Redis increment only if < certain number?

Question

How do I call .incr on a key and have it increment ONLY if the resulting number is < than a certain number without having to call .get beforehand?

The reason why is calling .get beforehand is problematic is because if I have multiple threads. There could possibly be 100 threads that have executed the first line below, they all get the value "0" and as a result, all increment. A race condition, if you will.

currentVal = $redis.get('key') #all threads could be done executing this but not yet the below if condition.

if(currentVal < 3)
   $redis.incr('key') #1
end

I've tried to do this before and the only way that seemed possible was with a Lua script. You could petition for a command like INCRBYTOMAX. — tadman
– tadman, Commented Dec 5, 2016 at 19:30
if currentVal is hard coded, couldn't you do the check after the incr then decr? — max pleaner
– max pleaner, Commented Dec 5, 2016 at 19:51

Itamar Haber · Accepted Answer · 2016-12-05 20:25:44Z

1

You can either use WATCH/MULTI/EXEC semantics for optimistic locking, or compose a Lua such as this one (not tested):

local r=redis.call('GET', KEYS[1])
if r < ARGV[1] then
  redis.call('INCR', KEYS[1])
end

answered Dec 5, 2016 at 20:25

Itamar Haber

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Magnielcz · Accepted Answer · 2022-05-03 09:11:26Z

0

I took the idea for lua script from Itamar Haber, improve it so it works, and added return values to know what happened on client side.

local r=redis.call('GET', KEYS[1])
if not r or tonumber(r) < tonumber(ARGV[1])
then
  redis.call('INCR', KEYS[1])
  return 1
else
  return 0
end

answered May 3, 2022 at 9:11

Magnielcz

315 bronze badges

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