fly-prefer-region header not working

[jerome]

@jasonkuhrt

How different are these two things:

They look like this:

@jasonkuhrt

1. Request routed to edge region X, continues with instance in region X

client -> region x edge -> region x worker -> instance

@jasonkuhrt

2. Request routed to edge region Y, via fly-prefer-region goes to instance in region X

client -> region y edge -> region x worker -> instance

The only difference is the hop in our internal network has extra latency unless the client is reaching region x by chance.

@jasonkuhrt

Also just to confirm, generally speaking (maybe there are exceptions…) an application can know a request is using fly-prefer-region if it sees a discrepancy between the fly-region header and the FLY_REGION environment variable. Right?

The fly-region header represents the edge region where the client was routed to (via BGP). Unless you have an instance in every single one of our regions and all your clients have perfect routing with us, then you can’t know if fly-prefer-region was used.

We also have some regions that are “edge-only” (but not for long). So you’d see jnb in the fly-region header and it couldn’t possible reach an instance in the same region since we don’t have “workers” there.