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Service bindings

​​ About Service bindings

Service bindings are an API that facilitate Worker-to-Worker communication via explicit bindings defined in your configuration. A Service binding allows you to send HTTP requests to another Worker without those requests going over the Internet. The request immediately invokes the downstream Worker, reducing latency as compared to a request to a third-party service. You can invoke other Workers directly from your code.

To use Service bindings in your code, you must first create a Service binding from one Worker to another.

​​ Interface

export default {
async fetch(req, env) {
return await env.BINDING.fetch(req);
interface Environment {
BINDING: Fetcher;
export default <ExportedHandler<Environment>> {
async fetch(req, env) {
return await env.BINDING.fetch(req);

Service bindings use the standard Fetch API. A Service binding will invoke the fetch() handler of a target Worker. To access a target Worker from a parent Worker, you must first configure the target Worker with a binding for that target Worker. The binding definition includes a variable name on which the fetch() method will be accessible. The fetch() method has the exact same signature as the global fetch. However, instead of sending an HTTP request to the Internet, the request is always sent to the Worker to which the Service binding points.

​​ Shared resources

Workers connected to one another via Service bindings share the CPU resources of the top-level request. A single thread is allocated and reused amongst these Workers. This means no idle resources are wasted while work is performed across various Workers.

​​ Lifecycle

Lifecycle is tied to the top-level Worker. If a child Worker is still processing, and the parent Worker does not await the completion of a child Worker, the child Worker will be terminated and cleaned up. It is important to use await and event.waitUntil to manage the lifecycle of any child processes invoked via Service bindings.

​​ Context

Service bindings live on the environment context. This means Service bindings can be used from within a Durable Object, as long as the environment context remains intact.

​​ Limits

Service bindings have the following limits:

  • Each request to a Worker via Service bindings count toward your subrequest limit.
  • Nested calls to child Workers increase the depth of your Worker Pipeline. Maximum Pipeline depth is 32, including the first Worker. Subsequent calls will throw an exception.
  • Simultaneous open connection limits are Pipeline-wide, meaning subrequests from multiple different Workers incur a global concurrent subrequest limit. However, a fetch call on a Service binding does not count as an open connection.