How to Create Custom Events for Python Asyncio - python-3.x

Question

The Mesos scheduler Marathon has an asynchronous HTTP API. E.g. when one deploys an app by posting a JSON to /v2/apps a deployment id is returned. The id can then be used to either poll the deployment state in /v2/deployments or by subscribing to /v2/events and look for the deployment_success event.
I would like to create an asynchronous Python client with coroutines. E.g. client.deploy_app(...) should return once the deployment_success event arrived but not block.
How can I implement these methods with asyncio? How can I create an event listener? It feels an event loop is made for this but I don't see how I register events.

Answer 1

Creating asynchronous post http requests that needed for /v2/apps can be done with aiohttp module:
import asyncio
import aiohttp
async def post(url, json):
async with aiohttp.ClientSession() as session:
async with session.post(url, json=json) as resp:
return await resp.json()
async def main():
res = await post('http://httpbin.org/post', {'test': 'object'})
print(res['json']) # {'test': 'object'}
loop = asyncio.get_event_loop()
try:
loop.run_until_complete(main())
loop.run_until_complete(loop.shutdown_asyncgens())
finally:
loop.close()
If you want to use /v2/events to track deployment success you should request for stream (see api doc). It can be achieved in aiohttp with it's asynchronous iteration: you just asynchronously read content line-by-line waiting for event you need, for example:
import asyncio
import aiohttp
async def stream(url):
async with aiohttp.ClientSession() as session:
async with session.get(url) as resp:
async for line in resp.content:
yield line
async def main():
async for line in stream('http://httpbin.org/stream/10'):
print(line) # check if line contains event you need
loop = asyncio.get_event_loop()
try:
loop.run_until_complete(main())
loop.run_until_complete(loop.shutdown_asyncgens())
finally:
loop.close()
If you want to use /v2/deployments you should request it periodically waiting some delay using asyncio.sleep. In this case your function won't be blocking:
import asyncio
import aiohttp
async def get(url):
async with aiohttp.ClientSession() as session:
async with session.get(url) as resp:
return await resp.json()
async def main():
while True:
# Make request to see if deplayment finished:
res = await get('http://httpbin.org/get')
print(res['origin']) # break if ok here
# Async wait before next try:
await asyncio.sleep(3)
loop = asyncio.get_event_loop()
try:
loop.run_until_complete(main())
loop.run_until_complete(loop.shutdown_asyncgens())
finally:
loop.close()