Kubernetes uvicorn fastapi path based routing Ingress not working

Question

I manage an Azure AKS cluster with several API's written in Python and Uvicorn (FastAPI). We use the ingress-nginx ingress controller.

Currently API's are reached through a unique url similar to: servicename.departmentname.companyname.com. I am attempting to transition unique url routes to a common root url + path routes. For instance, the above would then become departmentname.companyname.com/servicename.

However, this is consistently causing errors, either 404 not found error or "Failed to load API definition" error (see screenshot), depending on the exact setup I am testing.

I have looked into the FastApi documentation which mentions several approaches that might be relevant in this case, but so fat I haven't been able to understand the root cause of the problem. Any help or suggestions are welcome.

The current k8s uniqure url rout Ingress resource looks like this:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: servicename-api-ingress
  namespace: servicename-api-prod
  annotations:
    cert-manager.io/cluster-issuer: letsencrypt-clusterissuer
spec:
  ingressClassName: nginx
  rules:
  - host: servicename.departmentname.companyname.com
    http:
      paths:
      - path: /
        pathType: ImplementationSpecific
        backend:
          service:
            name: servicename-api-service
            port:
              number: 80
  tls:
  - hosts:
      - servicename.departmentname.companyname.com
    secretName: servicename-api-tls

The corresponding Dockerfile looks like this:

# build stage
FROM python:3.10

# install PDM
RUN pip install -U pip setuptools wheel
RUN pip install pdm

# copy files
COPY pyproject.toml pdm.lock README.md /project/
COPY src/ /project/src

# install dependencies and project
WORKDIR /project
RUN pdm install

# set command/entrypoint, adapt to fit your needs
CMD ["pdm", "run", "uvicorn", "companyname.servicename.api.main:app", "--host", "0.0.0.0", "--port", "8080"]

When transitioning to the root url + path route, the above Ingress resource would have to look something like the below, but this definition is not working.

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: servicename-api-ingress
  namespace: servicename-api-prod
  annotations:
    cert-manager.io/cluster-issuer: letsencrypt-clusterissuer
spec:
  ingressClassName: nginx
  rules:
  - host: departmentname.companyname.com
    http:
      paths:
      - path: /servicename/
        pathType: Prefix
        backend:
          service:
            name: servicename-api-service
            port:
              number: 80
  tls:
  - hosts:
      - departmentname.companyname.com
    secretName: servicename-api-tls

Answer 1

We have solved it like this:

In the Dockerfile we add a --root-path parameter to the run command, like this:

CMD ["pdm", "run", "uvicorn", "our.uvicorn.service.main:app", "--root-path", "/service/api/v1", "--host", "0.0.0.0", "--port", "8080"]

In the kubernetes ingress resource, we use regex to catch the right path:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: our-cool-ingress
  namespace: our-cool-namespace
  annotations:
    cert-manager.io/cluster-issuer: letsencrypt-clusterissuer
    nginx.ingress.kubernetes.io/use-regex: "true"
    nginx.ingress.kubernetes.io/rewrite-target: /$2
spec:
  ingressClassName: nginx
  rules:
  - host: our.cool.url.com
    http:
      paths:
      - path: /service/api/v1(/|$)(.*)
        pathType: Prefix
        backend:
          service:
            name: our-cool-service
            port:
              number: 80
  tls:
  - hosts:
      - our.cool.url.com
    secretName: our-cool-url-com-tls

Path can be anything you like, as long as the value is the same in both DOckerfile and ingress path, in this example "/service/api/v1".

Cheers, Mike