Simple(st) Datomic test program hangs at exit

Question

The following program, when run from an überjar, exits at the end only when using the in-memory Datomic database; when connecting to the Datomic server, it hangs indefinitely rather than exiting the JVM:

(ns myns.example
  (:use [datomic.api :only [db q] :as d])
  (:gen-class))

;; WORKS: (def uri "datomic:mem://testdb")

(def uri "datomic:free://localhost:4334/testdb2")

(defn -main []
  (println 1)
  (when (d/create-database uri)
    (d/connect uri))
  (shutdown-agents)
  (println 2))

Run as:

lein uberjar && java -cp target/myns-0.1.0-SNAPSHOT-standalone.jar myns.example

Outputs:

1
2

and hangs. It only hangs if the DB doesn't exist when the program starts.

Anyone know why, or how to fix? This is with both datomic-free-0.8.4020.26 and datomic-free-0.8.3941.

UPDATE -- the above program does actually terminate, but it takes a very long time (> 1 minute). I'd like to know why.

Answer 1

shutdown-agents takes up to one minute to complete (assuming no agents are running an action).

This is due to the way java.util.concurrent cached thread pools work.

Answer 2

Use datomic.api/shutdown

shutdown

function

Usage: (shutdown shutdown-clojure)

Shut down all peer resources. This method should be called as part of clean shutdown of a JVM process. Will release all Connections, and, if shutdown-clojure is true, will release Clojure resources. Programs written in Clojure can set shutdown-clojure to false if they manage Clojure resources (e.g. agents) outside of Datomic; programs written in other JVM languages should typically set shutdown-clojure to true.

Added in Datomic Clojure version 0.8.3861

(ns myns.example
  (:require [datomic.api :as d])
  (:gen-class))

(def uri "datomic:free://localhost:4334/testdb2")

(defn -main []
  (d/create-database uri)
  (let [conn (d/connect uri)]
    (try
      ;; do something
      (finally (d/shutdown true)))