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@bbbates
Created January 3, 2014 03:15
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Using LockService to guarantee leader with Clojure and JGroups
(ns leader-guarantee.core
(:import (org.jgroups.blocks.locking LockService)
(org.jgroups.protocols CENTRAL_LOCK)
(org.jgroups JChannel)
(java.util.concurrent.locks Lock)))
(defn- get-lock
[lock-service with-lock-fn]
(fn []
(let [lock (.getLock lock-service "leader")]
(do
(.lock lock) ;; <<< Lock is acquired here - will block until Leader
(with-lock-fn)))))
(defn when-leader*
"Executes with-lock-fn when the local application container becomes leader of the cluster-name cluster."
[cluster-name with-lock-fn]
(let [channel (doto (JChannel.) ;; Uses udp by default
(.. getProtocolStack (addProtocol (doto (CENTRAL_LOCK.) (.init))))
(.connect cluster-name))
lock-service (LockService. channel)]
;; Create a thread to get us the Leader Lock
(doto (Thread. (get-lock lock-service with-lock-fn) "LeaderAppointmentThread")
(.setDaemon true)
(.start))))
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