When initializing a requests’ Session, two HTTPAdapter will be created and mount to http and https.
This is how HTTPAdapter is defined:
class requests.adapters.HTTPAdapter(pool_connections=10, pool_maxsize=10,
max_retries=0, pool_block=False)
While I understand the meaning of pool_maxsize(which is the number of session a pool can save), I don’t understand what pool_connections means or what it does. Doc says:
Parameters: pool_connections – The number of urllib3 connection pools to cache.
But what does it mean “to cache”? And what’s the point using multiple connection pools?
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Answer
Requests uses urllib3 to manage its connections and other features.
Re-using connections is an important factor in keeping recurring HTTP requests performant. The urllib3 README explains:
Why do I want to reuse connections?
Performance. When you normally do a urllib call, a separate socket connection is created with each request. By reusing existing sockets (supported since HTTP 1.1), the requests will take up less resources on the server’s end, and also provide a faster response time at the client’s end. […]
To answer your question, “pool_maxsize” is the number of connections to keep around per host (this is useful for multi-threaded applications), whereas “pool_connections” is the number of host-pools to keep around. For example, if you’re connecting to 100 different hosts, and pool_connections=10, then only the latest 10 hosts’ connections will be re-used.