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Runtime Engines

Kiln supports two runtime server engines for serving your compiled application:

  1. default Engine (Default): Uses the framework’s official production HTTP server.
  2. bun-serve Engine: Uses Bun’s native Zig-powered HTTP dispatcher for zero-copy static asset delivery at 100,000+ req/sec.

Comparison Matrix

Section titled “Comparison Matrix”
Feature default Engine bun-serve Engine
Static Asset Performance Standard framework speed (~15k req/s) Native Zig speed (~100k+ req/s)
Time to First Byte (TTFB) ~5-15ms <1ms (Instant)
Zero-Copy Static File Transfers ❌ (Node.js buffers) ✅ (Bun.file kernel sendfile)
Dynamic SSR & Server Actions ✅ Native ✅ Native (proxied in-process)
Next.js 16 proxy.ts ✅ Intercepts all traffic ✅ Intercepts dynamic & API traffic
Best For 100% standard framework compatibility High-traffic public web applications

How to Select an Engine

Section titled “How to Select an Engine”

Via CLI Option:

Section titled “Via CLI Option:”
Terminal window
# Standard default engine
kiln --engine default -o ./bin/app


# High-speed Bun.serve engine
kiln --engine bun-serve -o ./bin/app

Via Environment Variable:

Section titled “Via Environment Variable:”
Terminal window
KILN_ENGINE=bun-serve kiln -o ./bin/app

How the bun-serve Engine Works

Section titled “How the bun-serve Engine Works”

When compiled with --engine bun-serve:

┌──────────────────────────────────────────────────────────┐
│                   Incoming HTTP Request                  │
└────────────────────────────┬─────────────────────────────┘
                             │
                             ▼
┌──────────────────────────────────────────────────────────┐
│             Bun.serve Native Zig Dispatcher              │
└──────────────┬────────────────────────────┬──────────────┘
               │                            │
  Matches Static Asset in         Dynamic Route, API,
  /_next/static/* or public/      Server Action, or proxy.ts
               │                            │
               ▼                            ▼
      ⚡ Instant Response            Proxied In-Process to
     (~100,000+ req/sec)              Framework Server
  1. Tier 1 (Static /_next/static/* assets): Served directly via Bun.file with Cache-Control: public, max-age=31536000, immutable.
  2. Tier 2 (Public static files): Served directly via Bun.file with zero JS heap allocation.
  3. Tier 3 (Dynamic SSR & APIs): Forwarded to the in-process framework server with full support for React 19 Server Actions and Next 16 proxy.ts.