問題描述：有一個參與度很高的活動，玩家的數據是每天重置的。時常有玩家抱怨，他們的活動數據會無緣無故地重置。經常就是，半夜玩了一段時間，白天再上線，打開界面看到，奮鬥的成果全沒了！What?
先簡要介紹該活動的重置機制實現。（以下爲Erlang）
-module(activity).
-export([get_data/1]).
-record(act, {
time = 0 :: integer(), % 玩家當天首次進入該活動的時間（Unix時間戳）
data = [] :: list() % 玩家的活動數據
}).
stamp() ->
{MS, S, _} = os:timestamp(),
MS * 1000000 + S.
%% 當天0點的Unix時間戳
midnight() ->
stamp() - calendar:time_to_seconds(time()).
need_reset(Time) ->
Time < midnight().
do_reset(Act) ->
Act#act{time = stamp(), data = []}.
%% 獲取玩家數據
get_data(Act) ->
Time = Act#act.time,
case need_reset(Time) of
true ->
do_reset(Act);
false ->
Act
end.
判斷是否需要重置的標準：每次獲取玩家數據時，若Time爲昨天或者更早之前的時間，則認爲這是過期的數據，需要重置。
咋一看，此邏輯並沒有問題。然而，有一次查到了某玩家的數據，在2點的時候是這樣的：
#act{
time = 1570464000, % 2019年10月8日00:00:00
data = [1, 2, 3, 4]
}.
而在13點的時候是這樣的：
#act{
time = 1570504000, % 2019年10月8日11:06:40
data = [1, 2]
}.
說明玩家半夜時在線，11點多時也在線。
爲什麼會這樣呢？玩家數據不應該被重置啊！於是，推測need_reset(Time)
會以極小的概率爲true，從而導致重置。爲了驗證猜想，設計了以下的代碼。
-module(test_reset).
-export([loop/0]).
stamp() ->
{MS, S, _} = os:timestamp(),
MS * 1000000 + S.
%% 當天0點的Unix時間戳
midnight() ->
stamp() - calendar:time_to_seconds(time()).
%% 判斷某個時間戳是否某天0點。
%% 由於服務器時間是北京時間，東八區，所以加8小時。
is_some_midnight(Time) ->
(Time + 8 * 60 * 60) rem 24 * 60 * 60 =:= 0.
loop() ->
Midnight = midnight(),
case is_some_midnight(Midnight) of
true ->
ok;
false ->
io:format("~p,", [Midnight])
end,
loop().
1> c(test_reset).
{ok,test_reset}
2> test_reset:loop().
1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,1570550401,
BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded
(v)ersion (k)ill (D)b-tables (d)istribution
q
果真如此。過了幾秒之後，test_reset:loop().就有輸出了。1570464001 是2019年10月8日00:00:01。這種情況下，need_reset(Time)爲false。
再驗證下，具體如何算多了1秒。
loop(N) ->
Os = {MS, S, _} = os:timestamp(),
Time = time(),
Stamp = MS * 1000000 + S,
Midnight = Stamp - calendar:time_to_seconds(Time),
case is_some_midnight(Midnight) of
true ->
ok;
false ->
io:format("~p,~p,~p,~p~n", [N, Os, Time, Midnight])
end,
loop(N + 1).
1> c(test_reset).
{ok,test_reset}
2> test_reset:loop(1).
1825750,{1570,585715,0},{9,48,34},1570550401
1825751,{1570,585715,251},{9,48,34},1570550401
1825752,{1570,585715,363},{9,48,34},1570550401
1825753,{1570,585715,536},{9,48,34},1570550401
1825754,{1570,585715,626},{9,48,34},1570550401
1825755,{1570,585715,714},{9,48,34},1570550401
1825756,{1570,585715,795},{9,48,34},1570550401
1825757,{1570,585715,877},{9,48,34},1570550401
1825758,{1570,585715,980},{9,48,34},1570550401
1825759,{1570,585715,1063},{9,48,34},1570550401
1825760,{1570,585715,1143},{9,48,34},1570550401
1825761,{1570,585715,1205},{9,48,34},1570550401
1825762,{1570,585715,1256},{9,48,34},1570550401
1825763,{1570,585715,1310},{9,48,34},1570550401
1825764,{1570,585715,1360},{9,48,34},1570550401
1825765,{1570,585715,1408},{9,48,34},1570550401
1825766,{1570,585715,1458},{9,48,34},1570550401
1825767,{1570,585715,1507},{9,48,34},1570550401
1825768,{1570,585715,1555},{9,48,34},1570550401
1825769,{1570,585715,1611},{9,48,34},1570550401
1825770,{1570,585715,1660},{9,48,34},1570550401
1825771,{1570,585715,1709},{9,48,34},1570550401
1825772,{1570,585715,1757},{9,48,34},1570550401
1825773,{1570,585715,1811},{9,48,34},1570550401
1825774,{1570,585715,1864},{9,48,34},1570550401
1825775,{1570,585715,1913},{9,48,34},1570550401
1825776,{1570,585715,1960},{9,48,34},1570550401
1825777,{1570,585715,2041},{9,48,34},1570550401
1825778,{1570,585715,2088},{9,48,34},1570550401
1825779,{1570,585715,2138},{9,48,34},1570550401
1825780,{1570,585715,2183},{9,48,34},1570550401
1825781,{1570,585715,2227},{9,48,34},1570550401
1825782,{1570,585715,2270},{9,48,34},1570550401
1825783,{1570,585715,2312},{9,48,34},1570550401
1825784,{1570,585715,2355},{9,48,34},1570550401
1825785,{1570,585715,2399},{9,48,34},1570550401
1825786,{1570,585715,2441},{9,48,34},1570550401
1825787,{1570,585715,2483},{9,48,34},1570550401
1825788,{1570,585715,2553},{9,48,34},1570550401
1825789,{1570,585715,2600},{9,48,34},1570550401
1825790,{1570,585715,2644},{9,48,34},1570550401
3655710,{1570,585716,0},{9,48,35},1570550401
3655711,{1570,585716,296},{9,48,35},1570550401
3655712,{1570,585716,471},{9,48,35},1570550401
3655713,{1570,585716,560},{9,48,35},1570550401
3655714,{1570,585716,671},{9,48,35},1570550401
3655715,{1570,585716,756},{9,48,35},1570550401
3655716,{1570,585716,845},{9,48,35},1570550401
3655717,{1570,585716,926},{9,48,35},1570550401
3655718,{1570,585716,1007},{9,48,35},1570550401
3655719,{1570,585716,1087},{9,48,35},1570550401
3655720,{1570,585716,1168},{9,48,35},1570550401
3655721,{1570,585716,1231},{9,48,35},1570550401
3655722,{1570,585716,1282},{9,48,35},1570550401
3655723,{1570,585716,1332},{9,48,35},1570550401
3655724,{1570,585716,1387},{9,48,35},1570550401
3655725,{1570,585716,1465},{9,48,35},1570550401
3655726,{1570,585716,1516},{9,48,35},1570550401
3655727,{1570,585716,1562},{9,48,35},1570550401
3655728,{1570,585716,1612},{9,48,35},1570550401
3655729,{1570,585716,1659},{9,48,35},1570550401
3655730,{1570,585716,1705},{9,48,35},1570550401
3655731,{1570,585716,1750},{9,48,35},1570550401
3655732,{1570,585716,1793},{9,48,35},1570550401
3655733,{1570,585716,1842},{9,48,35},1570550401
3655734,{1570,585716,1886},{9,48,35},1570550401
3655735,{1570,585716,1929},{9,48,35},1570550401
3655736,{1570,585716,1972},{9,48,35},1570550401
3655737,{1570,585716,2016},{9,48,35},1570550401
3655738,{1570,585716,2059},{9,48,35},1570550401
3655739,{1570,585716,2103},{9,48,35},1570550401
3655740,{1570,585716,2146},{9,48,35},1570550401
3655741,{1570,585716,2189},{9,48,35},1570550401
3655742,{1570,585716,2235},{9,48,35},1570550401
3655743,{1570,585716,2278},{9,48,35},1570550401
3655744,{1570,585716,2321},{9,48,35},1570550401
3655745,{1570,585716,2365},{9,48,35},1570550401
3655746,{1570,585716,2424},{9,48,35},1570550401
3655747,{1570,585716,2538},{9,48,35},1570550401
3655748,{1570,585716,2594},{9,48,35},1570550401
3655749,{1570,585716,2644},{9,48,35},1570550401
5481341,{1570,585717,0},{9,48,36},1570550401
5481342,{1570,585717,240},{9,48,36},1570550401
5481343,{1570,585717,339},{9,48,36},1570550401
5481344,{1570,585717,423},{9,48,36},1570550401
5481345,{1570,585717,511},{9,48,36},1570550401
5481346,{1570,585717,594},{9,48,36},1570550401
5481347,{1570,585717,676},{9,48,36},1570550401
BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded
(v)ersion (k)ill (D)b-tables (d)istribution
q
在CPU非常繁忙的情況下，連續幾百萬次的調用，os:timestamp()和time()的輸出偶爾就不一致了。看了下文檔和底層代碼，它們的實現方式是不同的。連續兩次調用時間函數，不一定能得到想要的結果。
修改此函數，改成只調用一次時間函數。
midnight1() ->
Timestamp = os:timestamp(),
{Date, _} = calendar:now_to_local_time(Timestamp),
Base = calendar:datetime_to_gregorian_seconds({{1970, 1, 1}, {0, 0, 0}}),
LocalDate = calendar:local_time_to_universal_time({Date, {0, 0, 0}}),
calendar:datetime_to_gregorian_seconds(LocalDate) - Base.
出錯概率會降低，但每次獲取玩家數據時都要計算當天0點的時間戳，仍然不靠譜。也該考慮下其他實現方式了。
思考：
在OTP 17及以前的文檔中（http://erlang.org/documentation/doc-6.4/erts-6.4/doc/html/time_correction.html），有一段是
2.4 Should I use erlang:now/0 or os:timestamp/0
The simple answer is to use erlang:now/0 for everything where you want to keep real time characteristics, but use os:timestamp for things like logs, user communication and debugging (typically timer:ts uses os:timestamp, as it is a test tool, not a real world application API). The benefit of using os:timestamp/0 is that it's faster and does not involve any global state (unless the operating system has one). The downside is that it will be vulnerable to wall clock time changes.
大概說，需要真正的時間特性時用erlang:now/0(單調遞增，間隔穩定，接近於wall clock，會校準)，在日誌，與用戶通信，調試時用 os:timestamp/0。後者當然比較快，但是wall clock時間變化時就有點不妙了。
在OTP 18及之後（http://erlang.org/documentation/doc-7.0/erts-7.0/doc/html/time_correction.html）說，However, you are strongly encouraged to use the new API instead of the old API based on erlang:now/0. erlang:now/0 has been deprecated since it is and forever will be a scalability bottleneck.
Don't
use erlang:now/0 in order to retrieve current Erlang system time.
Do
use erlang:system_time/1 in order to retrieve current Erlang system time on the time unit of your choice.
If you want the same format as returned by erlang:now/0, use erlang:timestamp/0.
新的時間API出來了。不要用 erlang:now/0，會帶來系統的瓶頸。用 erlang:system_time/1或者 erlang:timestamp/0。使用erlang:system_time(seconds).就可以獲取Unix時間戳了，不需要再又加法又乘法的了。
我的感想：
普通機器的時間永遠都是不準的。我的跑步專屬電子錶，每兩個月會快1分鐘。尊貴的勞力士，偶爾也要看電視對對時。手機，Windows系統的電腦，默認都會開啓網絡時間同步。連Linux機子，運維也要開啓ntp時間同步的定時任務。不信可以把時間同步關掉試試，看看一個月之後與實際時間相差多少，此舉還能檢測機器的質量呢。要有正確的時間觀念喲！
重要！在一個函數內或者連續的代碼中多次調用時間函數，並假設它們的返回值都一樣的（儘管以秒爲單位）。這是危險的。有一種可能就是，第一個時間函數在上一秒的末尾執行，第二個時間函數在下一秒的開始執行。這種情況下，返回值會相差1秒！