84 lines
7.0 KiB
HTML
84 lines
7.0 KiB
HTML
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Hard Problem? Invalidating the browser cache | CODING WITH ANDRZEJ
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<a href="http://localhost:1313"><h1>CODING WITH ANDRZEJ</h1></a>
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<h1>Hard Problem? Invalidating the browser cache</h1>
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<time datetime="2024-11-13T14:24:21+01:00">November 13, 2024</time>
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<p><strong>I had a bit of an issue with my <a href="https://demos.ajstepien.xyz">website</a> recently.</strong></p>
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<p>I pushed some changes incorporating images for the first time (I know, very swish), and everything seemed to be working just fine, but when I loaded the production site in Firefox, the images were not styled. Stranger still, they <em>were</em> styled when I loaded the same page in Chrome.</p>
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<p>The experienced computer touchers amongst you will be saying “this is obviously a cache problem”, and you’re right, it is obviously a cache problem. Pressing <code>CTR + SHIFT + R</code> (which forces Firefox to clear the cache and do a full reload) proved this thesis, and solved the problem handily for me, on my machine. But what about other people’s machines? I need to cache-bust.</p>
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<p>Post-processors such as Tailwind use fancy ‘fingerprinting’ techniques for this, but I want something simpler than that for this project. Something I can code myself, and understand at a deep level.</p>
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<h2 id="invalidating-cached-html">Invalidating cached HTML</h2>
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<p>The best way to deal with the caching problem is to tell the browser not to cache our HTML in the first place. Yes, this is kind of (100%) cheating, but c’mon bro, it’s just one little HTML file — browsers only cache those because most websites these days are glorified SPAs where the HTML rarely changes.</p>
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<p>I can stop the HTML getting cached by by adding the following meta tag, in this case to <code>index.html</code>.</p>
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<div class="highlight"><pre tabindex="0" class="chroma"><code class="language-html" data-lang="html"><span class="line"><span class="cl"> <span class="p"><</span><span class="nt">meta</span> <span class="na">http-equi</span><span class="err">"</span><span class="na">pragma</span><span class="err">"</span> <span class="na">content</span><span class="o">=</span><span class="s">"no-cache"</span> <span class="p">/></span>
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</span></span></code></pre></div><h2 id="invalidating-cached-css">Invalidating cached CSS</h2>
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<p>That’s all well and good, but what I really need is for the browser to recognize my CSS as a new file and load it anew from the server. I could change the file name whenever I want to bust the cache, but this would get tedious very quickly. What’s more, as far as Git is concerned, I’d be deleting the CSS file and writing a new one with every deployment. Surely there’s a better way?</p>
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<h3 id="using-a-query">Using a query</h3>
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<p>Of course there is. Look at this:</p>
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<div class="highlight"><pre tabindex="0" class="chroma"><code class="language-html" data-lang="html"><span class="line"><span class="cl"><span class="p"><</span><span class="nt">link</span> <span class="na">rel</span><span class="o">=</span><span class="s">"stylesheet"</span> <span class="na">href</span><span class="o">=</span><span class="s">"css/defaults.css?2"</span><span class="p">/></span>
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</span></span></code></pre></div><p>As I’m requesting the file via http, I can append a query. Awesome. Not awesome enough though. I’m too lazy to do this every time I push a commit, and, being human, I’ll probably forget at a critical moment. This can only mean one thing. It’s time to bash (🤣) out a quick build script.</p>
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<h3 id="automating-query-insertion">Automating query insertion</h3>
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<div class="highlight"><pre tabindex="0" class="chroma"><code class="language-bash" data-lang="bash"><span class="line"><span class="cl"><span class="cp">#!/usr/bin/env bash
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</span></span></span><span class="line"><span class="cl"><span class="cp"></span><span class="nv">COMMIT</span><span class="o">=</span><span class="s2">"</span><span class="k">$(</span>git rev-parse HEAD<span class="k">)</span><span class="s2">"</span>
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</span></span><span class="line"><span class="cl">sed -i <span class="s2">"s/css?=\w*/css?</span><span class="si">${</span><span class="nv">COMMIT</span><span class="si">}</span><span class="s2">/g"</span> index.html
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</span></span></code></pre></div><p>Let’s talk real quick about what’s happening here:</p>
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<p><code>COMMIT="$(git rev-parse HEAD)"</code> gets the commit id from Git and assigns it to the variable <code>$COMMIT</code>.</p>
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<p>Then, <code>sed -i "s/css?=\w*/css?${COMMIT}/g" index.html</code> does a find and replace on <code>index.html</code>. The regular expression <code>css?=\w*</code> matches ‘css?=’ plus any number of contiguous alphanumeric characters (everything until the next quote mark, basically) before replacing these alphanumeric characters with the commit id. The flag <code>-i</code> tells sed to edit the file in place. The <code>g</code> tells it to perform the operation on the whole file.</p>
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<p>Now, whenever we push a new commit, any CSS imports in <code>index.html</code> will be changed to something like this:</p>
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<div class="highlight"><pre tabindex="0" class="chroma"><code class="language-html" data-lang="html"><span class="line"><span class="cl"><span class="p"><</span><span class="nt">link</span> <span class="na">rel</span><span class="o">=</span><span class="s">"stylesheet"</span>
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</span></span><span class="line"><span class="cl"><span class="na">href</span><span class="o">=</span><span class="s">"css/styles.css?ab10c24280844c10c10c1adfb8b85b03b316f72b"</span>
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</span></span><span class="line"><span class="cl"><span class="p">/></span>
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</span></span></code></pre></div><p>Now I just need to add the build script to my Jenkinsfile, and the problem is solved.</p>
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<p>Pretty neat, huh?</p>
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<p>There’s just one thing bugging me: surely I do actually want the CSS to be cached <em>sometimes</em>. Caching exists for a reason, and I don’t want to sacrifice performance. Maybe I can modify the build script so that it only updates the CSS imports when the CSS files have changed… Sounds like a topic for another blogpost…</p>
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