On Friday, November 17, 2017 at 1:29:55 PM UTC-8, Pierre wrote:
>
> No this won't work......I don't know how to instantiate a lock that will
> act as a global variable. this is an enigma. tried to read this:
> http://martin.kleppmann.com/2016/02/08/how-to-do-distributed-locking.html
> but it's too smart for me.....what i want is '*a lock for
> humans*'............like
> it's said in the publicity
>
Distributed locks are for locking between multiple machines (like for
handling shards between ElasticSearch nodes).
For thread safety, you only need to lock on one machine. You can use a
FIFO socket or or signals (Chapter 17 of the Python docs). Oh, and invite
a few philosophers to dinner.
(The FIFO implementaton would be something like,
while fifo.read() != token:
sleep(1)
if fifo.read() == token:
do dangerous work # keep this as short as possible
fifo.write(token) # then give token back ASAP
sometimes the sleep() shoud be a busy wait, but probably not for a web
app. Busy waits are more common at driver level (the last time I did
that, I just used atomic reads and writes on a flag).
Oh, and obviously the fifo needs to have token at startup.
fifo = socket(blah blah bleh)
fifo.open()
fifo.write(token)
I haven't looked at your specific environment to see what extra
considerations are needed, and for so far for my apps it has been
sufficient to let the db engine handle locking (usually sqlite, which uses
a pretty coarse lock), so you may have holes in my advice.
/dps
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