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Announcing connect() — a new API for creating TCP sockets from Cloudflare Workers


8 min read
Announcing connect() — a new API for creating TCP sockets from Cloudflare Workers

Today, we are excited to announce a new API in Cloudflare Workers for creating outbound TCP sockets, making it possible to connect directly to any TCP-based service from Workers.

Standard protocols including SSH, MQTT, SMTP, FTP, and IRC are all built on top of TCP. Most importantly, nearly all applications need to connect to databases, and most databases speak TCP. And while Cloudflare D1 works seamlessly on Workers, and some hosted database providers allow connections over HTTP or WebSockets, the vast majority of databases, both relational (SQL) and document-oriented (NoSQL), require clients to connect by opening a direct TCP “socket”, an ongoing two-way connection that is used to send queries and receive data. Now, Workers provides an API for this, the first of many steps to come in allowing you to use any database or infrastructure you choose when building full-stack applications on Workers.

Database drivers, the client code used to connect to databases and execute queries, are already using this new API. pg, the most widely used JavaScript database driver for PostgreSQL, works on Cloudflare Workers today, with more database drivers to come.

The TCP Socket API is available today to everyone. Get started by reading the TCP Socket API docs, or connect directly to any PostgreSQL database from your Worker by following this guide.

First — what is a TCP Socket?

TCP (Transmission Control Protocol) is a foundational networking protocol of the Internet. It is the underlying protocol that is used to make HTTP requests (prior to HTTP/3, which uses QUIC), to send email over SMTP, to query databases using database–specific protocols like MySQL, and many other application-layer protocols.

A TCP socket is a programming interface that represents a two-way communication connection between two applications that have both agreed to “speak” over TCP. One application (ex: a Cloudflare Worker) initiates an outbound TCP connection to another (ex: a database server) that is listening for inbound TCP connections. Connections are established by negotiating a three-way handshake, and after the handshake is complete, data can be sent bi-directionally.

A socket is the programming interface for a single TCP connection — it has both a readable and writable “stream” of data, allowing applications to read and write data on an ongoing basis, as long as the connection remains open.

connect() — A simpler socket API

With Workers, we aim to support standard APIs that are supported across browsers and non-browser environments wherever possible, so that as many NPM packages as possible work on Workers without changes, and package authors don’t have to write runtime-specific code. But for TCP sockets, we faced a challenge — there was no clear shared standard across runtimes. Node.js provides the net and tls APIs, but Deno implements a different API — Deno.connect. And web browsers do not provide a raw TCP socket API, though a WICG proposal does exist, and it is different from both Node.js and Deno.

We also considered how a TCP socket API could be designed to maximize performance and ergonomics in a serverless environment. Most networking APIs were designed well before serverless emerged, with the assumption that the developer’s application is also the server, responsible for directly handling configuring TLS options and credentials.

With this backdrop, we reached out to the community, with a focus on maintainers of database drivers, ORMs and other libraries that create outbound TCP connections. Using this feedback, we’ve tried to incorporate the best elements of existing APIs and proposals, and intend to contribute back to future standards, as part of the Web-interoperable Runtimes Community Group (WinterCG).

The API we landed on is a simple function, connect(), imported from the new cloudflare:sockets module, that returns an instance of a Socket. Here’s a simple example showing it used to connect to a Gopher server. Gopher was one of the Internet’s early protocols that relied on TCP/IP, and still works today:

import { connect } from 'cloudflare:sockets';

export default {
  async fetch(req: Request) {
    const gopherAddr = "";
    const url = new URL(req.url);

    try {
      const socket = connect(gopherAddr);

      const writer = socket.writable.getWriter()
      const encoder = new TextEncoder();
      const encoded = encoder.encode(url.pathname + "\r\n");
      await writer.write(encoded);

      return new Response(socket.readable, { headers: { "Content-Type": "text/plain" } });
    } catch (error) {
      return new Response("Socket connection failed: " + error, { status: 500 });

We think this API design has many benefits that can be realized not just on Cloudflare, but in any serverless environment that adopts this design:

connect(address: SocketAddress | string, options?: SocketOptions): Socket

declare interface Socket {
  get readable(): ReadableStream;
  get writable(): WritableStream;
  get closed(): Promise<void>;
  close(): Promise<void>;
  startTls(): Socket;

declare interface SocketOptions {
  secureTransport?: string;
  allowHalfOpen: boolean;

declare interface SocketAddress {
  hostname: string;
  port: number;

Opportunistic TLS (StartTLS), without separate APIs

Opportunistic TLS, a pattern of creating an initial insecure connection, and then upgrading it to a secure one that uses TLS, remains common, particularly with database drivers. In Node.js, you must use the net API to create the initial connection, and then use the tls API to create a new, upgraded connection. In Deno, you pass the original socket to Deno.startTls(), which creates a new, upgraded connection.

Drawing on a previous W3C proposal for a TCP Socket API, we’ve simplified this by providing one API, that allows TLS to be enabled, allowed, or used when creating a socket, and exposes a simple method, startTls(), for upgrading a socket to use TLS.

// Create a new socket without TLS. secureTransport defaults to "off" if not specified.
const socket = connect("address:port", { secureTransport: "off" })

// Create a new socket, then upgrade it to use TLS.
// Once startTls() is called, only the newly created socket can be used.
const socket = connect("address:port", { secureTransport: "starttls" })
const secureSocket = socket.startTls();

// Create a new socket with TLS
const socket = connect("address:port", { secureTransport: "use" })

TLS configuration — a concern of host infrastructure, not application code

Existing APIs for creating TCP sockets treat TLS as a library that you interact with in your application code. The tls.createSecureContext() API from Node.js has a plethora of advanced configuration options that are mostly environment specific. If you use custom certificates when connecting to a particular service, you likely use a different set of credentials and options in production, staging and development. Managing direct file paths to credentials across environments and swapping out .env files in production build steps are common pain points.

Host infrastructure is best positioned to manage this on your behalf, and similar to Workers support for making subrequests using mTLS, TLS configuration and credentials for the socket API will be managed via Wrangler, and a connect() function provided via a capability binding. Currently, custom TLS credentials and configuration are not supported, but are coming soon.

Start writing data immediately, before the TLS handshake finishes

Because the connect() API synchronously returns a new socket, one can start writing to the socket immediately, without waiting for the TCP handshake to first complete. This means that once the handshake completes, data is already available to send immediately, and host platforms can make use of pipelining to optimize performance.

connect() API + DB drivers = Connect directly to databases

Many serverless databases already work on Workers, allowing clients to connect over HTTP or over WebSockets. But most databases don’t “speak” HTTP, including databases hosted on most cloud providers.

Databases each have their own “wire protocol”, and open-source database “drivers” that speak this protocol, sending and receiving data over a TCP socket. Developers rely on these drivers in their own code, as do database ORMs. Our goal is to make sure that you can use the same drivers and ORMs you might use in other runtimes and on other platforms on Workers.

Try it now — connect to PostgreSQL from Workers

We’ve worked with the maintainers of pg, one of the most popular database drivers in the JavaScript ecosystem, used by ORMs including Sequelize and knex.js, to add support for connect().

You can try this right now. First, create a new Worker and install pg:

wrangler init
npm install --save pg

As of this writing, you’ll need to enable the node_compat option in wrangler.toml:


name = "my-worker"
main = "src/index.ts"
compatibility_date = "2023-05-15"
node_compat = true

In just 20 lines of TypeScript, you can create a connection to a Postgres database, execute a query, return results in the response, and close the connection:


import { Client } from "pg";

export interface Env {
  DB: string;

export default {
  async fetch(
    request: Request,
    env: Env,
    ctx: ExecutionContext
  ): Promise<Response> {
    const client = new Client(env.DB);
    await client.connect();
    const result = await client.query({
      text: "SELECT * from customers",
    const resp = Response.json(result.rows);
    // Close the database connection, but don't block returning the response
    return resp;

To test this in local development, use the --experimental-local flag (instead of –local), which uses the open-source Workers runtime, ensuring that what you see locally mirrors behavior in production:

wrangler dev --experimental-local

What’s next for connecting to databases from Workers?

This is only the beginning. We’re aiming for the two popular MySQL drivers, mysql and mysql2, to work on Workers soon, with more to follow. If you work on a database driver or ORM, we’d love to help make your library work on Workers.

If you’ve worked more closely with database scaling and performance, you might have noticed that in the example above, a new connection is created for every request. This is one of the biggest current challenges of connecting to databases from serverless functions, across all platforms. With typical client connection pooling, you maintain a local pool of database connections that remain open. This approach of storing a reference to a connection or connection pool in global scope will not work, and is a poor fit for serverless. Managing individual pools of client connections on a per-isolate basis creates other headaches — when and how should connections be terminated? How can you limit the total number of concurrent connections across many isolates and locations?

Instead, we’re already working on simpler approaches to connection pooling for the most popular databases. We see a path to a future where you don’t have to think about or manage client connection pooling on your own. We’re also working on a brand new approach to making your database reads lightning fast.

What’s next for sockets on Workers?

Supporting outbound TCP connections is only one half of the story — we plan to support inbound TCP and UDP connections, as well as new emerging application protocols based on QUIC, so that you can build applications beyond HTTP with Socket Workers.

Earlier today we also announced Smart Placement, which improves performance by placing any Worker that makes multiple HTTP requests to an origin run as close as possible to reduce round-trip time. We’re working on making this work with Workers that open TCP connections, so that if your Worker connects to a database in Virginia and makes many queries over a TCP connection, each query is lightning fast and comes from the nearest location on Cloudflare’s global network.

We also plan to support custom certificates and other TLS configuration options in the coming months — tell us what is a must-have in order to connect to the services you need to connect to from Workers.

Get started, and share your feedback

The TCP Socket API is available today to everyone. Get started by reading the TCP Socket API docs, or connect directly to any PostgreSQL database from your Worker by following this guide.

We want to hear your feedback, what you’d like to see next, and more about what you’re building. Join the Cloudflare Developers Discord.

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Brendan Irvine-Broque|@irvinebroque
Matt Silverlock|@elithrar

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