Fly Proxy routes requests to individual Machines in your apps using a combination of concurrency settings specified on your app, current load, and closeness. This page describes the details of load balancing traffic to your apps on Fly.io.

Load balancing strategy

The basic load balancing strategy is:

• Send traffic to the least loaded, closest Machine
• If multiple Machines have identical load and closeness, randomly choose one

Load

Fly Proxy determines load using the concurrency settings configured for an app and the current traffic relative to those settings.

The table below describes how traffic may or may not be routed to a Machine based on configured soft_limit and hard_limit values.

Cross-region routing only happens when all Machines in the local region are unhealthy or at their hard_limit.

Closeness

Closeness is determined by RTT (round-trip time) between the Fly.io edge server receiving a connection or request, and the worker server where your Machine runs. Even within the same region, we use different datacenters with different RTTs. These RTTs are measured constantly between all servers.

You can observe live RTT values between Fly.io regions using our RTT app.

Example of load balancing for a web service

We have a hypothetical web service that we know can handle 25 concurrent requests with the configured CPU and memory settings. We set the following values in our fly.toml:

  [services.concurrency]
    type = "requests"
    hard_limit = 25
    soft_limit = 20

We set type = "requests" so Fly.io will use concurrent HTTP requests to determine when to adjust load. We prefer this to type = "connections", because our web service does work for each request and our users may make multiple requests over a single connection (e.g., with HTTP/2). Fly Proxy will also pool connections to a Machine for a short time (about 4 seconds) when using type = "requests" to avoid frequent opening and closing of connections to your app.

We set the soft_limit to 20, so Fly Proxy has some headroom to prefer less-loaded Machines within the same region before distributing traffic more evenly. Soft limits only affect routing within a region. They do not cause the proxy to shift traffic to other regions.

We deploy 10 Machines in four regions: ams (Amsterdam), bom (Mumbai), sea (Seattle), and sin (Singapore), with three of those in ams.

In this contrived example, all of the users are currently in Amsterdam, so the traffic is arriving at one of the Fly.io edges in Amsterdam. Here’s what happens as the number of concurrent HTTP requests from users in Amsterdam increases:

• 60 concurrent requests: Requests are divided evenly between the 3 Machines in the ams region. Closeness of the worker and edge will determine which of the 3 Machines each request goes to.
• 61+ concurrent requests: If all three Machines in ams are above their soft_limit, but still below their hard_limit, Fly Proxy continues routing to them. It does not route traffic to other regions based on soft limits alone.
• 200+ concurrent requests: All Machines are at their soft_limit, so Fly Proxy will start routing requests to the ams Machines again. For example, the 201st concurrent request will go to a Machine in the ams region that is currently at its soft_limit.
• 250 concurrent requests: All Machines are at their hard_limit. The 251st concurrent request will get queued by Fly Proxy until a Machine is below its hard_limit.

If traffic is far above the hard_limit for a long period of time, Fly Proxy might start returning 503 Service Unavailable responses for requests that are not able to be routed to a Machine.