Dynamic Request Routing with fly-replay

The fly-replay feature gives you fine-grained control over request routing in your Fly.io applications. It allows you to dynamically route requests between regions, specific Machines, or even different apps within your organization.

How fly-replay Works

When your app responds to a request with a fly-replay response, Fly Proxy will automatically replay the original request according to your specified routing rules. This enables advanced patterns like:

Routing write operations to primary database regions
Load balancing between specific Machines
Cross-app request routing within your organization
Implementing sticky sessions

Replay Header Format

Your app can add a fly-replay header to its response. The fly-replay header accepts the following fields:

Field	Description
`region`	The region(s) to route the request to. Accepts comma-separated list of region codes
`instance`	The ID of a specific Machine to route to
`prefer_instance`	The ID of a specific Machine to route to, if possible
`app`	The name of another app to route to
`state`	Optional string included in `fly-replay-src` header on replay
`elsewhere`	If `true`, excludes responding Machine from next load-balance
`timeout`	Duration to attempt the replay before giving up (e.g. `10s`, `800ms`)
`fallback`	If the replay fails, route back to the original Machine. `force_self` or `prefer_self` (see Replay Timeout and Fallback)

Example Usage

Route to a specific region:

fly-replay: region=sjc

Route to one of the given regions, in order of preference:

fly-replay: region="iad,ord,us,na"

Route to a preferred region, or fallback to any available machine:

fly-replay: region="sjc,any"

Route to specific Machine:

fly-replay: instance=00bb33ff

Route to another app:

fly-replay: app=target-app

By default, cross-app routing works within the same organization and network. To route across networks, see Cross-Network Routing. To route to apps in other organizations, see Cross-Organization Routing.

You can combine multiple fields:

fly-replay: region="sjc,any";app=target-app

Route to another app with a timeout and fallback to the original Machine:

fly-replay: app=my-worker;timeout=10s;fallback=force_self

Note: A comma-separated list of regions must be quoted.

Geographic groups and aliases

When replaying to a region, you can use geographic aliases like us, eu, or sa to target a broader area.

Alias	Area
`apac`	Asia-Pacific
`eu`	Europe
`na`	North America
`sa`	South America
`us`, `usa`	United States
`any`	Earth

Replay Timeout and Fallback

You can set a timeout and fallback on a replay to handle cases where the replay target is unreachable. timeout sets how long the proxy tries to reach the replay target. The actual duration may slightly exceed this value. Accepts duration strings like 10s, 500ms. Without fallback, a timeout makes the replay error faster instead of waiting for the default error timeout. fallback tells the proxy to route the request back to the Machine that issued the replay if the replay fails due to timeout, exhausted retries, or no available candidate:

force_self: Route back to the exact Machine that issued the replay. Returns a proxy error if that Machine is no longer available.
prefer_self: Try the original Machine first, but fall back to any Machine in the original app if it is unavailable.

When a fallback triggers, the original Machine receives the request again with a fly-replay-failed request header containing details about the failed replay attempt. Since this is still the original request, your app can respond with a useful error instead of the client receiving a generic proxy error.

Note: Fallback requests cannot themselves issue fly-replay responses.

Replay JSON Format

Your app can set the response content-type to application/vnd.fly.replay+json and include replay instructions in the response body. For a complex replay the JSON body is easier to compose than the header format, and also supports more functionality.

JSON Structure

The application/vnd.fly.replay+json replay body accepts the following fields:

Field	Description
`region`	The region(s) to route the request to. Accepts comma-separated list of region codes
`instance`	The ID of a specific Machine to route to
`prefer_instance`	The ID of a specific Machine to route to, if possible
`app`	The name of another app to route to
`state`	Optional string included in `fly-replay-src` header on replay
`elsewhere`	If `true`, excludes responding Machine from next load-balance
`timeout`	Duration to attempt the replay before giving up (e.g. `"10s"`, `"800ms"`)
`fallback`	If the replay fails, route back to the original Machine. `"force_self"` or `"prefer_self"` (see Replay Timeout and Fallback)
`transform.path`	Rewrite the path and query parameters of the request
`transform.delete_headers`	Delete headers from the request, hiding them from the replay target
`transform.set_headers`	Set new headers on the request, overwriting headers of the same name
`cache.prefix`	Cache the replay for matching requests (see Replay Caching)
`cache.ttl`	Cache the replay for this many seconds (see Replay Caching)
`cache.invalidate`	Invalidate the cache for the current route (see Replay Caching)

Example Usage

Route to another app, and modify the request:

{
  "app": "target-app",
  "region": "iad,us",
  "transform": {
    "path": "/new/path?param=value",
    "delete_headers": ["x-unwanted-header", "cookie"],
    "set_headers": [
      { "name": "x-custom-header", "value": "new-value" },
      { "name": "authorization", "value": "Bearer token123" }
    ]
  }
}

Route to another app with a timeout and fallback:

{
  "app": "my-worker",
  "timeout": "10s",
  "fallback": "force_self"
}

Replay Caching

Replay caching allows Fly Proxy to remember and reuse replay decisions, reducing both load on your application and the latency of replayed requests. There are two types of replay caching:

Path-based: Cache replays for specific URL paths
Session-based: Cache replays for specific cookie or header values

Note: Replay caching is an optimization, not a guarantee. Your app should not depend on this mechanism to function. The app issuing fly-replay still serves as the ultimate source of truth, and we may decide to consult that app at any moment even if a replay cache has previously been set. To ensure reliable operation, the app issuing fly-replay should still have multiple instances deployed in multiple regions.

Path-based Replay Caching

Replay caching for paths is configured per-request during a replay. To cache a replay for a path:

Issue a fly-replay header as usual
Set fly-replay-cache: /some/path/*
- This needs to be a path-matching pattern, and implicitly ends with a wildcard /*. This is where the cached replay is applied.
- The cached replay is unique to the request domain. The domain may also be set explicitly: example.com/some/path/*
- The domain part should not include ports; use example.com, not example.com:80.
- The pattern must also match the current request.
Set fly-replay-cache-ttl-secs: number_of_seconds

For apps using the JSON format, specify a cache object as part of the replay:

{
  "app": "target-app",
  "cache": {
    "prefix": "/some/path/*",
    "ttl": 60
  }
}

Session-based Replay Caching

If your app’s sessions are identified by a cookie or authorization header, and should consistently route to the same target, Fly Proxy can cache that replay. Unlike path-based caching (which requires your app to return fly-replay-cache headers), session-based caching is configured in your fly.toml. When your app issues a fly-replay response for a request with a matching cookie or header, Fly Proxy will cache that replay decision and automatically apply it to subsequent requests with the same session identifier.

Configuration

Configure session-based replay caching in your fly.toml. This can be set under http_service.http_options or services.ports.http_options:

[http_service]
  internal_port = 8080
  force_https = true

  [[http_service.http_options.replay_cache]]
    path_prefix = "/"
    ttl_seconds = 300
    type = "cookie"
    name = "session_id"

  [[http_service.http_options.replay_cache]]
    path_prefix = "/api"
    ttl_seconds = 600
    type = "header"
    name = "Authorization"

In this example:

Replays for requests to / (and its subpaths) are cached for 5 minutes based on the session_id cookie value
Replays for requests to /api (and its subpaths) are cached for 10 minutes based on the Authorization header value
When multiple rules match, the longest matching path takes precedence

If your app accepts requests for multiple hostnames, you can narrow the configuration by specifying a hostname in path_prefix:

  [[http_service.http_options.replay_cache]]
    path_prefix = "api.example.com/api"

Caches are not shared between hostnames, even when path_prefix doesn’t specify a hostname. Each hostname maintains its own cache. For more details on configuration options, see the fly.toml configuration reference.

Invalidating the Replay Cache

If the replay target does eventually change, the replay target may proactively invalidate the cache by:

Issuing a fly-replay back to the origin
Setting fly-replay-cache: invalidate

Or, using the JSON format:

{
  "app": "origin-app",
  "cache": {
    "invalidate": true
  }
}

This works for invalidating both the path-based cache, and the session-based cache.

Bypassing Replay Cache

Sometimes, you might want a client to bypass a cached replay without actually invalidating the cache. For example, you might have an API endpoint that mostly needs to be replayed to a “leader” instance. But depending on application logic, it could sometimes be handled by any instance. The recommended approach is to clearly separate endpoints that require replay from those that don’t. However, when that is not possible, the replaying app can be configured to allow bypass. For path-based replay caching, additionally set the header:

fly-replay-cache-allow-bypass: yes

or set the JSON field "allow_bypass": true. For session-based replay caching, set allow_bypass to true in your fly.toml configuration for your replay_cache rule. Then, when your client makes a request, it can set the header:

fly-replay-cache-control: skip

to skip any cached replay that might be present.

Note: This behavior is opt-in because enabling it by default could make your app vulnerable to malicious clients creating unexpected load on the replaying machine.

Was my request served by the cache?

If your app needs to know whether a request was served from the replay cache, it can check the fly-replay-cache-status header. This header is sent to the replay target app or machine.

Absent: the request was not replayed and was delivered directly to the app or machine.
miss: the request didn’t use replay cache; it first hit another machine and got replayed here.
hit: the request did use the replay cache! It never reached the source app or machine and was redirected straight to the target.
bypass: even though the cache might have been set, it was explicitly skipped by setting the fly-replay-cache-control: skip header.

Implementation Details

Requirements and Limitations

Your app must use the http handler
Requests larger than 1MB cannot be replayed
Field combinations must be logically valid (e.g., don’t specify both app and instance if instance isn’t in that app)

For large uploads that exceed the 1MB limit, consider:

Using direct-to-storage uploads where possible
Using the fly-prefer-region header instead

For fly-replay-cache, the following limitations apply:

The state field cannot be set in the fly-replay intended to be cached
Transformations for the headers or path cannot be defined.
The TTL needs to be a minimum of 10 seconds
Only one step of lookup is performed in the cache; as such, if the target app issues another fly-replay-cache, the caching behavior in this case is undefined
The timeout and fallback fields cannot be set in the fly-replay intended to be cached
The fly-replay-src header (described below) will not be set for requests replayed through the cache

The fly-replay-src Header

When a request is replayed, Fly Proxy adds a fly-replay-src header containing metadata about the original request:

Field	Description
`instance`	ID of Machine that sent fly-replay
`region`	Region request was replayed from
`t`	Timestamp (microseconds since Unix epoch)
`state`	Contents of original state field, if any

This header is useful for tracking request paths and implementing consistency patterns. See the official Ruby client for an example of using these fields to prevent read-your-write inconsistency. This header is not set when the request is replayed through a cached fly-replay entry (fly-replay-cache).

The fly-preferred-instance-unavailable Header

If you replay with prefer_instance set, Fly Proxy will attempt to route to this Machine. This may not happen for a number of reasons, for example the Machine may not be found, or found but at its configured hard_limit. In these cases, the request will be delivered to a different Machine that matches the remaining fields in your replay. Along with the other Fly.io-specific headers, a fly-preferred-instance-unavailable header will be set containing the ID of the instance that could not be reached.

The fly-replay-failed Header

When a replay fallback triggers, Fly Proxy delivers the request back to the original Machine with a fly-replay-failed request header. This header contains semicolon-separated metadata about the failed replay attempt:

Field	Description
`instance`	ID of Machine the replay was targeting
`app`	App the replay was targeting
`region`	Region the replay was targeting
`replay_source`	ID of the Machine that originally issued the replay
`reason`	Why the replay failed: `timeout`, `retries_exhausted`, or `no_candidate`
`elapsed_ms`	Time in milliseconds spent attempting the replay

Example header value:

fly-replay-failed: instance=00bb33ff;app=target-app;region=iad;replay_source=11aa44ee;reason=timeout;elapsed_ms=10000

Your app can use this header to detect that a fallback occurred and respond accordingly, for example by serving a helpful error to the client.

Web Socket Considerations

It is worth noting that an application returning fly-replay headers should not negotiate a web socket upgrade itself. Some frameworks automatically handle this process. Instead, the application or instance receiving the requests should handle the upgrade.

Alternative Routing Headers

For cases where fly-replay isn’t suitable, Fly.io provides two alternative request headers:

fly-prefer-region: ams

Attempts to route directly to specific region(s). You can specify multiple regions as comma-separated values for fallback preferences:

fly-prefer-region: iad,ord,us

Falls back to the next region in the list, or to the nearest region with healthy Machines if none of the specified regions are available. Useful for large uploads that can’t be replayed.

fly-force-region: ams

Attempts to route directly to specific region(s). Unlike fly-prefer-region, the Fly Proxy will not fall back to requesting the nearest region if it cannot reach any machine in the listed region(s). Like fly-prefer-region, you may also specify multiple regions in order of preference:

fly-force-region: iad,ord,us

The fly-prefer-instance-id Header

fly-prefer-instance-id: 90801679a10038

Attempts to route to a specific Machine. Falls back to any matching Machine if the target Machine cannot be found, or is unable to accept requests. If the request is delivered to a different Machine, the fly-preferred-instance-unavailable request header will be set.

The fly-force-instance-id Header

fly-force-instance-id: 90801679a10038

Forces routing to a specific Machine. The Fly Proxy will attempt to reach the Machine multiple times if the Machine is unhealthy. No fallback if Machine is ultimately unavailable.

Note: Get Machine IDs using fly status or fly Machines list.

Common Use Cases

Multi-Region Databases

When using global read replicas, use fly-replay to ensure write operations go to the primary region:

fly-replay: region=sjc

See our blueprint for Multi-region databases and fly-replay for a complete implementation guide.

Cross-App Routing

Use fly-replay to implement routing layers or FaaS-style architectures:

fly-replay: app=customer-function-app

This allows you to build router apps that can dynamically route requests to other apps. By default, cross-app routing works within the same organization and the same private network.

Cross-Network Routing

By default, fly-replay with app= only routes to apps within the same private network. If you have multiple networks within your organization and need to replay requests across them, you can enable cross-network replays for your organization.

Note: Cross-network replay restrictions only apply to same-org replays. Cross-organization replays (via the allowlist) are not affected by this setting.

Enabling Cross-Network Replays

Only organization admins can manage this setting. You can enable cross-network replays via the dashboard or CLI. Dashboard: Go to your organization’s settings page and find the Routing section. Use the Cross-Network Replay toggle to enable or disable cross-network replays. CLI: Check the current status:

fly orgs cross-network-replays status --org <your-org>

Enable cross-network replays:

fly orgs cross-network-replays enable --org <your-org>

Disable cross-network replays:

fly orgs cross-network-replays disable --org <your-org>

Cross-Organization Routing

By default, fly-replay with app= only works for apps within the same organization. To allow replay requests from apps in other organizations, you must configure an allowlist on the target organization.

Configuring Allowed Replay Sources

Only organization admins can manage the replay sources allowlist, and must also have read permissions or higher on the source organization being added. You can configure replay sources via the dashboard or CLI. Dashboard: Go to your organization’s settings page and find the Routing section. Under Cross-Organization Routing, you can add or remove organizations that are allowed to send replay requests to apps in your organization. CLI: List organizations currently allowed to send replays:

fly orgs replay-sources list --org <target-org>

Add organizations to the allowlist interactively:

fly orgs replay-sources add --org <target-org>

Remove organizations from the allowlist interactively:

fly orgs replay-sources remove --org <target-org>

Fly docs

Dynamic Request Routing with fly-replay

How fly-replay Works

Replay Header Format

Example Usage

Geographic groups and aliases

Replay Timeout and Fallback

Replay JSON Format

JSON Structure

Example Usage

Replay Caching

Path-based Replay Caching

Session-based Replay Caching

Configuration

Invalidating the Replay Cache

Bypassing Replay Cache

Was my request served by the cache?

Implementation Details

Requirements and Limitations

The fly-replay-src Header

The fly-preferred-instance-unavailable Header

The fly-replay-failed Header

Web Socket Considerations

Alternative Routing Headers

The fly-prefer-region Header

The fly-force-region Header

The fly-prefer-instance-id Header

The fly-force-instance-id Header

Common Use Cases

Multi-Region Databases

Cross-App Routing

Cross-Network Routing

Enabling Cross-Network Replays

Cross-Organization Routing

Configuring Allowed Replay Sources

​How fly-replay Works

​Replay Header Format

​Example Usage

​Geographic groups and aliases

​Replay Timeout and Fallback

​Replay JSON Format

​JSON Structure

​Example Usage

​Replay Caching

​Path-based Replay Caching

​Session-based Replay Caching

​Configuration

​Invalidating the Replay Cache

​Bypassing Replay Cache

​Was my request served by the cache?

​Implementation Details

​Requirements and Limitations

​The fly-replay-src Header

​The fly-preferred-instance-unavailable Header

​The fly-replay-failed Header

​Web Socket Considerations

​Alternative Routing Headers

​The fly-prefer-region Header

​The fly-force-region Header

​The fly-prefer-instance-id Header

​The fly-force-instance-id Header

​Common Use Cases

​Multi-Region Databases

​Cross-App Routing

​Cross-Network Routing

​Enabling Cross-Network Replays

​Cross-Organization Routing

​Configuring Allowed Replay Sources

How fly-replay Works

Replay Header Format

Example Usage

Geographic groups and aliases

Replay Timeout and Fallback

Replay JSON Format

JSON Structure

Example Usage

Replay Caching

Path-based Replay Caching

Session-based Replay Caching

Configuration

Invalidating the Replay Cache

Bypassing Replay Cache

Was my request served by the cache?

Implementation Details

Requirements and Limitations

The fly-replay-src Header

The fly-preferred-instance-unavailable Header

The fly-replay-failed Header

Web Socket Considerations

Alternative Routing Headers

The fly-prefer-region Header

The fly-force-region Header

The fly-prefer-instance-id Header

The fly-force-instance-id Header

Common Use Cases

Multi-Region Databases

Cross-App Routing

Cross-Network Routing

Enabling Cross-Network Replays

Cross-Organization Routing

Configuring Allowed Replay Sources