This means that if an agent is doing something that takes a few seconds, it holds up all the other agents. In the real world this could mean that email doesn't get sent until a search engine is finished doing an overnight crawl. Threads are better than agents for server apps, they truly run concurrently, they wait for nothing. There's a new version of system.shutdown.shutdownscript in the root updates process. It calls scheduler.shutdown as Frontier exits.
Pointers See the docs page for the original scheduler .