Hey there, let's work through your TCP connection problem with aiohttp's ClientSession. I’ve dealt with similar async HTTP connection hiccups before, so here are targeted fixes and checks to get your requests working smoothly:

Common Issues & Fixes #

1. Misconfigured TCPConnector or System Limits #

Your limit=10240 is extremely high—most systems don’t allow that many open file descriptors by default. This can cause connection failures when the system hits its resource limit:

• Adjust the connection limit: Start with a reasonable number (like 100) and scale up only if needed.
• Add keepalive settings: Help maintain healthy connections in the pool.
• Check system limits: Run ulimit -n in your terminal to see the max open files. If it’s lower than your connector limit, increase it temporarily (or permanently via system configs).

2. Improper ClientSession Lifecycle #

Your global _session isn’t being properly managed. Asyncio requires careful handling of session resources to avoid leaked connections:

• Always use async with to create your ClientSession—this ensures it’s automatically closed when done, cleaning up connections.
• Avoid global sessions for long-running code unless you explicitly handle closing them with await _session.close() when the event loop ends.

3. Unread Responses Blocking Connection Reuse #

If you don’t read the full response from your server, aiohttp might hold onto the TCP connection instead of returning it to the pool. Even if the server only sends an ack, you need to consume the response:

• Add await resp.text() or await resp.read() inside your request context to fully process the server’s reply.

4. Uvloop Compatibility Glitches #

While uvloop boosts performance, it can occasionally have compatibility issues with specific aiohttp versions. Try switching back to the default asyncio event loop to rule this out:

• Comment out asyncio.set_event_loop_policy(uvloop.EventLoopPolicy()) and test again.

Corrected Example Code #

Here’s a revised version of your code incorporating these fixes:

import aiohttp
import time
import asyncio
import uvloop

# Optional: Keep uvloop if it works, or comment out to test default loop
asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())

def _get_session_headers() -> dict:
    return {}

async def test(session, data, value):
    try:
        async with session.request('POST', "http://target:8080/path", data=data) as resp:
            # Critical: Read the response to release the connection
            ack_response = await resp.text()
            print(f"Received ack: {ack_response} for request {value}")
    except aiohttp.ClientError as e:
        print(f"Connection failed for request {value}: {str(e)}")

async def main():
    start_time = time.time()
    
    # Configure connector with sensible limits and keepalive
    conn = aiohttp.TCPConnector(limit=100, keepalive_timeout=30)
    
    # Use async with to manage session lifecycle
    async with aiohttp.ClientSession(connector=conn, headers=_get_session_headers()) as session:
        # Example: Launch multiple test requests
        tasks = [test(session, f"payload_{i}", i) for i in range(10)]
        await asyncio.gather(*tasks)
    
    print(f"Total execution time: {time.time() - start_time:.2f} seconds")

if __name__ == "__main__":
    asyncio.run(main())

Additional Debugging Steps #

• Verify server reachability: Use curl http://target:8080/path or telnet target 8080 to confirm the server is accepting connections.
• Enable debug logging: Add this at the top of your code to get detailed connection logs:

  import logging
  logging.basicConfig(level=logging.DEBUG)
  

  This will show you exactly where the connection is failing (e.g., DNS resolution, timeout, refused connection).

内容的提问来源于stack exchange，提问作者rajan sthapit