distribution.lua

-- Quick and Dirty script to get Redis'keys TTL distribution per hours
--   How to run it?
--     With a redis-cli:
--       `redis-cli -h my.redis.host.local --eval distribution.lua`
--
local logtable = {} -- Array to store our logs, returned by the script, printed to the screen
-- A small log function
local function logit(msg)
  logtable[#logtable+1] = msg
end
logit("Hello from my script") -- Example to log a message

-- Parameters to get distribution per hours
local max = 24 * 7  -- max 7d
local increment = 4 -- increment by 4h
--
local results = {} -- declare an array to store all measures
for j = 0, max, increment
do
    results[j] = 0
end
local abovemax = 0 -- count nb of key above our max
local nottl = 0
--
local cursor = "0" -- The scan function need a cursor
repeat -- start a loop until the cursor is back to the original value.
    -- Using the scan function is much safer and consume less resource than the `key *`
    -- SCAN is not deterministic, inside the loop we can't have atomic writes.
    local t = redis.call("SCAN", cursor, "COUNT", 10000); -- return an array {cursor,results}
    local list = t[2]; -- our result are in the 2nd position, t[1] is the cursor ; IDK what's inside t[0]
    for i = 1, #list do -- iterate on our results
        local vttl = redis.call("TTL", list[i]) -- call another redis function to get the TTL on each key
        -- logic to have the distribution
        if vttl < 1 then
          nottl = nottl + 1
        elseif vttl > (max * 3600) then
          abovemax = abovemax + 1
        else
          for j = 0, max, increment
          do
            if vttl > (j * 3600) and vttl < ((j + increment) * 3600) then
              results[j] = results[j] + 1
              break
            end
          end
        end
        -- end logic
    end
    cursor = t[1]; -- Update the cursor to be able to do the next SCAN, if we don't update this scan will.
until cursor == "0"; -- Cusor back to the original position, scan is completed
-- Construct human readable output
logit("END")
logit("No TTL: " .. nottl .. " keys")
for j = 0, max, increment
do
    logit("Up to " .. ( j + increment ) ..  " hours: " .. results[j] .. " keys")
end
logit("More than" .. max .. " hours:" .. abovemax .. " keys")
-- return output filled by `logit` function
return logtable