Building servers with non-blocking IO has been quite popular these days. Tests have shown that it does actually improve scalability of web applications. However, my experience show that it comes at a cost. In this post I am going to discuss some negative aspects of writing asynchronous code based on Scala’s Futures.
Debugging exceptions in asynchronous programs is a pain. When issuing an asynchronous IO operation you provide a callback that should be executed when the operation returns. In most implementations, this callback might be executed on any thread (not necessarly the same thread that invoked the operation). Since call stack is local to the thread, the stacktrace that you get when handling an exception is not very informative. It will not trace back to the servlet so you may have hard time figuring out where what actually happened.
Exception in thread "main" java.lang.IllegalArgumentException