Dynamic Request Routing

Traffic sent to multi-region Fly.io apps is automatically routed to the region nearest to the client, thanks to Anycast-enabled IP addresses. Read more about Anycast on Wikipedia.

Sometimes it’s useful, or necessary, to route requests to other regions, other apps, or both. You can use custom request and response HTTP headers to achieve this.

The fly-replay response header

Apps may append a fly-replay header to responses. This instructs Fly Proxy to redeliver (replay) the original request in another region, or to replay it to another app in the same organization.

The content of the fly-replay header fields tells Fly Proxy which magic to perform, and the proxy takes it from there. If the target region doesn’t host any healthy Machines for the target app, the proxy throws an error.

Limitations

Attempting to replay requests larger than 1MB will throw an error. If you need certain requests - like file uploads - to be handled by a specific region, consider the following workarounds.

If your service is simply proxying uploads to object storage, some services like S3 allow uploading directly from the browser. Rails supports this out of the box.

If you can perform uploads via fetch or XMLHttpRequest, you can prepend the fly-prefer-region or fly-force-instance-id header to send the request directly to a specific region.

fly-replay use cases

Here are a few example use cases for the fly-replay response header.

Replaying writes in secondary regions

Fly.io Postgres supports global read replicas for speeding up reads in regions close to users.

Replay a request to the region where the HA database leader lives:

fly-replay: region=sjc

Fly Proxy will get the request to a Machine in that region and let the instances in the cluster take care of getting it to the writeable leader.

Check out our blueprint for Multi-region databases and fly-replay.

Replay requests to other apps

fly-replay can replay requests across apps in the same organization. Think of a router app for a FaaS that wants to spin up a customer Machine on demand.

To send the request to a different app in the same organization, in the nearest region that has a Machine:

fly-replay: app=app-in-same-org

Replay requests to specific Machines

Replay the request to a specific Machine by ID:

fly-replay: instance=00bb33ff

Pass state to replay targets

The request replay target may need to know why the request was routed to it:

fly-replay: region=sjc;state=captured_write

Fields can be stacked; for instance, to send the request on to an Machine in the app “app-in-same-org” in the sjc region:

fly-replay: region=sjc;app=app-in-same-org

Some combinations of fields can conflict. For example, don’t specify an app name and a Machine ID that doesn’t belong to that app.

Related: Multi-region PostgreSQL; Run Ordinary Rails Apps Globally

The fly-replay-src request header

When replaying an HTTP request, Fly Proxy appends the fly-replay-src header with information about the Machine that sent the fly-replay.

See how the official Fly.io Ruby client uses the state and t fields to prevent read-your-own-write inconsistency.

The fly-prefer-region request header

Clients accessing Fly.io apps may set the fly-prefer-region header to attempt sending the request directly to a desired target region. This is useful for cases where fly-replay isn’t practical, such as large file uploads which can’t be replayed once buffered by Fly Proxy.

If the target region has no healthy Machines, the region nearest to the client with healthy Machines will field the request.

Example:

fly-prefer-region: ams

The fly-force-instance-id request header

Clients accessing Fly.io apps may set the fly-force-instance-id header to ensure that the request is sent to only a specific Machine.

If the Machine is deleted or not found, no other Machines will be tried.

Example:

fly-force-instance-id: 90801679a10038