The journey to fast production asset builds with Webpack

Posted by on February 3, 2020

Etsy has switched from using a RequireJS-based JavaScript build system to using Webpack. This has been a crucial cornerstone in the process of modernizing Etsy’s JavaScript ecosystem. We’ve learned a lot from addressing our multiple use-cases during the migration, and this post is the first of two parts documenting our learnings. Here, we specifically cover the production use-case — how we set up Webpack to build production assets in a way that meets our needs. The second post can be found here.

We’re proud to say that our Webpack-powered build system, responsible for over 13,200 assets and their source maps, finishes in four minutes on average. This fast build time is the result of countless hours of optimizing. What follows is our journey to achieving such speed, and what we’ve discovered along the way.

Production Expectations

One of the biggest challenges of migrating to Webpack was achieving production parity with our pre-existing JavaScript build system, named Builda. It was built on top of the RequireJS optimizer, a build tool predating Webpack, with extensive customization to speed up builds and support then-nascent standards like JSX. Supporting Builda became more and more untenable, though, with each custom patch we added to support JavaScript’s evolving standards. By early 2018, we consequently decided to switch to Webpack; its community support and tooling offered a sustainable means to keep up with JavaScript modernization. However, we had spent many years optimizing Builda to accommodate our production needs. Assets built by Webpack would need to have 100% functional parity with assets built by Builda in order for us to have confidence around switching.

Our primary expectation for any new build system is that it takes less than five minutes on average to build our production assets. Etsy is one of the earliest and biggest proponents of continuous deployment, where our fast build/deploy pipeline allows us to ship changes up to 30 times a day. Builda was already meeting this expectation, and we would negatively impact our deployment speed/frequency if a new system took longer to build our JavaScript. Build times, however, tend to increase as a codebase grows, productionization becomes more complex, and available computing resources are maxed out.

At Etsy, our frontend consists of over 12,000 modules that eventually get bundled into over 1,200 static assets. Each asset needs to be localized and minified, both of which are time-consuming tasks. Furthermore, our production asset builds were limited to using 32 CPU cores and 64GB of RAM. Etsy had not yet moved to the cloud when we started migrating to Webpack, and these specs were of the beefiest on-premise hosts available. This meant we couldn’t just add more CPU/RAM to achieve faster builds.

So, to recap:

We got this.


From the start, we knew that localization would be a major obstacle to achieving sub-five-minute build times. Localization strings are embedded in our JavaScript assets, and at Etsy we officially support eleven locales. This means we need to produce eleven copies of each asset, where each copy contains localization strings of a specific locale. Suddenly, building over 1,200 assets balloons into building over 1,200 × 11 = 13,200 assets.

General caching solutions help reduce build times, idempotent of localization’s multiplicative factor. After we solved the essential problems of resolving our module dependencies and loading our custom code with Webpack, we incorporated community solutions like cache-loader and babel-loader’s caching options. These solutions cache intermediary artifacts of the build process, which can be time-consuming to calculate. As a result, asset builds after the initial one finish much faster. Still though, we needed more than caching to build localized assets quickly.

One of the first search results for Webpack localization was the now-deprecated i18n-webpack-plugin. It expects a separate Webpack configuration for each locale, leading to a separate production asset build per locale. Even though Webpack supports multiple configurations via its MultiCompiler mode, the documentation crucially points out that “each configuration is only processed after the previous one has finished processing.” At this stage in our process, we measured that a single production asset build without minification was taking ~3.75 minutes with no change to modules and a hot cache (a no-op build). It would take us ~3.75 × 11 = ~41.25 minutes to process all localized configurations for a no-op build.

We also ruled out using this plugin with a common solution like parallel-webpack to process configurations in parallel. Each parallel production asset build requires additional CPU and RAM, and the sum far exceeded the 32 CPU cores and 64GB of RAM available. Even when we limited the parallelism to stay under our resource limits, we were met with overall build times of ~15 minutes for a no-op build. It was clear we need to approach localization differently.

Localization inlining

To localize our assets, we took advantage of two characteristics about our localization. First, the way we localize our JavaScript code is through a module abstraction. An engineer defines a module that contains only key-value pairs. The value is the US-English version of the text that needs to be localized, and the key is a succinct description of the text. To use the localized strings, the engineer imports the module in their source code. They then have access to a function that, when passed a string corresponding to one of the keys, returns the localized value of the text.

example of how we include localizations in our JavaScript

For a different locale, the message catalog contains analogous localization strings for the locale. We programmatically handle generating analogous message catalogs with a custom Webpack loader that applies whenever Webpack encounters an import for localizations. If we wanted to build Spanish assets, for example, the loader would look something like this:

example of how we would load Spanish localizations into our assets

Second, once we build the localized code and output localized assets, the only differing lines between copies of the same asset from different locales are the lines with localization strings; the rest are identical. When we build the above example with English and Spanish localizations, the diff of the resulting assets confirms this:

diff of the localized copies of an asset

Even when caching intermediary artifacts, our Webpack configuration would spend over 50% of the overall build time constructing the bundled code of an asset. If we provided separate Webpack configurations for each locale, we would repeat this expensive asset construction process eleven times.

diagram of running Webpack for each locale

We could never finish this amount of work within our build-time constraints, and as we saw before, the resulting localized variants of each asset would be identical except for the few lines with localizations. What if, rather than locking ourselves into loading a specific locale’s localization and repeating an asset build for each locale, we returned a placeholder where the localizations should go?

code to load placeholders in place of localizations

We tried this placeholder loader approach, and as long as it returned syntactically valid JavaScript, Webpack could continue with no issue and generate assets containing these placeholders, which we call “sentinel assets”. Later on in the build process a custom plugin takes each sentinel asset, finds the placeholders, and replaces them with corresponding message catalogs to generate a localized asset.

diagram of our build process with localization inlining

We call this approach “localization inlining”, and it was actually how Builda localized its assets too. Although our production asset builds write these sentinel assets to disk, we do not serve them to users. They are only used to derive the localized assets.

With localization inlining, we were able to generate all of our localized assets from one production asset build. This allowed us to stay within our resource limits; most of Webpack’s CPU and RAM usage is tied to calculating and generating assets from the modules it has loaded. Adding additional files to be written to disk does not increase resource utilization as much as running an additional production asset build does.

Now that a single production asset build was responsible for over 13,200 assets, though, we noticed that simply writing this many assets to disk substantially increased build times. It turns out, Webpack only uses a single thread to write a build’s assets to disk. To address this bottleneck, we included logic to write a new localized asset only if the localizations or the sentinel asset have changed — if neither have changed, then the localized asset hasn’t changed either. This optimization greatly reduced the amount of disk writing after the initial production asset build, allowing subsequent builds with a hot cache to finish up to 1.35 minutes faster. A no-op build without minification consistently finished in ~2.4 minutes. With a comprehensive solution for localization in place, we then focused on adding minification.


Out of the box, Webpack includes the terser-webpack-plugin for asset minification. Initially, this plugin seemed to perfectly address our needs. It offered the ability to parallelize minification, cache minified results to speed up subsequent builds, and even extract license comments into a separate file.

When we added this plugin to our Webpack configuration, though, our initial asset build suddenly took over 40 minutes and used up to 57GB of RAM at its peak. We expected the initial build to take longer than subsequent builds and that minification would be costly, but this was alarming. Enabling any form of production source maps also dramatically increased the initial build time by a significant amount. Without the terser-webpack-plugin, the initial production asset build with localizations would finish in ~6 minutes. It seemed like the plugin was adding an unknown bottleneck to our builds, and ad hoc monitoring with htop during the initial production asset build seemed to confirmed our suspicions:

htop during minification

At some points during the minification phase, we appeared to only use a single CPU core. This was surprising to us because we had enabled parallelization in terser-webpack-plugin’s options. To get a better understanding of what was happening, we tried running strace on the main thread to profile the minification phase:

strace during minification

At the start of minification, the main thread spent a lot of time making memory syscalls (mmap and munmap). Upon closer inspection of terser-webpack-plugin’s source code, we found that the main thread needed to load the contents of every asset to generate parallelizable minification jobs for its worker threads. If source maps were enabled, the main thread also needed to calculate each asset’s corresponding source map. These lines explained the flood of memory syscalls we noticed at the start.

Further into minification, the main thread started making recvmsg and write syscalls to communicate between threads. We corroborated these syscalls when we found that the main thread needed to serialize the contents of each asset (and source maps if they were enabled) to send it to a worker thread to be minified. After receiving and deserializing a minification result received from a worker thread, the main thread was also solely responsible for caching the result to disk. This explained the stat, open, and other write syscalls we observed because the Node.js code promises to write the contents. The underlying epoll_wait syscalls then poll to check when the writing finishes so that the promise can be resolved.

The main thread can become a bottleneck when it has to perform these tasks for a lot of assets, and considering our production asset build could produce over 13,200 assets, it was no wonder we hit this bottleneck. To minify our assets, we would have to think of a different way.


We opted to minify our production assets outside of Webpack, in what we call “post-processing”. We split our production asset build into two stages, a Webpack stage and a post-processing stage. The former is responsible for generating and writing localized assets to disk, and the latter is responsible for performing additional processing on these assets, like minification:

running Webpack with a post-processing stage
diagram of our build process with localization inlining and post-processing

For minification, we use the same terser library the terser-webpack-plugin uses. We also baked parallelization and caching into the post-processing stage, albeit in a different way than the plugin. Where Webpack’s plugin reads the file contents on the main thread and sends the whole contents to the worker threads, our parallel-processing jobs send just the file path to the workers. A worker is then responsible for reading the file, minifying it, and writing it to disk. This reduces memory usage and facilitates more efficient parallel-processing. To implement caching, the Webpack stage passes along the list of assets written by the current build to tell the post-processing stage which files are new. Sentinel assets are also excluded from post-processing because they aren’t served to users.

Splitting our production asset builds into two stages does have a potential downside: our Webpack configuration is now expected to output un-minified text for assets. Consequently, we need to audit any third-party plugins to ensure they do not transform the outputted assets in a format that breaks post-processing. Nevertheless, post-processing is well worth it because it allows us to achieve the fast build times we expect for production asset builds.

Bonus: source maps

We don’t just generate assets in under five minutes on average — we also generate corresponding source maps for all of our assets too. Source maps allow engineers to reconstruct the original source code that went into an asset. They do so by maintaining a mapping of the output lines of a transformation, like minification or Webpack bundling, to the input lines. Maintaining this mapping during the transformation process, though, inherently adds time.

Coincidentally, the same localization characteristics that enable localization inlining also enable faster source map generation. As we saw earlier, the only differences between localized assets are the lines containing localization strings. Subsequently, these lines with localization strings are the only differences between the source maps for these localized assets. For the rest of the lines, the source map for one localized asset is equivalently accurate for another because each line is at the same line number between localized assets.

If we were to generate source maps for each localized asset, we would end up repeating resource-intensive work only to result in nearly identical source maps across locales. Instead, we only generate source maps for the sentinel assets the localized assets are derived from. We then use the sentinel asset’s source map for each localized asset derived from it, and accept that the mapping for the lines with localization strings will be incorrect. This greatly speeds up source map generation because we are able to reuse a single source map that applies to many assets.

For the minification transformation that occurs during post-processing, terser accepts a source map alongside the input to be minified. This input source map allows terser to account for prior transformations when generating source mappings for its minification. As a result, the source map for its minified results still maps back to the original source code before Webpack bundling. In our case, we pass terser the sentinel asset’s source map for each localized asset derived from it. This is only possible because we aren’t using terser-webpack-plugin, which (understandably) doesn’t allow mismatched asset/source map pairings.

diagram of our complete build process with localization inlining, post-processing, and source map optimizations

Through these source map optimizations, we are able to maintain source maps for all assets while only adding ~1.7 minutes to our build time average. Our unique approach can result in up to a 70% speedup in source map generation compared to out-of-the-box options offered by Webpack, a dramatic reduction in the time.


Our journey to achieving fast production builds can be summed up into three principles: reduce, reuse, recycle.

While some implementation details may become obsolete as Webpack and the frontend evolve, these principles will continue to guide us towards faster production builds.

This post is the first in a two-part series on our migration to a modern JavaScript build system. The second part can be found here.

Posted by on February 3, 2020
Category: engineering, infrastructure Tags: ,


Wow, excellent article. Thanks for sharing!

Thanks for sharing such an insightful article.

Excellent write up, thanks for sharing!

What fidelity of source maps are you producing on the webpack side?

Also curious to know what speedup factor you saw from replacing terser-webpack-plugin with your own post-processing step?

This is exactly what I was looking for – the terser plugin was eating up too much memory during build. Also, interesting localization inlining solution. Fantastic article