Persistent connections maintain a pool of connections that can be reused by Send HTTP Request activities to avoid opening and closing the connection. Persistent Connection Manager is created by HTTP Client shared resource. Hence, each HTTP Client shared resource has its own persistent connection pool.
A Send HTTP Request activity requires a connection to the HTTP server. The activity exclusively uses the connection until the HTTP server sends the response message. If you have many process instances connecting to a HTTP server, each Send HTTP Request activity opens a connection, holds the connection until the activity completes, and then closes the connection. Opening and closing a large number of connections causes a significant overhead. Persistent connections play a significant role in such scenarios.
Using persistent connection manager you can specify the total number of connections and the maximum number of connections per HTTP server. The total number of connections signify the aggregate connections to all HTTP servers handled by that connection pool.