Core Concepts

Let’s dive deep into the internals of Turbopack to figure out why it’s so fast.

The Turbo engine

Turbopack is so fast because it’s built on a reusable library for Rust which enables incremental computation known as the Turbo engine. Here’s how it works:

Function-level caching

In a Turbo engine-powered program, you can mark certain functions as ‘to be remembered’. When these functions are called, the Turbo engine will remember what they were called with, and what they returned. It’ll then save it in an in-memory cache.

Here’s a simplified example of what this might look like in a bundler:

We start with calling readFile on two files, api.ts and sdk.ts. We then bundle those files, concat them together, and end up with the fullBundle at the end. The results of all of those function calls get saved in the cache for later.

Let’s imagine that we’re running on a dev server. You save the sdk.ts file on your machine. Turbopack receives the file system event, and knows it needs to recompute readFile("sdk.ts"):

Since the result of sdk.ts has changed, we need to bundle it again, which then needs to be concatenated again.

Crucially, api.ts hasn’t changed. We read its result from the cache and pass that to concat instead. So we save time by not reading it and re-bundling it again.

Now imagine this in a real bundler, with thousands of files to read and transformations to execute. The mental model is the same. You can save enormous amounts of work by remembering the result of function calls and not re-doing work that’s been done before.

The cache

The Turbo engine currently stores its cache in memory. This means the cache will last as long as the process running it - which works well for a dev server. When you run next dev --turbo in Next.js 13+, you’ll start a cache with the Turbo engine. When you cancel your dev server, the cache gets cleared.

In the future, we’re planning to persist this cache - either to the filesystem, or to a remote cache like Turborepo’s. This will mean that Turbopack could remember work done across runs and machines.

How does it help?

This approach makes Turbopack extremely fast at computing incremental updates to your apps. This optimizes Turbopack for handling updates in development, meaning your dev server will always respond snappily to changes.

In the future, a persistent cache will open the door to much faster production builds. By remembering work done across runs, new production builds could only rebuild changed files - potentially leading to enormous time savings.