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Caching

The examples in this guide stem from getting started, output management and code splitting.

So we're using webpack to bundle our modular application which yields a deployable /dist directory. Once the contents of /dist have been deployed to a server, clients (typically browsers) will hit that server to grab the site and its assets. The last step can be time consuming, which is why browsers use a technique called caching. This allows sites to load faster with less unnecessary network traffic, however it can also cause headaches when you need new code to be picked up.

This guide focuses on the configuration needed to ensure files produced by webpack compilation can remain cached unless their contents has changed.

Output Filenames

A simple way to ensure the browser picks up changed files is by using output.filename substitutions. The [hash] substitution can be used to include a build-specific hash in the filename, however it's even better to use the [contenthash] substitution which is the hash of the content of a file, which is different for each asset.

Let's get our project set up using the example from getting started with the plugins from output management, so we don't have to deal with maintaining our index.html file manually:

project

webpack-demo
|- package.json
|- webpack.config.js
|- /dist
|- /src
  |- index.js
|- /node_modules

webpack.config.js

  const path = require('path');
  const CleanWebpackPlugin = require('clean-webpack-plugin');
  const HtmlWebpackPlugin = require('html-webpack-plugin');

  module.exports = {
    entry: './src/index.js',
    plugins: [
      new CleanWebpackPlugin(['dist']),
      new HtmlWebpackPlugin({
-       title: 'Output Management'
+       title: 'Caching'
      })
    ],
    output: {
-     filename: 'bundle.js',
+     filename: '[name].[contenthash].js',
      path: path.resolve(__dirname, 'dist')
    }
  };

Running our build script, npm run build, with this configuration should produce the following output:

...
                       Asset       Size  Chunks                    Chunk Names
main.7e2c49a622975ebd9b7e.js     544 kB       0  [emitted]  [big]  main
                  index.html  197 bytes          [emitted]
...

As you can see the bundle's name now reflects its content (via the hash). If we run another build without making any changes, we'd expect that filename to stay the same. However, if we were to run it again, we may find that this is not the case:

...
                       Asset       Size  Chunks                    Chunk Names
main.205199ab45963f6a62ec.js     544 kB       0  [emitted]  [big]  main
                  index.html  197 bytes          [emitted]
...

This is because webpack includes certain boilerplate, specifically the runtime and manifest, in the entry chunk.

Output may differ depending on your current webpack version. Newer versions may not have all the same issues with hashing as some older versions, but we still recommend the following steps to be safe.

Extracting Boilerplate

As we learned in code splitting, the SplitChunksPlugin can be used to split modules out into separate bundles. webpack provides an optimization feature which does split out runtime code into a separate chunk(s) according to the options provided, simply use optimization.runtimeChunk set to single for creating one runtime bundle:

webpack.config.js

  const path = require('path');
  const CleanWebpackPlugin = require('clean-webpack-plugin');
  const HtmlWebpackPlugin = require('html-webpack-plugin');

  module.exports = {
    entry: './src/index.js',
    plugins: [
      new CleanWebpackPlugin(['dist']),
      new HtmlWebpackPlugin({
        title: 'Caching'
    ],
    output: {
      filename: '[name].[contenthash].js',
      path: path.resolve(__dirname, 'dist')
    },
+   optimization: {
+     runtimeChunk: 'single'
+   }
  };

Let's run another build to see the extracted runtime bundle:

Hash: 82c9c385607b2150fab2
Version: webpack 4.12.0
Time: 3027ms
                          Asset       Size  Chunks             Chunk Names
runtime.cc17ae2a94ec771e9221.js   1.42 KiB       0  [emitted]  runtime
   main.e81de2cf758ada72f306.js   69.5 KiB       1  [emitted]  main
                     index.html  275 bytes          [emitted]
[1] (webpack)/buildin/module.js 497 bytes {1} [built]
[2] (webpack)/buildin/global.js 489 bytes {1} [built]
[3] ./src/index.js 309 bytes {1} [built]
    + 1 hidden module

It's also good practice to extract third-party libraries, such as lodash or react, to a separate vendor chunk as they are less likely to change than our local source code. This step will allow clients to request even less from the server to stay up to date. This can be done by using the cacheGroups option of the SplitChunksPlugin demonstrated in Example 2 of SplitChunksPlugin. Lets add optimization.splitChunks with cacheGroups with next params and build:

webpack.config.js

  var path = require('path');
  const CleanWebpackPlugin = require('clean-webpack-plugin');
  const HtmlWebpackPlugin = require('html-webpack-plugin');

  module.exports = {
    entry: './src/index.js',
    plugins: [
      new CleanWebpackPlugin(['dist']),
      new HtmlWebpackPlugin({
        title: 'Caching'
      }),
    ],
    output: {
      filename: '[name].[contenthash].js',
      path: path.resolve(__dirname, 'dist')
    },
    optimization: {
-     runtimeChunk: 'single'
+     runtimeChunk: 'single',
+     splitChunks: {
+       cacheGroups: {
+         vendor: {
+           test: /[\\/]node_modules[\\/]/,
+           name: 'vendors',
+           chunks: 'all'
+         }
+       }
+     }
    }
  };

Let's run another build to see our new vendor bundle:

...
                          Asset       Size  Chunks             Chunk Names
runtime.cc17ae2a94ec771e9221.js   1.42 KiB       0  [emitted]  runtime
vendors.a42c3ca0d742766d7a28.js   69.4 KiB       1  [emitted]  vendors
   main.abf44fedb7d11d4312d7.js  240 bytes       2  [emitted]  main
                     index.html  353 bytes          [emitted]
...

We can now see that our main bundle does not contain vendor code from node_modules directory and is down in size to 240 bytes!

Module Identifiers

Let's add another module, print.js, to our project:

project

webpack-demo
|- package.json
|- webpack.config.js
|- /dist
|- /src
  |- index.js
+ |- print.js
|- /node_modules

print.js

+ export default function print(text) {
+   console.log(text);
+ };

src/index.js

  import _ from 'lodash';
+ import Print from './print';

  function component() {
    var element = document.createElement('div');

    // Lodash, now imported by this script
    element.innerHTML = _.join(['Hello', 'webpack'], ' ');
+   element.onclick = Print.bind(null, 'Hello webpack!');

    return element;
  }

  document.body.appendChild(component());

Running another build, we would expect only our main bundle's hash to change, however...

...
                           Asset       Size  Chunks                    Chunk Names
  vendor.a7561fb0e9a071baadb9.js     541 kB       0  [emitted]  [big]  vendor
    main.b746e3eb72875af2caa9.js    1.22 kB       1  [emitted]         main
manifest.1400d5af64fc1b7b3a45.js    5.85 kB       2  [emitted]         manifest
                      index.html  352 bytes          [emitted]
...

... we can see that all three have. This is because each module.id is incremented based on resolving order by default. Meaning when the order of resolving is changed, the IDs will be changed as well. So, to recap:

  • The main bundle changed because of its new content.
  • The vendor bundle changed because its module.id was changed.
  • And, the manifest bundle changed because it now contains a reference to a new module.

The first and last are expected -- it's the vendor hash we want to fix. Luckily, there are two plugins we can use to resolve this issue. The first is the NamedModulesPlugin, which will use the path to the module rather than a numerical identifier. While this plugin is useful during development for more readable output, it does take a bit longer to run. The second option is the HashedModuleIdsPlugin, which is recommended for production builds:

webpack.config.js

  const path = require('path');
+ const webpack = require('webpack');
  const CleanWebpackPlugin = require('clean-webpack-plugin');
  const HtmlWebpackPlugin = require('html-webpack-plugin');

  module.exports = {
    entry: './src/index.js',
    plugins: [
      new CleanWebpackPlugin(['dist']),
      new HtmlWebpackPlugin({
        title: 'Caching'
      }),
+      new webpack.HashedModuleIdsPlugin()
    ],
    output: {
      filename: '[name].[contenthash].js',
      path: path.resolve(__dirname, 'dist')
    },
    optimization: {
      runtimeChunk: 'single',
      splitChunks: {
        cacheGroups: {
          vendor: {
            test: /[\\/]node_modules[\\/]/,
            name: 'vendors',
            chunks: 'all'
          }
        }
      }
    }
  };

Now, despite any new local dependencies, our vendor hash should stay consistent between builds:

...
                          Asset       Size  Chunks             Chunk Names
   main.216e852f60c8829c2289.js  340 bytes       0  [emitted]  main
vendors.55e79e5927a639d21a1b.js   69.5 KiB       1  [emitted]  vendors
runtime.725a1a51ede5ae0cfde0.js   1.42 KiB       2  [emitted]  runtime
                     index.html  353 bytes          [emitted]
Entrypoint main = runtime.725a1a51ede5ae0cfde0.js vendors.55e79e5927a639d21a1b.js main.216e852f60c8829c2289.js
...

And let's modify our src/index.js to temporarily remove that extra dependency:

src/index.js

  import _ from 'lodash';
- import Print from './print';
+ // import Print from './print';

  function component() {
    var element = document.createElement('div');

    // Lodash, now imported by this script
    element.innerHTML = _.join(['Hello', 'webpack'], ' ');
-   element.onclick = Print.bind(null, 'Hello webpack!');
+   // element.onclick = Print.bind(null, 'Hello webpack!');

    return element;
  }

  document.body.appendChild(component());

And finally run our build again:

...
                          Asset       Size  Chunks             Chunk Names
   main.ad717f2466ce655fff5c.js  274 bytes       0  [emitted]  main
vendors.55e79e5927a639d21a1b.js   69.5 KiB       1  [emitted]  vendors
runtime.725a1a51ede5ae0cfde0.js   1.42 KiB       2  [emitted]  runtime
                     index.html  353 bytes          [emitted]
Entrypoint main = runtime.725a1a51ede5ae0cfde0.js vendors.55e79e5927a639d21a1b.js main.ad717f2466ce655fff5c.js
...

We can see that both builds yielded 55e79e5927a639d21a1b in the vendor bundle's filename.

Conclusion

Caching gets messy. Plain and simple. However the walk-through above should give you a running start to deploying consistent, cacheable assets. See the Further Reading section below to learn more.


Further Reading


Contributors