Push messaging using Fanout

Implements Fanout handlers for WebSockets, SSE, etc.


Use this starter

Using the Fastly CLI, create a new project using this starter somewhere on your computer:

$ fastly compute init --from=https://github.com/fastly/compute-starter-kit-rust-fanout

Or click the button below to create a GitHub repository, provision a Fastly service, and set up continuous deployment:

Deploy to Fastly


The app expects a configured backend named "self" that points back to app itself. For example, if the service has a domain foo.edgecompute.app, then you'll need to create a backend on the service named "self" with the destination host set to foo.edgecompute.app and port 443. Also set 'Override Host' to the same host value.


The app exposes the following endpoints to clients:

  • /ws: bi-directional WebSocket
  • /stream: HTTP streaming of text/plain
  • /sse: SSE (streaming of text/event-stream)
  • /response: Long-polling

Connecting to any endpoint will subscribe the connection to channel "test". The WebSocket endpoint echos back any messages it receives from the client.

Data can be sent to the connections via the GRIP publish endpoint at https://fanout.fastly.com/{service-id}/publish/. For example, here's a curl command to send a WebSocket message:

curl \
--user {service-id}:{secret} \
-d '{"items":[{"channel":"test","formats":{"ws-message":{"content":"hello"}}}]}' \

How it works

For each call, the app is actually invoked twice.

  1. Initially, a client request arrives at the app without having been routed through the Fanout proxy yet. The app checks for this via the presence of a Grip-Sig header. If that header is not present, the app calls req.handoff_fanout("self") and exits. This tells the subsystem that the connection should be routed through Fanout, and is used for HTTP requests controlled by GRIP.

  2. Since self refers to the same app, a second request is made to the same app, this time coming through Fanout. The app checks for this, and then handles the request accordingly (in handle()).


This app is not currently supported in Fastly's local development server, as the development server does not support Fanout features. To experiment with Fanout, you will need to publish this project to your Fastly Compute@Edge service. using the fastly compute publish command.

Next steps

Starters are a good way to bootstrap a project. For more specific use cases, and answers to common problems, try our library of code examples.