> ## Documentation Index
> Fetch the complete documentation index at: https://cerebrium.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom Python Web Servers

> Run ASGI/WSGI Python apps on Cerebrium

Cerebrium's default runtime covers most app needs. For more control, use ASGI or WSGI servers through the custom runtime feature - enabling custom authentication, dynamic batching, frontend dashboards, public endpoints, and WebSocket connections.

## Setting Up Custom Servers

A basic FastAPI server running as a custom server on Cerebrium:

```python theme={null}
from fastapi import FastAPI
app = FastAPI()

@app.post("/hello")
def hello():
    return {"message": "Hello Cerebrium!"}

@app.get("/health")
def health():
    return "OK"

@app.get("/ready")
def ready():
    return "OK"
```

Configure this server in `cerebrium.toml` by adding a custom runtime section:

```toml theme={null}
[cerebrium.runtime.custom]
port = 5000
entrypoint = ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "5000"]
healthcheck_endpoint = "/health"
readycheck_endpoint = "/ready"

[cerebrium.dependencies.pip]
pydantic = "latest"
numpy = "latest"
loguru = "latest"
fastapi = "latest"
```

The configuration requires three key parameters:

* `entrypoint`: The command that starts your server
* `port`: The port your server listens on
* `healthcheck_endpoint`: The endpoint used to confirm instance health. If unspecified, defaults to a TCP ping on the configured port. If the health check registers a non-200 response, it will be considered *unhealthy*, and be restarted should it not recover timely.
* `readycheck_endpoint`: The endpoint used to confirm if the instance is ready to receive. If unspecified, defaults to a TCP ping on the configured port. If the ready check registers a non-200 response, it will not be a viable target for request routing.

<Info>
  For ASGI applications like FastAPI, include the appropriate server package
  (like `uvicorn`) in your dependencies. After deployment, your endpoints become
  available at
  `https://api.aws.us-east-1.cerebrium.ai/v4/[project-id]/[app-name]/your/endpoint`.
</Info>

The [FastAPI Server Example](https://github.com/CerebriumAI/examples) provides a complete implementation.

## Request Headers

Custom web servers receive the Cerebrium run ID in the `X-Request-Id` header on every request. This corresponds to the internal `run_id` and is useful for tracking and debugging.