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HTTP Client

While pyvoy is primarily a Python application server, it also provides an HTTP client implementation using Envoy, which is commonly required by apps. For ASGI, it is an implementation of pyqwest's Transport, and for WSGI, of pyqwest's SyncTransport, which means it can be slotted into a pyqwest Client or SyncClient as-is. Envoy's HTTP client implementation provides many advanced features such as DNS load balancing and circuit breakers needed for advanced production applications.

Envoy HTTP client depends on specifying upstream addresses in configuration; this means the pyvoy HTTP client is not appropriate for accessing arbitrary servers. In practice, most applications access a fixed set of backend servers which can be configured as upstreams. pyvoy provides the --upstream flag to specify a backend host.

uv run pyvoy package.app --upstream auth-svc=http://localhost:8081 --upstream user-svc=http://localhost:8082

Your code initializes a pyvoy HTTPTransport with the name passed to upstream. It is stateless so can be defined anywhere including module constants. Then, just use it as normal - if you use Connect-Python, you can pass in the client and benefit from Envoy's features. Notably, when using gRPC protocol, having DNS load balancing can improve connectivity significantly.

from pyqwest import Client
from pyvoy.asgi.httpclient import HTTPTransport

auth_client = Client(transport=HTTPTransport("auth-svc"))
user_client = Client(transport=HTTPTransport("user-svc"))

async def app(scope, recv, send):
    res = await auth_client.post("/fetch-token")
    user = await user_client.post("/get-user", headers={"Authorization": f"Bearer {res.text()}"})

from pyqwest import SyncClient
from pyvoy.wsgi.httpclient import HTTPTransport

auth_client = SyncClient(transport=HTTPTransport("auth-svc"))
user_client = SyncClient(transport=HTTPTransport("user-svc"))

def app(environ, start_response):
    res = auth_client.post("/fetch-token")
    user = user_client.post("/get-user", headers={"Authorization": f"Bearer {res.text()}"})

Limitations

  • The transport is implemented using callbacks provided by pyvoy's server request handler. This means client requests cannot outlive the server request, notably fire-and-forget type of requests that create a new asyncio.Task without waiting for it in the server request flow will not work. Use a normal pyqwest transport for this use case.

  • Envoy currently does not expose functionality to implement backpressure for HTTP clients so pyvoy's client also cannot offer this. If you have large streaming payloads and require backpressure, use a normal pyqwest transport. We hope to work with the Envoy team to provide this feature in the future.