Your http server will need to stateful.
Say you use a query string of http://xxx.xxx.xx/api/log?lines=100 and the server, having remembered that in the previous call it served lines 0 to 99, serves lines 100 to 199 to this call.
The http_poller input is not stateful and has no facility to remember what the last processed line number was and adjust the query string for example.