Capability bindings

Bindings are how workers access external resources and configuration. Each binding gives the worker a JavaScript API object that provides access to a specific capability. This capability-based approach makes it clear what each worker can access and prevents SSRF attacks.

How bindings work

Bindings appear differently depending on your worker syntax:

ES modules: Bindings are properties on the env object passed to handlers
Service workers: Bindings are global variables

// ES modules syntax
export default {
  async fetch(request, env, ctx) {
    // Access bindings via env object
    const apiKey = env.API_KEY;
    const response = await env.backend.fetch(request);
    return response;
  }
};

// Service worker syntax
addEventListener('fetch', event => {
  // Access bindings as globals
  const apiKey = API_KEY;
  const response = await backend.fetch(request);
  event.respondWith(response);
});

Defining bindings

Bindings are defined in the bindings list of your worker configuration:

const myWorker :Workerd.Worker = (
  modules = [
    (name = "index.js", esModule = embed "index.js"),
  ],
  compatibilityDate = "2023-02-28",

  bindings = [
    (name = "API_KEY", text = "secret-value"),
    (name = "backend", service = "backend-api"),
    (name = "DATABASE", durableObjectNamespace = "Counter"),
  ],
);

Binding types

Simple data bindings

These bindings provide static data to your worker:

text

Text

A string value.

(name = "API_KEY", text = "my-secret-key"),
(name = "ENVIRONMENT", text = "production"),

const apiKey = env.API_KEY;  // "my-secret-key"

data

Data

Binary data as an ArrayBuffer.

(name = "SECRET_BYTES", data = embed "secret.bin"),

const bytes = env.SECRET_BYTES;  // ArrayBuffer

json

Text

JSON data, parsed into a JavaScript object.

(name = "CONFIG", json = embed "config.json"),
(name = "SETTINGS", json = "{\"debug\":true}"),

const config = env.CONFIG;  // Parsed object
console.log(config.debug);  // true

wasmModule

Data

WebAssembly module instance. Only supported in service workers syntax.

Deprecated. Use ES modules syntax and embed Wasm modules as modules instead.

(name = "WASM", wasmModule = embed "module.wasm"),

Cryptographic key binding

cryptoKey

CryptoKey

A CryptoKey instance for WebCrypto API operations. You can prevent the worker from extracting raw key material by setting extractable = false.

(name = "SIGNING_KEY", cryptoKey = (
  raw = embed "key.bin",
  algorithm = (name = "HMAC"),
  extractable = false,
  usages = ["sign", "verify"],
)),

Key formats:

raw: Raw key material
hex: Hex-encoded key
base64: Base64-encoded key
pkcs8: Private key in PEM-encoded PKCS#8
spki: Public key in PEM-encoded SPKI
jwk: Key in JSON format

Algorithm:

name: Just a name like "AES-GCM"
json: Object encoded as JSON

Usage values: encrypt, decrypt, sign, verify, deriveKey, deriveBits, wrapKey, unwrapKey

const signature = await crypto.subtle.sign(
  { name: 'HMAC' },
  env.SIGNING_KEY,
  data
);

Service bindings

service

ServiceDesignator

Binding to a named service (worker, external server, or network).

(name = "backend", service = "backend-api"),
(name = "admin", service = (name = "api-worker", entrypoint = "admin")),

// Call another worker
const response = await env.backend.fetch(request);

// Call with custom request
const response = await env.backend.fetch('https://api.example.com/data');

Durable Object bindings

durableObjectNamespace

DurableObjectNamespaceDesignator

Binding to a Durable Object namespace. In the common case of a class in the same worker, you can specify just the class name as a string.

(name = "COUNTER", durableObjectNamespace = "Counter"),
(name = "ROOMS", durableObjectNamespace = (className = "ChatRoom")),

// Get an object ID
const id = env.COUNTER.idFromName("global");

// Get the object stub
const counter = env.COUNTER.get(id);

// Call methods
const response = await counter.fetch(request);

durableObjectClass

ServiceDesignator

A Durable Object class binding without a storage namespace. Used to implement facets.

(name = "FACET", durableObjectClass = "MyFacet"),

Storage bindings

kvNamespace

ServiceDesignator

KV namespace binding. Requests are converted to HTTP requests targeting the service.

(name = "KV", kvNamespace = "kv-service"),

// KV operations
await env.KV.put("key", "value");
const value = await env.KV.get("key");
await env.KV.delete("key");

r2Bucket

ServiceDesignator

R2 bucket binding for object storage.

(name = "BUCKET", r2Bucket = "r2-service"),

// R2 operations
await env.BUCKET.put("file.txt", "content");
const object = await env.BUCKET.get("file.txt");
await env.BUCKET.delete("file.txt");

r2Admin

ServiceDesignator

R2 admin API binding for bucket management.

(name = "R2_ADMIN", r2Admin = "r2-admin-service"),

Queue binding

queue

ServiceDesignator

Queue binding for message queuing.

(name = "QUEUE", queue = "queue-service"),

// Send messages
await env.QUEUE.send({ data: "message" });
await env.QUEUE.sendBatch([{ body: "msg1" }, { body: "msg2" }]);

Analytics Engine binding

analyticsEngine

ServiceDesignator

Analytics Engine binding for storing analytics events. Requires --experimental flag.

(name = "ANALYTICS", analyticsEngine = "analytics-service"),

env.ANALYTICS.writeDataPoint({
  indexes: ['user:123'],
  doubles: [response_time],
  blobs: ['GET /api'],
});

Hyperdrive binding

hyperdrive

group

Hyperdrive binding for PostgreSQL connection pooling and caching.

(name = "DB", hyperdrive = (
  designator = "hyperdrive-service",
  database = "mydb",
  user = "dbuser",
  password = "dbpass",
  scheme = "postgres",
)),

// Use with postgres client
const client = new Client({ connectionString: env.DB.connectionString });
await client.connect();

Memory cache binding

memoryCache

group

In-memory cache shared across isolates in the same process.

(name = "CACHE", memoryCache = (
  id = "shared-cache",
  limits = (
    maxKeys = 1000,
    maxValueSize = 1048576,  # 1 MiB
    maxTotalValueSize = 10485760,  # 10 MiB
  ),
)),

// Cache operations (simplified API)
await env.CACHE.put("key", value);
const value = await env.CACHE.get("key");

Advanced bindings

fromEnvironment

Text

Takes the value from a system environment variable. The binding value is null if the environment variable isn’t set.

(name = "NODE_ENV", fromEnvironment = "NODE_ENV"),

const nodeEnv = env.NODE_ENV;  // Value from $NODE_ENV

wrapped

WrappedBinding

Wraps a collection of inner bindings in a common API wrapper. The wrapper is an internal module that exports a function accepting the inner bindings.

(name = "shop", wrapped = (
  moduleName = "burrito-shop:binding",
  entrypoint = "default",
  innerBindings = [
    (name = "recipes", json = embed "recipes.json"),
  ],
)),

The wrapper module exports a function:

export default function(env) {
  // env contains innerBindings
  return {
    // Return API object
  };
}

workerLoader

group

Binding for dynamically loading workers at runtime. Workers are cached by name.

(name = "LOADER", workerLoader = (
  id = "worker-cache",  # Optional: share cache across bindings
)),

// Load and execute worker
const worker = await env.LOADER.load("worker-name", workerCode);
const response = await worker.fetch(request);

unsafeEval

Void

Enables access to the UnsafeEval API for dynamic code evaluation.

Use with extreme caution. Dynamic code evaluation can introduce security vulnerabilities.

(name = "UNSAFE_EVAL", unsafeEval = void),

workerdDebugPort

Void

Provides a connect() method to dynamically connect to any workerd instance’s debug port. For local development and testing only.

(name = "DEBUG_PORT", workerdDebugPort = void),

const client = await env.DEBUG_PORT.connect("localhost:1234");
const fetcher = await client.getEntrypoint("service", "entrypoint");

Parameter bindings

Parameter bindings declare that a worker requires a binding but doesn’t specify its value. Another worker can inherit this worker and fill in the binding.

const baseWorker :Workerd.Worker = (
  modules = [(name = "index.js", esModule = embed "index.js")],
  compatibilityDate = "2023-02-28",
  bindings = [
    (name = "API_KEY", parameter = (type = (text = void))),
    (name = "backend", parameter = (type = (service = void), optional = false)),
  ],
);

const derivedWorker :Workerd.Worker = (
  inherit = "base-worker",
  bindings = [
    (name = "API_KEY", text = "actual-secret-key"),
    (name = "backend", service = "backend-api"),
  ],
);

parameter

group

Declares a required or optional parameter.

type: Expected type of the parameter (text, service, durableObjectNamespace, etc.)
optional: If true, derived workers need not specify it

Workers with non-optional unfilled parameters can only be used for inheritance, not invoked directly.

Complete example

using Workerd = import "/workerd/workerd.capnp";

const config :Workerd.Config = (
  services = [
    (name = "main", worker = .mainWorker),
    (name = "backend", external = .backendServer),
  ],

  sockets = [
    ( name = "http", address = "*:8080", http = (), service = "main" ),
  ]
);

const mainWorker :Workerd.Worker = (
  modules = [(name = "index.js", esModule = embed "index.js")],
  compatibilityDate = "2023-02-28",

  durableObjectNamespaces = [
    (className = "Counter", uniqueKey = "a1b2c3d4"),
  ],

  durableObjectStorage = (inMemory = void),

  bindings = [
    # Simple data
    (name = "ENVIRONMENT", text = "production"),
    (name = "CONFIG", json = embed "config.json"),
    
    # Services
    (name = "backend", service = "backend"),
    
    # Durable Objects
    (name = "COUNTER", durableObjectNamespace = "Counter"),
    
    # Cache
    (name = "CACHE", memoryCache = (
      id = "main-cache",
      limits = (maxKeys = 1000, maxValueSize = 1048576, maxTotalValueSize = 10485760),
    )),
    
    # Environment variable
    (name = "NODE_ENV", fromEnvironment = "NODE_ENV"),
  ],
);

const backendServer :Workerd.ExternalServer = (
  address = "10.0.1.5:8080",
  http = (),
);

Getting Started

Core Concepts

Configuration

Runtime APIs

Node.js Compatibility

Advanced

Development

Capability bindings

How bindings work

Defining bindings

Binding types

Simple data bindings

Cryptographic key binding

Service bindings

Durable Object bindings

Storage bindings

Queue binding

Analytics Engine binding

Hyperdrive binding

Memory cache binding

Advanced bindings

Parameter bindings

Complete example

Next steps

Workers

Services

Build docs developers (and LLMs) love

Getting Started

Core Concepts

Configuration

Runtime APIs

Node.js Compatibility

Advanced

Development

Documentation Index

​How bindings work

​Defining bindings

​Binding types

​Simple data bindings

​Cryptographic key binding

​Service bindings

​Durable Object bindings

​Storage bindings

​Queue binding

​Analytics Engine binding

​Hyperdrive binding

​Memory cache binding

​Advanced bindings

​Parameter bindings

​Complete example

​Next steps

Workers

Services

Build docs developers (and LLMs) love

How bindings work

Defining bindings

Binding types

Simple data bindings

Cryptographic key binding

Service bindings

Durable Object bindings

Storage bindings

Queue binding

Analytics Engine binding

Hyperdrive binding

Memory cache binding

Advanced bindings

Parameter bindings

Complete example

Next steps