Making HTTP Requests

Spin provides an interface for you to make outgoing HTTP requests.

Why do I need a Spin interface? Why can't I just use my language's HTTP library?

The current version of the WebAssembly System Interface (WASI) doesn’t provide a sockets interface, so HTTP libraries can’t be built to Wasm. The Spin interface means Wasm modules can bypass this limitation by asking Spin to perform the HTTP request on their behalf.

Using HTTP From Applications

The outbound HTTP interface depends on your language.

Under the surface, Spin uses the wasi-http interface. If your tools support the Wasm Component Model, you can work with that directly; but for most languages the Spin SDK is more idiomatic.

Please note: When using Spin v2.6.0 or newer, Spin app guest modules can no longer set the Host header on outbound requests. (The Wasmtime runtime, that underpins Spin, has recently updated how headers are handled).

Want to go straight to the reference documentation? Find it here.

To send requests, use the spin_sdk::http::send function. This takes a request argument and returns a response (or error). It is async, so within an async inbound handler you can have multiple outbound sends running concurrently.

Support for streaming request and response bodies is experimental. We currently recommend that you stick with the simpler non-streaming interfaces if you don’t require streaming.

send is quite flexible in its request and response types. The request may be:

  • http::Request - typically constructed via http::Request::builder()
  • spin_sdk::http::Request - typically constructed via spin_sdk::http::Request::get(), spin_sdk::http::Request::post(), or spin_sdk::http::Request::builder()
    • You can also use the builder type directly - build will be called automatically for you
  • spin_sdk::http::OutgoingRequest - constructed via OutgoingRequest::new()
  • Any type for which you have implemented the TryInto<spin_sdk::http::OutgoingRequest> trait

Generally, you should use OutgoingRequest when you need to stream the outbound request body; otherwise, the Request types are usually simpler.

The response may be:

Generally, you should use IncomingResponse when you need to stream the response body; otherwise, the Response types are usually simpler.

Here is an example of doing outbound HTTP in a simple request-response style:

use spin_sdk::{
    http::{IntoResponse, Request, Method, Response},

// The trigger handler (in this case an HTTP handler) has to be async
// so we can `await` the outbound send.
async fn handle_request(_req: Request) -> anyhow::Result<impl IntoResponse> {

    // Create the outbound request object
    let request = Request::builder()

    // Send the request and await the response
    let response: Response = spin_sdk::http::send(request).await?;

    // Use the outbound response body
    let response_len = response.body().len();

    // Return the response to the inbound request
        .header("content-type", "text/plain")
        .body(format!("The test page was {response_len} bytes"))

For an example of receiving the response in a streaming style, see this example in the Spin repository.

HTTP operations are available via the standard JavaScript fetch function. The Spin runtime maps this to the underlying Wasm interface. For example:

const response = await fetch("");


  • Although the underlying Spin interface is blocking, the fetch function is defined by JavaScript as async. You must await the response, but the request will always block, and the promise will resolve as soon as the request is returned.

You can find a complete example of using outbound HTTP in the JavaScript SDK repository on GitHub (TypeScript, JavaScript).

Want to go straight to the reference documentation? Find it here.

HTTP functions and classes are available in the http module. The function name is send. The request type is Request, and the response type is Response. For example:

from spin_sdk.http import Request, Response, send
response = send(Request("GET", "", {}, None))


  • For compatibility with idiomatic Python, types do not necessarily match the underlying Wasm interface. For example, method is a string.
  • Request and response bodies are bytes. (You can pass literal strings using the b prefix.) Pass None for no body.
  • Request and response headers are dictionaries.
  • Errors are signalled through exceptions.

You can find a complete example for using outbound HTTP in the Python SDK repository on GitHub.

Want to go straight to the reference documentation? Find it here.

HTTP functions are available in the package. See Go Packages for reference documentation. The general function is named Send, but the Go SDK also surfaces individual functions, with request-specific parameters, for the Get and Post operations. For example:

import (
	spinhttp ""

res1, err1 := spinhttp.Get("")
res2, err2 := spinhttp.Post("", "application/json", json)

request, err := http.NewRequest("PUT", "", bytes.NewBufferString(user1))
request.Header.Add("content-type", "application/json")
res3, err3 := spinhttp.Send(req)


  • In the Post function, the body is an io.Reader. The Spin SDK reads this into the underlying Wasm byte array.
  • The NewRequest function is part of the standard library. The Send method adapts the standard request type to the underlying Wasm interface.
  • Errors are returned through the usual Go multiple return values mechanism.

You can find a complete example for using outbound HTTP in the Spin repository on GitHub.

Granting HTTP Permissions to Components

By default, Spin components are not allowed to make outgoing HTTP requests. This follows the general Wasm rule that modules must be explicitly granted capabilities, which is important to sandboxing. To grant a component permission to make HTTP requests to a particular host, use the allowed_outbound_hosts field in the component manifest:

allowed_outbound_hosts = [ "", "" ]

The Wasm module can make HTTP requests only to the specified hosts. If a port is specified, the module can make requests only to that port; otherwise, the module can make requests only on the default port for the scheme. Requests to other hosts (or ports) will fail with an error.

You can use a wildcard to allow requests to any subdomain of a domain:

allowed_outbound_hosts = [ "https://*" ]

You can also pass an IPv4 CIDR address:

allowed_outbound_hosts = [ "" ]

For development-time convenience, you can also pass the string "https://*:*" in the allowed_outbound_hosts collection. This allows the Wasm module to make HTTP requests to any host and on any port. However, once you’ve determined which hosts your code needs, you should remove this string and list the hosts instead. Other Spin implementations may restrict host access and disallow components that ask to connect to anything and everything!

Configuration-Based Permissions

You can use application variables in the allowed_outbound_hosts field. However, this feature is not yet available on Fermyon Cloud.

Making HTTP Requests Within an Application

If your Spin application functions as a set of microservices, you’ll often want to make requests directly from one component to another within the same application. It’s best not to use a full URL for this, because that’s not portable across different deployment environments - the URL in the cloud is different from the one in your development environment. Instead, Spin provides two ways to make inter-component requests:

  • By component ID (“local service chaining”)
  • By route

Both of these work only from HTTP components. That is, if you want to make an intra-application request from, say, a Redis trigger, you must use a full URL.

Local Service Chaining

To make an HTTP request to another component in your application, use the special <component-id>.spin.internal host name. For example, an outbound HTTP request to authz.spin.internal will be handled by the authz component.

In this way of doing self-requests, the request is passed in memory without ever leaving the Spin host process. This is extremely fast, as the two components are wired almost directly together, but may reduce deployment flexibility depending on the nature of the microservices. Also, public components that are the target of service chaining requests may see URLs in both routed and chained forms: therefore, if they parse the URL (for example, extracting a resource identifier from the path), they must ensure both forms are correctly handled.

Service chaining is the only way to call private endpoints.

You must still grant permission by including the relevant spin.internal hosts in allowed_outbound_hosts:

allowed_outbound_hosts = ["http://authz.spin.internal", "https://reporting.spin.internal"]

To allow local chaining to any component in your application, use a subdomain wildcard:

allowed_outbound_hosts = ["*://*.spin.internal"]

Local service chaining is not currently supported on Fermyon Cloud.

Intra-Application HTTP Requests by Route

To make an HTTP request to another route with your application, you can pass just the route as the URL. For example, if you make an outbound HTTP request to /api/customers/, Spin prepends the route with whatever host the application is running on. It also replaces the URL scheme (http or https) with the scheme of the current HTTP request. For example, if the application is running in the cloud, Spin changes /api to https://.../api.

In this way of doing self-requests, the request undergoes normal HTTP processing once Spin has prepended the host. For example, in a cloud deployment, the request passes through the network, and potentially back in through a load balancer or other gateway. The benefit of this is that it allows load to be distributed across the environment, but it may count against your use of bandwidth.

You must still grant permission by including self in allowed_outbound_hosts:

allowed_outbound_hosts = ["http://self", "https://self"]