fastapi-etag

Quickstart

Basic etag support for FastAPI, allowing you to benefit from conditional caching in web browsers and reverse-proxy caching layers.

This does not generate etags that are a hash of the response content, but instead lets you pass in a custom etag generating function per endpoint that is called before executing the route function.
This lets you bypass expensive API calls when client includes a matching etag in the If-None-Match header, in this case your endpoint is never called, instead returning a 304 response telling the client nothing has changed.

The etag logis is implemented with a fastapi dependency that you can add to your routes or entire routers.

Here's how you use it:

# app.py

from fastapi import FastAPI
from starlette.requests import Request
from fastapi_etag import Etag, add_exception_handler

app = FastAPI()
add_exception_handler(app)


async def get_hello_etag(request: Request):
    return "etagfor" + request.path_params["name"]


@app.get("/hello/{name}", dependencies=[Depends(Etag(get_hello_etag))])
async def hello(name: str):
    return {"hello": name}

Run this example with uvicorn: uvicorn --port 8090 app:app

Let's break it down:

add_exception_handler(app)

The dependency raises a special CacheHit exception to exit early when there's a an etag match, this adds a standard exception handler to the app to generate a correct 304 response from the exception.

async def get_hello_etag(request: Request):
    name = request.path_params.get("name")
    return f"etagfor{name}"

This is the function that generates the etag for your endpoint.
It can do anything you want, it could for example return a hash of a last modified timestamp in your database.
It can be either a normal function or an async function.
Only requirement is that it accepts one argument (request) and that it returns either a string (the etag) or None (in which case no etag header is added)

@app.get("/hello/{name}", dependencies=[Depends(Etag(get_hello_etag))])
def hello(name: str):
	...

The Etag dependency is called like any fastapi dependency. It always adds the etag returned by your etag gen function to the response.
If client passes a matching etag in the If-None-Match header, it will raise a CacheHit exception which triggers a 304 response before calling your endpoint.

Now try it with curl:

curl -i "http://localhost:8090/hello/bob"
HTTP/1.1 200 OK
date: Mon, 30 Dec 2019 21:55:43 GMT
server: uvicorn
content-length: 15
content-type: application/json
etag: W/"etagforbob"

{"hello":"bob"}

Etag header is added

Now including the etag in If-None-Match header (mimicking a web browser):

curl -i -X GET "http://localhost:8090/hello/bob" -H "If-None-Match: W/\"etagforbob\""
HTTP/1.1 304 Not Modified
date: Mon, 30 Dec 2019 21:57:37 GMT
server: uvicorn
etag: W/"etagforbob"

It now returns no content, only the 304 telling us nothing has changed.

Add response headers

If you want to add some extra response headers to the 304 and regular response, you can add the extra_headers argument with a dict of headers:

@app.get(
    "/hello/{name}",
    dependencies=[
        Depends(
            Etag(
                get_hello_etag,
                extra_headers={"Cache-Control": "public, max-age: 30"},
            )
        )
    ],
)
def hello(name: str):
	...

This will add the cache-control header on all responses from the endpoint.

Contributing

See CONTRIBUTING.md