-3-3"><a id="__codelineno-3-3" name="__codelineno-3-3" href="#__codelineno-3-3"></a> <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">MAX_SIZE</span><span class="p">):</span>
</span><span id="__span-3-4"><a id="__codelineno-3-4" name="__codelineno-3-4" href="#__codelineno-3-4"></a> <span class="bp">self</span><span class="o">.</span><span class="n">MAX_SIZE</span> <span class="o">=</span> <span class="n">MAX_SIZE</span>
</span><span id="__span-3-5"><a id="__codelineno-3-5" name="__codelineno-3-5" href="#__codelineno-3-5"></a> <span class="bp">self</span><span class="o">.</span><span class="n">size</span> <span class="o">=</span> <span class="mi">0</span>
</span><span id="__span-3-6"><a id="__codelineno-3-6" name="__codelineno-3-6" href="#__codelineno-3-6"></a> <span class="bp">self</span><span class="o">.</span><span class="n">lookup</span> <span class="o">=</span> <span class="p">{}</span> <span class="c1"># key: query, value: node</span>
</span><span id="__span-3-7"><a id="__codelineno-3-7" name="__codelineno-3-7" href="#__codelineno-3-7"></a> <span class="bp">self</span><span class="o">.</span><span class="n">linked_list</span> <span class="o">=</span> <span class="n">LinkedList</span><span class="p">()</span>
</span><span id="__span-3-8"><a id="__codelineno-3-8" name="__codelineno-3-8" href="#__codelineno-3-8"></a>
</span><span id="__span-3-9"><a id="__codelineno-3-9" name="__codelineno-3-9" href="#__codelineno-3-9"></a> <span class="k">def</span> <span class="nf">get</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">query</span><span class="p">)</span>
</span><span id="__span-3-10"><a id="__codelineno-3-10" name="__codelineno-3-10" href="#__codelineno-3-10"></a><span class="w"> </span><span class="sd">&quot;&quot;&quot;Get the stored query result from the cache.</span>
</span><span id="__span-3-11"><a id="__codelineno-3-11" name="__codelineno-3-11" href="#__codelineno-3-11"></a>
</span><span id="__span-3-12"><a id="__codelineno-3-12" name="__codelineno-3-12" href="#__codelineno-3-12"></a><span class="sd"> Accessing a node updates its position to the front of the LRU list.</span>
</span><span id="__span-3-13"><a id="__codelineno-3-13" name="__codelineno-3-13" href="#__codelineno-3-13"></a><span class="sd"> &quot;&quot;&quot;</span>
</span><span id="__span-3-14"><a id="__codelineno-3-14" name="__codelineno-3-14" href="#__codelineno-3-14"></a> <span class="n">node</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">lookup</span><span class="p">[</span><span class="n">query</span><span class="p">]</span>
</span><span id="__span-3-15"><a id="__codelineno-3-15" name="__codelineno-3-15" href="#__codelineno-3-15"></a> <span class="k">if</span> <span class="n">node</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
</span><span id="__span-3-16"><a id="__codelineno-3-16" name="__codelineno-3-16" href="#__codelineno-3-16"></a> <span class="k">return</span> <span class="kc">None</span>
</span><span id="__span-3-17"><a id="__codelineno-3-17" name="__codelineno-3-17" href="#__codelineno-3-17"></a> <span class="bp">self</span><span class="o">.</span><span class="n">linked_list</span><span class="o">.</span><span class="n">move_to_front</span><span class="p">(</span><span class="n">node</span><span class="p">)</span>
</span><span id="__span-3-18"><a id="__codelineno-3-18" name="__codelineno-3-18" href="#__codelineno-3-18"></a> <span class="k">return</span> <span class="n">node</span><span class="o">.</span><span class="n">results</span>
</span><span id="__span-3-19"><a id="__codelineno-3-19" name="__codelineno-3-19" href="#__codelineno-3-19"></a>
</span><span id="__span-3-20"><a id="__codelineno-3-20" name="__codelineno-3-20" href="#__codelineno-3-20"></a> <span class="k">def</span> <span class="nf">set</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">results</span><span class="p">,</span> <span class="n">query</span><span class="p">):</span>
</span><span id="__span-3-21"><a id="__codelineno-3-21" name="__codelineno-3-21" href="#__codelineno-3-21"></a><span class="w"> </span><span class="sd">&quot;&quot;&quot;Set the result for the given query key in the cache.</span>
</span><span id="__span-3-22"><a id="__codelineno-3-22" name="__codelineno-3-22" href="#__codelineno-3-22"></a>
</span><span id="__span-3-23"><a id="__codelineno-3-23" name="__codelineno-3-23" href="#__codelineno-3-23"></a><span class="sd"> When updating an entry, updates its position to the front of the LRU list.</span>
</span><span id="__span-3-24"><a id="__codelineno-3-24" name="__codelineno-3-24" href="#__codelineno-3-24"></a><span class="sd"> If the entry is new and the cache is at capacity, removes the oldest entry</span>
</span><span id="__span-3-25"><a id="__codelineno-3-25" name="__codelineno-3-25" href="#__codelineno-3-25"></a><span class="sd"> before the new entry is added.</span>
</span><span id="__span-3-26"><a id="__codelineno-3-26" name="__codelineno-3-26" href="#__codelineno-3-26"></a><span class="sd"> &quot;&quot;&quot;</span>
</span><span id="__span-3-27"><a id="__codelineno-3-27" name="__codelineno-3-27" href="#__codelineno-3-27"></a> <span class="n">node</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">lookup</span><span class="p">[</span><span class="n">query</span><span class="p">]</span>
</span><span id="__span-3-28"><a id="__codelineno-3-28" name="__codelineno-3-28" href="#__codelineno-3-28"></a> <span class="k">if</span> <span class="n">node</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
</span><span id="__span-3-29"><a id="__codelineno-3-29" name="__codelineno-3-29" href="#__codelineno-3-29"></a> <span class="c1"># Key exists in cache, update the value</span>
</span><span id="__span-3-30"><a id="__codelineno-3-30" name="__codelineno-3-30" href="#__codelineno-3-30"></a> <span class="n">node</span><span class="o">.</span><span class="n">results</span> <span class="o">=</span> <span class="n">results</span>
</span><span id="__span-3-31"><a id="__codelineno-3-31" name="__codelineno-3-31" href="#__codelineno-3-31"></a> <span class="bp">self</span><span class="o">.</span><span class="n">linked_list</span><span class="o">.</span><span class="n">move_to_front</span><span class="p">(</span><span class="n">node</span><span class="p">)</span>
</span><span id="__span-3-32"><a id="__codelineno-3-32" name="__codelineno-3-32" href="#__codelineno-3-32"></a> <span class="k">else</span><span class="p">:</span>
</span><span id="__span-3-33"><a id="__codelineno-3-33" name="__codelineno-3-33" href="#__codelineno-3-33"></a> <span class="c1"># Key does not exist in cache</span>
</span><span id="__span-3-34"><a id="__codelineno-3-34" name="__codelineno-3-34" href="#__codelineno-3-34"></a> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">size</span> <span class="o">==</span> <span class="bp">self</span><span class="o">.</span><span class="n">MAX_SIZE</span><span class="p">:</span>
</span><span id="__span-3-35"><a id="__codelineno-3-35" name="__codelineno-3-35" href="#__codelineno-3-35"></a> <span class="c1"># Remove the oldest entry from the linked list and lookup</span>
</span><span id="__span-3-36"><a id="__codelineno-3-36" name="__codelineno-3-36" href="#__codelineno-3-36"></a> <span class="bp">self</span><span class="o">.</span><span class="n">lookup</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">linked_list</span><span class="o">.</span><span class="n">tail</span><span class="o">.</span><span class="n">query</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
</span><span id="__span-3-37"><a id="__codelineno-3-37" name="__codelineno-3-37" href="#__codelineno-3-37"></a> <span class="bp">self</span><span class="o">.</span><span class="n">linked_list</span><span class="o">.</span><span class="n">remove_from_tail</span><span class="p">()</span>
</span><span id="__span-3-38"><a id="__codelineno-3-38" name="__codelineno-3-38" href="#__codelineno-3-38"></a> <span class="k">else</span><span class="p">:</span>
</span><span id="__span-3-39"><a id="__codelineno-3-39" name="__codelineno-3-39" href="#__codelineno-3-39"></a> <span class="bp">self</span><span class="o">.</span><span class="n">size</span> <span class="o">+=</span> <span class="mi">1</span>
</span><span id="__span-3-40"><a id="__codelineno-3-40" name="__codelineno-3-40" href="#__codelineno-3-40"></a> <span class="c1"># Add the new key and value</span>
</span><span id="__span-3-41"><a id="__codelineno-3-41" name="__codelineno-3-41" href="#__codelineno-3-41"></a> <span class="n">new_node</span> <span class="o">=</span> <span class="n">Node</span><span class="p">(</span><span class="n">query</span><span class="p">,</span> <span class="n">results</span><span class="p">)</span>
</span><span id="__span-3-42"><a id="__codelineno-3-42" name="__codelineno-3-42" href="#__codelineno-3-42"></a> <span class="bp">self</span><span class="o">.</span><span class="n">linked_list</span><span class="o">.</span><span class="n">append_to_front</span><span class="p">(</span><span class="n">new_node</span><span class="p">)</span>
</span><span id="__span-3-43"><a id="__codelineno-3-43" name="__codelineno-3-43" href="#__codelineno-3-43"></a> <span class="bp">self</span><span class="o">.</span><span class="n">lookup</span><span class="p">[</span><span class="n">query</span><span class="p">]</span> <span class="o">=</span> <span class="n">new_node</span>
</span></code></pre></div>
<h4 id="when-to-update-the-cache">When to update the cache<a class="headerlink" href="#when-to-update-the-cache" title="Permanent link">&para;</a></h4>
<p>The cache should be updated when:</p>
<ul>
<li>The page contents change</li>
<li>The page is removed or a new page is added</li>
<li>The page rank changes</li>
</ul>
<p>The most straightforward way to handle these cases is to simply set a max time that a cached entry can stay in the cache before it is updated, usually referred to as time to live (TTL).</p>
<p>Refer to <a href="https://github.com/donnemartin/system-design-primer#when-to-update-the-cache">When to update the cache</a> for tradeoffs and alternatives. The approach above describes <a href="https://github.com/donnemartin/system-design-primer#cache-aside">cache-aside</a>.</p>
<h2 id="step-4-scale-the-design">Step 4: Scale the design<a class="headerlink" href="#step-4-scale-the-design" title="Permanent link">&para;</a></h2>
<blockquote>
<p>Identify and address bottlenecks, given the constraints.</p>
</blockquote>
<p><a class="glightbox" href="http://i.imgur.com/4j99mhe.png" data-type="image" data-width="auto" data-height="auto" data-desc-position="bottom"><img alt="Imgur" src="http://i.imgur.com/4j99mhe.png" /></a></p>
<p><strong>Important: Do not simply jump right into the final design from the initial design!</strong></p>
<p>State you would 1) <strong>Benchmark/Load Test</strong>, 2) <strong>Profile</strong> for bottlenecks 3) address bottlenecks while evaluating alternatives and trade-offs, and 4) repeat. See <a href="../scaling_aws/">Design a system that scales to millions of users on AWS</a> as a sample on how to iteratively scale the initial design.</p>
<p>It's important to discuss what bottlenecks you might encounter with the initial design and how you might address each of them. For example, what issues are addressed by adding a <strong>Load Balancer</strong> with multiple <strong>Web Servers</strong>? <strong>CDN</strong>? <strong>Master-Slave Replicas</strong>? What are the alternatives and <strong>Trade-Offs</strong> for each?</p>
<p>We'll introduce some components to complete the design and to address scalability issues. Internal load balancers are not shown to reduce clutter.</p>
<p><em>To avoid repeating discussions</em>, refer to the following <a href="https://github.com/donnemartin/system-design-primer#index-of-system-design-topics">system design topics</a> for main talking points, tradeoffs, and alternatives:</p>
<ul>
<li><a href="https://github.com/donnemartin/system-design-primer#domain-name-system">DNS</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#load-balancer">Load balancer</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#horizontal-scaling">Horizontal scaling</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#reverse-proxy-web-server">Web server (reverse proxy)</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#application-layer">API server (application layer)</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#cache">Cache</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#consistency-patterns">Consistency patterns</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#availability-patterns">Availability patterns</a></li>
</ul>
<h3 id="expanding-the-memory-cache-to-many-machines">Expanding the Memory Cache to many machines<a class="headerlink" href="#expanding-the-memory-cache-to-many-machines" title="Permanent link">&para;</a></h3>
<p>To handle the heavy request load and the large amount of memory needed, we'll scale horizontally. We have three main options on how to store the data on our <strong>Memory Cache</strong> cluster:</p>
<ul>
<li><strong>Each machine in the cache cluster has its own cache</strong> - Simple, although it will likely result in a low cache hit rate.</li>
<li><strong>Each machine in the cache cluster has a copy of the cache</strong> - Simple, although it is an inefficient use of memory.</li>
<li><strong>The cache is <a href="https://github.com/donnemartin/system-design-primer#sharding">sharded</a> across all machines in the cache cluster</strong> - More complex, although it is likely the best option. We could use hashing to determine which machine could have the cached results of a query using <code>machine = hash(query)</code>. We'll likely want to use <a href="https://github.com/donnemartin/system-design-primer#under-development">consistent hashing</a>.</li>
</ul>
<h2 id="additional-talking-points">Additional talking points<a class="headerlink" href="#additional-talking-points" title="Permanent link">&para;</a></h2>
<blockquote>
<p>Additional topics to dive into, depending on the problem scope and time remaining.</p>
</blockquote>
<h3 id="sql-scaling-patterns">SQL scaling patterns<a class="headerlink" href="#sql-scaling-patterns" title="Permanent link">&para;</a></h3>
<ul>
<li><a href="https://github.com/donnemartin/system-design-primer#master-slave-replication">Read replicas</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#federation">Federation</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#sharding">Sharding</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#denormalization">Denormalization</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#sql-tuning">SQL Tuning</a></li>
</ul>
<h4 id="nosql">NoSQL<a class="headerlink" href="#nosql" title="Permanent link">&para;</a></h4>
<ul>
<li><a href="https://github.com/donnemartin/system-design-primer#key-value-store">Key-value store</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#document-store">Document store</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#wide-column-store">Wide column store</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#graph-database">Graph database</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#sql-or-nosql">SQL vs NoSQL</a></li>
</ul>
<h3 id="caching">Caching<a class="headerlink" href="#caching" title="Permanent link">&para;</a></h3>
<ul>
<li>Where to cache<ul>
<li><a href="https://github.com/donnemartin/system-design-primer#client-caching">Client caching</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#cdn-caching">CDN caching</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#web-server-caching">Web server caching</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#database-caching">Database caching</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#application-caching">Application caching</a></li>
</ul>
</li>
<li>What to cache<ul>
<li><a href="https://github.com/donnemartin/system-design-primer#caching-at-the-database-query-level">Caching at the database query level</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#caching-at-the-object-level">Caching at the object level</a></li>
</ul>
</li>
<li>When to update the cache<ul>
<li><a href="https://github.com/donnemartin/system-design-primer#cache-aside">Cache-aside</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#write-through">Write-through</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#write-behind-write-back">Write-behind (write-back)</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#refresh-ahead">Refresh ahead</a></li>
</ul>
</li>
</ul>
<h3 id="asynchronism-and-microservices">Asynchronism and microservices<a class="headerlink" href="#asynchronism-and-microservices" title="Permanent link">&para;</a></h3>
<ul>
<li><a href="https://github.com/donnemartin/system-design-primer#message-queues">Message queues</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#task-queues">Task queues</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#back-pressure">Back pressure</a></li>
<li><a href="https://github.com/donnemartin/system-design-primer#microservices">Microservices</a></li>
</ul>
<h3 id="communications">Communications<a class="headerlink" href="#communications" title="Permanent link">&para;</a></h3>
<ul>
<li>Discuss tradeoffs:<ul>
<li>External communication with clients - <a href="https://github.com/donnemartin/system-design-primer#representational-state-transfer-rest">HTTP APIs following REST</a></li>
<li>Internal communications - <a href="https://github.com/donnemartin/system-design-primer#remote-procedure-call-rpc">RPC</a></li>
</ul>
</li>
<li><a href="https://github.com/donnemartin/system-design-primer#service-discovery">Service discovery</a></li>
</ul>
<h3 id="security">Security<a class="headerlink" href="#security" title="Permanent link">&para;</a></h3>
<p>Refer to the <a href="https://github.com/donnemartin/system-design-primer#security">security section</a>.</p>
<h3 id="latency-numbers">Latency numbers<a class="headerlink" href="#latency-numbers" title="Permanent link">&para;</a></h3>
<p>See <a href="https://github.com/donnemartin/system-design-primer#latency-numbers-every-programmer-should-know">Latency numbers every programmer should know</a>.</p>
<h3 id="ongoing">Ongoing<a class="headerlink" href="#ongoing" title="Permanent link">&para;</a></h3>
<ul>
<li>Continue benchmarking and monitoring your system to address bottlenecks as they come up</li>
<li>Scaling is an iterative process</li>
</ul>
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