TCP/IP Client Service

Ignition
4

Something like this:

from java.lang import Thread
from java.util.concurrent import ConcurrentHashMap

g = system.util.persistent("long-lived-thread")
# The .setdefault method will not replace an existing key, and
# is thread-safe (concurrent).
chm = g.setdefault('chm', ConcurrentHashMap())

tagpath = '[default]path/to/boolean/tag'

# Rely on closures/globals to make above available in the thread
def liveLongAndProsper():
	me = Thread.currentThread()
	# A ConcurrentHashMap's .put() method has the handy behavior of
	# atomically replacing and **returning** the previous value for
	# the given key.
	priorThread = chm.put('running', me)

	# If something was already running, interrupt it and join it.
	if priorThread:
		priorThread.interrupt()
		priorThread.join()

	# Not quite an infinite loop
	while not Thread.interrupted():
		v = system.tag.read(tagpath).value
		system.tag.writeSynchronous(tagpath, not v)
		# Sleep is only appropriate in a dedicated thread like this one
		Thread.sleep(5000)

system.util.invokeAsynchronous(liveLongAndProsper)

So this turns on when anything references its script module. And lives “forever”, or a successor kills it. You can turn it off with this:

priorThread = someScriptModule.chm.get('running')
if priorThread:
	priorThread.interrupt()

It won’t start on an inactive gateway, but won’t die. You’ll want the loop to check the gateway redundancy status if that’s important.

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