Does AsyncHttpClient knows how many threads to allocate for all the HTTP requests
The AsyncHttpClient client (and other non-blocking IO clients for the matter), don't need to allocate a thread per request, and the client need not resize its thread pool even if you bombard it with requests. You do initiate many connections if you don't use HTTP keep-alive, or call multiple hosts, but it can all be handled by a single threaded client (there may be more than one IO thread, depending on the implementation).
However, it's always a good idea to limit the max requests per host, and max requests per domain, to avoid overloading a service on a specific host, or a site, and avoid getting blocked. This is why HTTP clients add a maxConnectionsPerXxx setting.
AHC has two types of threads:
- For I/O operation. On your screen, it's AsyncHttpClient-x-x threads. AHC creates 2*core_number of those.
- For timeouts. On your screen, it's AsyncHttpClient-timer-1-1 thread. Should be only one.
And as you mentioned:
maxConnections just means number of open connections which does not directly affect the number of threads
Source: issue on GitHub: https://github.com/AsyncHttpClient/async-http-client/issues/1658