redis:每天重置计数器 [英] redis: reset counter every day

问题描述

我希望每天使用Redis重置计数器.我是Redis的新手，所以我想确保我充分了解事务和管道的工作方式.

I am looking to reset a counter every day using Redis. I am new to Redis so I want to make sure I well understood how transactions and pipes work.

以下代码是否可以确保在多进程环境中工作时，我总是得到一对唯一的(日期，数字)，还是我需要使用Redis锁?

Does the following code ensure that I will always get a unique couple of (date, number) while working in a multi processes environment or do I need to use a Redis lock?

import datetime
import redis

r = redis.Redis(...)

def get_utc_date_now():
    return datetime.datetime.utcnow().date()

def get_daily_counter(r, dt_key='dt', counter_key='counter'):

    def incr_daily_number(pipe):
        dt_now = get_utc_date_now().isoformat()  # e.g.: "2014-10-18"
        dt = pipe.get(dt_key)    
        pipe.multi()    
        if dt != dt_now:
            pipe.set(dt_key, dt_now)
            pipe.set(counter_key, 0)
        pipe.get(dt_key)
        pipe.incr(counter_key)

    result = r.transaction(incr_daily_number, dt_key)
    return result[-2:]

# Get the (dt, number) couple
# 2014-10-18, 1
# 2014-10-18, 2
# etc.
dt, number = get_daily_counter(r)

更新

尝试使用LUA脚本:

UPDATE

Try with LUA Script:

r = redis.Redis(...)

incr_with_reset_on_change_lua_script = """

local dt = redis.call('GET', KEYS[2])
if dt ~= ARGV[2] then
  redis.call('MSET', KEYS[1], ARGV[1], KEYS[2], ARGV[2])
end
return redis.call('INCR', KEYS[1])

"""
# Incr KEYS1 but reset first if KEYS2 has changed.
incr_with_reset_on_change = r.register_script(incr_with_reset_on_change_lua_script)

counter_key = 'dcounterA'
watch_key = 'dcounterA_dt'
watch_value = get_utc_date_now().isoformat()

number = incr_with_reset_on_change(keys=[counter_key, watch_key], args=[reset_value, watch_value])

推荐答案

考虑在午夜发生的两个并发事务.两者都可以执行get(dt_key)，但是其中一个将首先执行MULTI/EXEC块.它将重置计数器，设置新日期，增加计数器.第二个也将输入其MULTI/EXEC块中，但是由于'dt'的值已更改，执行将失败，并且将再次调用incr_daily_number.这次get(dt_key)将返回新日期，因此当执行MULTI/EXEC块时，计数器将递增而不进行任何复位.两项交易将返回具有不同计数器值的新日期.

Consider two concurrent transactions occuring at midnight. Both can execute get(dt_key), but one will execute the MULTI/EXEC block first. It will reset the counter, set the new date, increment the counter. The second one will enter also in its MULTI/EXEC block, but because the value of 'dt' has changed, the execution will fail, and incr_daily_number will be called again. This time get(dt_key) will return the new date, so when the MULTI/EXEC block will be executed, the counter will be incremented without any reset. The two transactions will return the new date with different counter values.

所以，我相信这里没有比赛条件，并且(日期，数字)对将是唯一的.

So, I believe there is no race condition here, and that the (date,number) couples will be unique.

您还可以使用服务器端Lua脚本(其执行始终是原子的)来实现此目的.通常比较方便.

You could also have implemented this using a server-side Lua script (whose execution is always atomic). It is usually more convenient.

请注意，实际上，没有Redis锁之类的东西. API中可用的锁定机制由Python客户端提供，而不是由Redis服务器提供.如果您看一下它的实现，您将会意识到它也是基于SETNX + WATCH/MULTI/EXEC块或Lua脚本编写的.

Note that actually, there is no such thing as a Redis lock. The locking mechanism available in the API is provided by the Python client - not by the Redis server. If you look at its implementation, you will realize it is also based on SETNX + WATCH/MULTI/EXEC blocks or Lua scripting.

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