Lua function in Redis - error attempt to compare nil with number

Question

I've a Redis cluster v7.x with 3 masters each having a slave. I've defined a Lua function as following:

local keyGet = redis.call('GET', timeSlotKey)

local currentValue = 0
if keyGet ~= nil then
    currentValue = tonumber(keyGet)
else
    currentValue = 0
end

redis.log(redis.LOG_NOTICE, currentValue)
if currentValue < 5 then
    redis.call('INCR', timeSlotKey)
    redis.call('EXPIRE', timeSlotKey, expiryDuration)
    return 1
else
    return 0
end

When I call the function from command line it throws an error:

attempt to compare nil with number at line if currentValue < 5 then

What is the issue here? How to use the return value of GET call from the Redis?

Answer 1

Finally, after trying other things, I added

redis.setresp(3)

inside the lua function and it is working now.

Answer 2

The issue is that currentValue is set to nil in the call to tonumber(keyGet).

From the lua manual for tonumber:

Tries to convert its argument to a number. If the argument is already a number or a string convertible to a number, then tonumber returns this number; otherwise, it returns nil.

So, even though keyGet is not nil, whatever it is, it is still not convertible into a number.

Add a line to log the value of keyGet after local keyGet = redis.call(...)

redis.log(redis.LOG_NOTICE, keyGet)

Hopefully, that will tell you why the value of keyGet isn't working with tonumber.