asyncio

When Guido van Rossum started to work on adding async features in Python 3, part of the community pushed for a Gevent-like solution, because it made a lot of sense to write applications in a synchronous, sequential fashion rather than having to add explicit callbacks like in Tornado or Twisted.

But Guido picked the explicit technique, and experimented in a project called Tulip inspired by Twisted. Eventually, the asyncio module was born out of that side project and added into Python.

In hindsight, implementing an explicit event loop mechanism in Python instead of going the Gevent way makes a lot of sense. The way the Python core developers coded asyncio, and how they elegantly extended the language with the async and await keywords to implement coroutines, made asynchronous applications built with vanilla Python 3.5+ code look very elegant and close to synchronous programming.

By doing this, Python did a great job at avoiding the callback syntax mess we sometimes see in Node.js or Twisted (Python 2) applications.

And beyond coroutines, Python 3 has introduced a full set of features and helpers in the asyncio package to build asynchronous applications, refer to https://docs.python.org/3/library/asyncio.html.

Python is now as expressive as languages like Lua to create coroutine-based applications, and there are now a few emerging frameworks that have embraced those features, and will only work with Python 3.5+ to benefit from this.

KeepSafe's aiohttp (http://aiohttp.readthedocs.io) is one of them, and building the same microservice, fully asynchronous, with it would simply need these few elegant lines:

    from aiohttp import web  
    import time 
 
    async def handle(request): 
        return web.json_response({'time': time.time()}) 
 
    if __name__ == '__main__': 
        app = web.Application() 
        app.router.add_get('/', handle) 
        web.run_app(app) 

In this small example, we're very close to how we would implement a synchronous app. The only hint we're using async is the async keyword, which marks the handle function as being a coroutine.

And that's what's going to be used at every level of an async Python app going forward. Here's another example using aiopg, a PostgreSQL library for asyncio from the project documentation:

    import asyncio 
    import aiopg 
 
    dsn = 'dbname=aiopg user=aiopg password=passwd host=127.0.0.1' 
 
    async def go(): 
        pool = await aiopg.create_pool(dsn) 
        async with pool.acquire() as conn: 
            async with conn.cursor() as cur: 
                await cur.execute("SELECT 1") 
                ret = [] 
                async for row in cur: 
                    ret.append(row) 
                assert ret == [(1,)] 
 
    loop = asyncio.get_event_loop() 
    loop.run_until_complete(go())  

With a few async and await prefixes, the function that performs an SQL query and sends back the result looks a lot like a synchronous function.

But asynchronous frameworks and libraries based on Python 3 are still emerging, and if you are using asyncio or a framework like aiohttp, you will need to stick with particular asynchronous implementations for each feature you need.

If you need to use a library that is not asynchronous in your code, to use it from your asynchronous code means that you will need to go through some extra and challenging work if you want to prevent blocking the event loop.

If your microservices deal with a limited number of resources, it could be manageable. But it's probably a safer bet at the time of this writing to stick with a synchronous framework that's been around for a while rather than an asynchronous one. Let's enjoy the existing ecosystem of mature packages, and wait until the asyncio ecosystem gets more sophisticated.

And there are many great synchronous frameworks to build microservices with Python, like Bottle, Pyramid with Cornice, or Flask.