running multiple remotetask instances might cause "at least once" semantic instead of "exactly once"

[Jodok Batlogg]

today benji pointed me to a interesting problem of lovely.remotetask / zc.queue:
this explains the troubles we had on the some videoportals when running multiple remotetask instances.

if lovely.remotetask pulls one item from zc.queue, the item won't disappear from the queue as long the transaction isn't beeing commited. because of this it's possible that another instance might pick up exactly the same job.
if you are running multiple remotetask instances that definitely causes troubles...

in case we want to run multiple instances we should think about a pattern that a kind of two-phase removal of items from the queue.

[Jürgen Kartnaller (jukart)]

This is a known issue.

That's why we use more than one remote task service in the book portal each serving other tasks.