Since agents run in a thread pool, the way exceptions are signaled and handled becomes an issue. Does the offending message function simply stop running? Does the agent stop? What happens to the Exception instance?
Clojure has several mechanisms to deal with errors in agent functions. We'll walk through them in this recipe.
The agent's error mode and error handler determine how it handles errors. The error mode can be set when the agent is created or with the function
set-error-mode!, and the error handler is set with set-error-handler!.
The default error mode is :fail. If we don't specify an error mode when we create an agent, this is what it gets. With this error mode, when one of the agent's message functions throws an exception, the agent stops processing any more messages and stores the exception. We can retrieve the exception with agent-error, and we can start processing it again with restart-agent:
user=> (def agent-99...