PathBase vs Prefix (HttpSys)

[whoaskedfrfr]

Hey! I'm confused about what's the difference of prefix and path base.

Recently I made a change in a code base from:

webBuilder.UseUrls("https://*:888/api");

to

webBuilder.UseUrls("https://*:888");

/* ... */

appBuilder.UsePathBase("/api");

Everything worked fine until it reached an environment with other apps making it crash with The prefix 'https://*:888/' is already registered.

I was under the impression the same thing. Can someone explain to me what are the differences?

[engineering87]

Hi there,
UseUrls method specifies the URLs on which Kestrel server should listen for incoming HTTP requests and combines the protocol, host, port and path prefix (/api) to define where the server listens for requests.
In your case:

webBuilder.UseUrls("https://*:888/api");

restricts the server to handle requests starting with /api on port 888.

UsePathBase method modifies the request path that the application sees and allowing the application to act as if it's hosted at a subpath. It doesn’t affect where the server listens (like UseUrls) but changes how the requests are routed internally within the application.

By removing /api from UseUrls, your application began listening on https://*:888/, which is more general than https://*:888/api. If another application is already registered to listen on "https://*:888/", it causes a conflict, resulting in the error: "The prefix 'https://*:888/' is already registered."

So to solve the issue, you have to assign different ports for each application to avoid conflicts or use a reverse proxy to route requests to different applications based on the path.