Topcoat enables Rust to render server-side web apps

Rust guru rejiggers web app reactivity for potentially speedier page loading

Popular for other uses, Rust isn't typically deployed for web development. That could change, however.

With AI here to do the heavy lifting, coders may want to consider the programming language, given its runtime performance and safety guarantees, suggested Carl Lerche, an Amazon Web Services principal engineer, speaking at ScyllaDB’s 2026 P99 virtual conference last week. 

Lerche was there to demonstrate his latest project, Topcoat, a framework for running web applications written in Rust. 

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Lerche is perhaps best known for creating Tokio, which is the most popular asynchronous Rust runtime in that ecosystem.

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“Three years ago, had [I] gone out and said that Rust could be the best language for building web apps, you probably would have called me crazy,” he told the virtual audience. 

Web developers gravitate towards the languages they find most familiar. These days, that means JavaScript, or possibly Ruby. 

A number of web frameworks already use Rust. Both Leptos and Dioxus compile Rust into WebAssembly, which most browsers run. This approach is great for heavy computational jobs, though the upfront loading time can be a real drag. 

Topcoat compiles your Rust code into more web-friendly JavaScript. For Lerche, JavaScript remains the best way to interact with the browser's Document Object Model (DOM).

“Generally, the approach taken by any Rust framework can be traced back to another ecosystem, usually a very popular one. You can't really point to a single existing framework as [Topcoat’s] main source of inspiration,” noted frontend developer Ken Kohlman, in a summary video on his “Code to the Moon” YouTube channel.

Walk on the server side

Lerche admitted that Topcoat isn’t for building complex applications, on the scale of, say, Figma, but it can easily be used for the run-of-the-mill, browser-based, database-driven applications, which is to say 90 percent of the web today.

Unlike most present-day JavaScript frameworks, Topcoat eschews the current definition of being reactive, where the program logic is dumped onto the browser when the session begins. Instead, most of the logic executes on the server, with only the results delivered to the browser (Developers can still write client-side logic using a subset of Rust, which Topcoat automatically compiles into JavaScript for the browser).

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It’s an old idea recast with better tech. Server-side rendering predates reactive programming (see Apache’s server-side includes). With server-side rendering, Lerche admitted, “It might sound like we're going back to the old days of full page refreshes for every click [but] you can actually get almost all of what you think of as reactive with server-side rendering.”

But server-side programming has been making a comeback lately, especially for those devs disgruntled with React and other frameworks. HTMX, Datastar, and Phoenix LiveView all render web pages on the server, sending them back to the browser nearly completed. 

Topcoat is just tweaking their ideas to fit well with Rust, Lerche said. When a user smashes a button on a web page, the whole page isn’t refreshed. Just the HTML snippet for that component is returned and refreshed by the server.

JavaScript in the front, Rust in the back  

Topcoat saves load times by rendering all the components at once, rather than one by one. Also, data loading does not block the loading of the page. The server can send back a web page with placeholders into which the data will stream.  

One big dollop of secret sauce that Topcoat has is that it provides a way for the component, which is already on the server, to speak directly to the database, thanks to an earlier project from Lerche called Toasty, a Rust-based asynchronous database object-relational mapper (which he spoke about at P99 2024). 

“You can do your rendering right with the data immediately, so you don't have to figure out how to serialize it, send it to the client, deserialize it,” he said. 

By pushing data loading deep down into the page render, the server can send the head of the HTML template to the client immediately, allowing the browser to start preloading assets (like CSS or images) while the server renders the rest of the page.

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“The simplicity is incredible, and it is something that we have been sorely missing in this space. I feel like someone who doesn't really know Rust could get off the ground pretty quickly with this thing,” Kohlman said. He did agree with Lerche that Topcoat is best for smaller web projects, given that Topcoat only uses a subset of the Rust syntax. 

“There's something to be said for having the full Rust syntax at your disposal,” he said. ®