Autocomplete server-side implementation

Question

What is a fast and efficient way to implement the server-side component for an autocomplete feature in an html input box?

I am writing a service to autocomplete use

Answer 1

With a set that large I would try something like a Lucene index to find the terms you want, and set a timer task that gets reset after every key stroke, with a .5 second delay. This way if a user types multiple characters fast it doesn't query the index every stroke, only when the user pauses for a second. Useability testing will let you know how long that pause should be.

Timer findQuery = new Timer();
...
public void keyStrokeDetected(..) {
   findQuery.cancel();
   findQuery = new Timer();
   String text = widget.getEnteredText();
   final TimerTask task = new TimerTask() {
      public void run() {
         ...query Lucene Index for matches
      }
   };
   findQuery.schedule(task, 350); //350 ms delay
}

Some pseduocode there, but that's the idea. Also if the query terms are set the Lucene Index can be pre-created and optimized.