Spec Server

Virtualization - Hardware spec for a small business virtual server

The WSGI specification has worked well since it was introduced, and allowed for great flexibility in Python framework and web server choice. However, its design is irrevocably tied to the HTTP-style request/response cycle, and more and more protocols are becoming a standard part of web programming that do not follow this pattern (most notably, WebSocket).

ASGI attempts to preserve a simple application interface, but provide an abstraction that allows for data to be sent and received at any time, and from different application threads or processes.

It also take the principle of turning protocols into Python-compatible, asynchronous-friendly sets of messages and generalises it into two parts; a standardised interface for communication and to build servers around (this document), and a set of standard .

Its primary goal is to provide a way to write HTTP/2 and WebSocket code, alongside normal HTTP handling code, however, and part of this design is ensuring there is an easy path to use both existing WSGI servers and applications, as a large majority of Python web usage relies on WSGI and providing an easy path forwards is critical to adoption. Details on that interoperability are covered in HTTP & WebSocket ASGI Message Format (Draft Spec).

The end result of this process has been a specification for generalised inter-process communication between Python processes, with a certain set of guarantees and delivery styles that make it suited to low-latency protocol processing and response. It is not intended to replace things like traditional task queues, but it is intended that it could be used for things like distributed systems communication, or as the backbone of a service-oriented architecure for inter-service communication.

Overview

ASGI consists of three different components - protocol servers, a channel layer, and application code. Channel layers are the core part of the implementation, and provide an interface to both protocol servers and applications.

A channel layer provides a protocol server or an application server with a send callable, which takes a channel name and message dict, and a receive callable, which takes a list of channel names and returns the next message available on any named channel.

Thus, rather than under WSGI, where you point the protocol server to the application, under ASGI you point both the protocol server and the application to a channel layer instance. It is intended that applications and protocol servers always run in separate processes or threads, and always communicate via the channel layer.

ASGI tries to be as compatible as possible by default, and so the only implementation of receive that must be provided is a fully-synchronous, nonblocking one. Implementations can then choose to implement a blocking mode in this method, and if they wish to go further, versions compatible with the asyncio or Twisted frameworks (or other frameworks that may become popular, thanks to the extension declaration mechanism).

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