mongoc_connection_pooling(3) on Linux
The text da883ae9f9ee, as Debian 10 (Buster) shipped it.
Connection Pooling The MongoDB C driver has two connection modes: single-threaded and pooled. Single-threaded mode is optimized for embedding the driver within languages like PHP. Multi-threaded programs should use pooled mode: this mode minimizes the total connection count, and in pooled mode a background thread monitors the MongoDB server topology, so the program need not block to scan it.