Implementation of ZPOP with redis-lua + CAS transactions

zpop.lua

require 'luarocks.require'
require 'redis'

-- See "WATCH explained" from http://redis.io/topics/transactions

--[[
./redis-cli ZADD zset 1 a
./redis-cli ZADD zset 2 b
./redis-cli ZADD zset 3 c
]]

local redis = Redis.connect()

function zpop(client, key)
    assert(type(key) == 'string', 'key must be a string')
    local element
    local options = { watch = key, cas = true }
    local tx_reply = client:transaction(options, function(t)
        element = t:zrange(key, 0, 0)[1]
        if element then
            t:multi()
            t:zrem(key, element)
        end
    end)
    return element
end

local value = zpop(redis, 'zset')
print(value)