Node.js runs on a single thread. That's usually fine. The event loop handles I/O concurrency without you thinking about locks, races, or deadlocks. But "single-threaded" has a cost that only shows up under pressure: if your JavaScript monopolizes the CPU, nothing else runs. No timers fire. No network callbacks execute. No I/O completes.

We ran into this with Inngest Connect, a persistent WebSocket connection between your app and the Inngest server. Connect is an alternative to our HTTP-based model (our serve function) that reduces TCP handshake overhead and avoids long-lived HTTP requests during long-running steps. Workers send heartbeats over the WebSocket so the server knows they're alive.

• The problem: Users reported "no available worker" errors despite their workers running.
• The cause: CPU-heavy user code was monopolizing the main thread, starving the event loop, and blocking heartbeats. The server assumed the workers were dead and stopped routing work to them.
• The fix: Move Connect internals into a worker thread.

But getting there taught us a few things about how worker threads actually work in Node.js, and how they compare to threading models in other languages.

Connect's worker thread isolation is a new feature in v4 of the Inngest TypeScript SDK. Upgrade to get it automatically.

This post focuses on Node.js, but Bun and Deno also support worker threads.

Event loop starvation

Node's event loop processes callbacks in phases: timers, I/O polling, setImmediate, close callbacks. Between each phase, it checks for microtasks (resolved promises, queueMicrotask). The critical property: the loop can only advance when the current JavaScript execution yields.

A synchronous function that runs for 30 seconds blocks everything for 30 seconds. That includes setTimeout callbacks, incoming network data, and any other scheduled work. The timers don't fire at all until the thread is free.

Consider a heartbeat scheduled with setInterval every 10 seconds. The callback is queued, ready to fire. Then a CPU-heavy function starts and runs for 30 seconds straight. The heartbeat callback sits in the timer queue the entire time, and multiple intervals pass without a single one going out. By the time the function returns, the server has already timed out and marked the worker as dead.

The standard advice is "don't block the event loop." But sometimes you're running user code and you don't control what it does. You need the critical path (heartbeats, connection management) isolated from user workloads. That's where worker threads come in.

What worker threads give you

The worker_threads module lets you spin up additional JavaScript execution contexts within the same process. Each worker gets its own V8 isolate, its own heap, and its own event loop. Critically, one worker's CPU-bound code does not block another worker's event loop.

Let's look at the basic API. You'll notice that the two threads communicate by sending messages to each other.

main.js

javascript

import { Worker } from "node:worker_threads";

// Spawns a new thread running worker.js
const worker = new Worker("./worker.js", {
  workerData: { greeting: "hello" }, // Initial data, cloned into the worker
});

// Receive messages from the worker
worker.on("message", (msg) => {
  console.log("from worker:", msg);
});

// Send a message to the worker
worker.postMessage({ type: "ping" });

worker.js

javascript

import { parentPort, workerData } from "node:worker_threads";

// workerData is the cloned initial payload
console.log(workerData.greeting); // "hello"

// parentPort is the channel back to the main thread
parentPort.on("message", (msg) => {
  if (msg.type === "ping") {
    parentPort.postMessage({ type: "pong" });
  }
});

Two independent event loops, each able to run JavaScript without blocking the other.

The constraints

Worker threads solve the isolation problem, but they come with constraints that feel jarring if you've used concurrency primitives in other languages.

You can't pass logic to a worker

In Go, you spawn a goroutine with a function:

go

go func() {
  fmt.Println("hello from goroutine")
}()

In Rust, you spawn a thread with a closure:

rust

std::thread::spawn(|| {
  println!("hello from thread");
});

In Python, you pass a function to a new thread:

threading.Thread(target=lambda: print("hello from thread")).start()

In each of these languages, you hand arbitrary logic directly to the concurrency primitive. The function (or closure, or lambda) carries its logic and captured state together.

A caveat on Python: the GIL (Global Interpreter Lock) prevents threads from executing Python bytecode in parallel, so threads won't speed up CPU-bound work. They're still useful for protecting I/O from being blocked by other threads, which is similar to our heartbeat problem. Python 3.13 introduced an experimental free-threaded mode that removes the GIL, so this limitation is on its way out.

Node.js worker threads don't work this way. You can't pass a function to new Worker(). The structured clone algorithm, which serializes data between threads, can't serialize functions. Instead, you point the worker at a file:

javascript

const worker = new Worker("./my-worker.js");

// There is no equivalent of this:
// const worker = new Worker(() => { doStuff() });

Think of the structured clone algorithm as JSON.stringify/JSON.parse with broader type support (Map, Set, Date, ArrayBuffer, circular references). The key similarity: neither can serialize functions.

This means every worker thread is an independent program with its own entry point, imports, and initialization. You design the communication protocol up front and exchange serialized messages, which makes the experience closer to writing a microservice than spawning a concurrent task.

Communication is message passing

In Go, goroutines share the same address space, and in Python, threads share the same heap. In both languages, passing data between concurrent tasks is cheap because it stays in memory.

Node.js workers are isolated by default. They communicate via postMessage and event listeners. Data is serialized using the structured clone algorithm, meaning most JavaScript values (objects, arrays, typed arrays, Map, Set) are deep-copied between threads. Every message is serialized on one side and deserialized on the other. For small messages this is negligible, but large payloads (big JSON blobs, deeply nested objects) pay a real cost in both CPU time and memory, since the data exists in both heaps simultaneously.

If you need shared state instead of message passing, SharedArrayBuffer lets threads share raw memory, and Atomics provides thread-safe operations on it. This avoids serialization entirely, but you're limited to typed arrays of numbers. It's well suited for shared counters, flags, or coordination primitives, but not for passing structured messages like the ones Connect exchanges between threads.

Bundlers can't see your worker file

This one is subtle and annoying. Bundlers (webpack, esbuild, Rollup) perform static analysis to discover imports and produce optimized bundles. When they encounter a standard import or require, they follow the dependency graph automatically. But new Worker("./worker.js") isn't an import. It's a string argument to a constructor.

Modern bundlers recognize a specific pattern:

javascript

new Worker(new URL("./worker.js", import.meta.url));

If you use this exact syntactic form with a string literal, webpack 5+ will detect it, resolve the file, and emit it as a separate bundle. But any indirection breaks the detection. None of the following examples are detected by webpack:

javascript

const path = "./worker.js";
new Worker(new URL(path, import.meta.url));

new Worker(new URL(`./workers/${name}.js`, import.meta.url));

const url = new URL("./worker.js", import.meta.url);
new Worker(url);

TypeScript adds another layer. The file extension you reference in the new URL() call depends on your toolchain: webpack resolves .ts through its loader pipeline, plain tsc expects .js (because the URL resolves at runtime against compiled output), and esbuild doesn't auto-detect workers at all. Each requires a different approach.

If you're building a library that uses worker threads internally, this gets worse. Your library's worker file needs to survive your consumer's bundler, which you don't control. The file must be explicitly included in the build output as a separate entry point, typically through a build script or bundler plugin.

For our TypeScript SDK, we added the worker file as an explicit entry point in our tsdown config so it gets compiled and included in the package output. We also had to make the file extension dynamic (.js or .ts) based on the file type of the caller, since consumers using ts-node or tsx run against .ts files directly while compiled environments expect .js.

They aren't lightweight

Each worker thread is a full V8 isolate with its own heap and event loop. That means roughly 10 MB of memory overhead per worker and a startup cost in the tens of milliseconds. Goroutines start at a few KB, and a Go program can comfortably run thousands of them. OS-level threads in Rust, C, or Python are an order of magnitude cheaper than a V8 isolate. You won't be running a pool of hundreds of Node.js workers the way you might with goroutines or threads.

This makes worker threads best suited for long-lived workers that justify the overhead, not short-lived tasks you spin up and tear down frequently.

How we used worker threads for Inngest Connect

The architecture shift

Back to the original problem. Connect maintains a persistent WebSocket to the Inngest server. The server pushes function invocations over this connection, and the SDK executes them. Heartbeats flow back to confirm the worker is alive. Any user running CPU-heavy functions with Connect could hit the starvation problem, since a single long computation would block heartbeats and cause the server to drop the connection.

The architecture before worker threads looked like this:

Main Thread

User code execution

Connect internals

‣ WebSocket conn

‣ Heartbeats

‣ Reconnection

‣ Auth handshakes

Everything shared one event loop, so when a user's function did something CPU-intensive (heavy computation, data transformation, image processing) the heartbeat timer couldn't fire and the server timed out.

After worker threads:

Main Thread

User code execution

messages←→

Worker Thread

Connect internals

‣ WebSocket conn

‣ Heartbeats

‣ Etc.

The Connect internals (WebSocket, heartbeats, etc.) live in a worker thread, while user code execution stays on the main thread. The two communicate via message passing.

Now a CPU-heavy function can saturate the main thread's event loop for as long as it wants. The worker thread's event loop keeps ticking independently, so heartbeats go out on time and the server knows the worker is alive.

Message passing in practice

The main traffic is forwarded WebSocket frames: invocations flow from the worker thread to the main thread for execution, and results flow back.

Logging unexpectedly became a message passing problem, too. The Inngest SDK lets users pass a custom logger (Winston, Pino, any compatible logger). That logger lives on the main thread, and since it's an object with methods, the structured clone algorithm can't serialize it. The worker thread can't call logger.info() directly.

So log messages became part of the protocol. The worker thread posts structured log entries (level, message, context) to the main thread, which feeds them into the user's logger. From the user's perspective, logs from the worker thread look like logs from anywhere else in the SDK. From our perspective, it's another message type in the protocol.

This pattern generalizes: anything that relies on user-provided objects (loggers, callbacks, configuration with function values) has to stay on the main thread. The worker thread can only request that the main thread use them on its behalf.

Respawning with backoff

We also had to handle the worker thread dying. A bug in the connection logic, an unhandled exception, or a V8 out-of-memory error could crash the worker. If the main thread doesn't notice and respawn it, the user's app silently loses its connection to Inngest. So the main thread watches for the worker's exit event and spins up a replacement.

But naive respawning is dangerous. If the worker hits a pathological error (a bad server response it can't parse, a misconfiguration that crashes on startup) it could die immediately after spawning, over and over. Without a backoff, the main thread would enter a tight respawn loop, burning CPU and flooding logs. We added exponential backoff to the respawn logic: the first restart is immediate, the second waits a short interval, and each subsequent restart doubles the delay up to a cap. A successful startup resets the backoff. This keeps the system self-healing under transient failures without spiraling under persistent ones.

The tradeoffs

Every constraint described above applied:

• We couldn't move "just the heartbeat logic" into a worker. We had to move all the connection management, because the worker thread is a separate file with its own entry point and its own initialization.
• All communication between the SDK's public API (main thread) and the connection internals (worker thread) had to be designed as a message protocol.
• The worker thread file had to be explicitly compiled and included in the SDK's published package, because bundlers couldn't discover it through static analysis.

But the result was worth it. The "no available worker" errors stopped.

Wrapping up

Worker threads are a genuine tool for isolating work in Node.js, but the programming model is fundamentally different from goroutines, Rust threads, or Python's threading module. You're not spawning a concurrent task with a closure. You're launching a separate program and communicating over a serialized message channel. That's more work up front, but it gives you hard isolation, which is exactly what you need when the main thread can't be trusted to yield.

If you're hitting event loop starvation in your own Node.js apps, worker threads are worth considering. If you're building with Inngest, Connect handles all of this for you. And if you're new to Inngest, start here to see how it works.