Lua script for Redis which sums the values of keys

Question

I am building out my first Redis server side script (for debugging) and my lack of Lua experience has me quite stuck.

Essentially have a dataset of K/V pairs (containing ~1000 values) from which I want to list all the KEYS that match a pattern. For example in redis-cli:

> KEYS "carlos:*"
1) "carlos:1"
2) "carlos:2"
3) "carlos:3"
4) "carlos:4"

Based on the above output I want to return the sum of those keys by executing a Lua script. Currently I have the following on my sum.lua

local sum = 0
local matches = redis.call('KEYS', 'carlos:*')

for unpack(matches)
   sum = sum + redis.call('GET', matches)
end

return sum

While the above script is likely incorrect, trying even redis.call('KEYS', 'carlos:*') by itself produces the following error

root@carlos:~# redis-cli EVAL "$(cat sum.lua)"

(error) ERR wrong number of arguments for 'eval' command

I have tried a number of iterations of my syntax to no avail. Any ideas?

Thanks

Answer 1

1. EVAL requires a minimum of two arguments; the script and the number of keys you are passing to the script. In this case, you are passing zero keys, meaning the script can be invoked as follows:

   redis-cli EVAL "$(cat sum.lua)" 0
   

   or:

   redis-cli --eval sum.lua
   

2. Your loop structure for iterating over the values returned from KEYS was incorrect; I have fixed it for you.

3. You need to convert the value returned from GET from a string to a number using Lua's tonumber function.

With the above changes made, the following script should work for you:

local sum = 0
local matches = redis.call('KEYS', 'carlos:*')

for _,key in ipairs(matches) do
    local val = redis.call('GET', key)
    sum = sum + tonumber(val)
end

return sum