Debian 的 64-bit time_t 計畫

在「Debian: 64-bit time_t transition in progress」這邊看到 Debian 的 mailing list 上的討論:「64-bit time_t transition in progress」,另外官方也有整理 wiki page:「64-bit time」。

因為技術上無法表示 2038/01/19 以後的時間,確定會 breaking ABI 將 time_t 從 32-bit 變成 64-bit,而現在要想辦法搞定 32-bit 平台上面可以處理這樣的改變:

The goal of this transition is to ensure that 32-bit architectures in trixie (whether they are currently release architectures, or out of archive, etc) will be capable of handling current and future timestamps referring to times beyond 2038.

離 2038/01/17 還有約 13 年多...

Python 3.12 將淘汰 datetime.datetime 的 utcnow() 與 utcfromtimestamp()

Simon Willison 這邊看到「It's Time For A Change: datetime.utcnow() Is Now Deprecated」,引用的文章是「It's Time For A Change: datetime.utcnow() Is Now Deprecated」這篇。



from datetime import datetime, timezone
datetime.fromtimestamp(timestamp, timezone.utc)


import datetime
datetime.datetime.fromtimestamp(timestamp, datetime.timezone.utc)

要稍微注意一下這個歷史遺跡要被拆了... (StackOverflow 上面應該有很多用到這兩個 function 的解答)

分散式系統的 clock

前幾天在 Hacker News 上看到「Clocks and Causality – Ordering Events in Distributed Systems (2022) (」這篇,講分散式系統上 clock 的設計,作者也有跑出來在 Hacker News 上面跟大家聊一下 (帳號是 thoughtlede),原文在「Clocks and Causality - Ordering Events in Distributed Systems」這邊。

文章裡面主要講空間是 O(1)Lamport timestamp 與空間是 O(n)Vector clock (這邊的 n 相對於節點數量),以及這兩個對應的擴充版本:

作者會整理這些資料的原因是因為在研究 CRDT 的時候看到演算法中常常會需要處理分散式系統裡面事件的順序,所以花了一些時間整理常見的方式:

Author here. Pleasantly surprised to see the article here.

Some context behind the article. I studied CRDTs for a few months, and noticed that different CRDT designs use logical clocks in different and clever ways. And I haven't seen anyone narrate all those ways of use in one article. My attempt with this article was to dredge up those flavors of logical clocks into one article and give them names for future reference.

(To respond to a couple of other comments, I ignored atomic (and gps-based) clocks in this discussion, as indicated in my footnote 3).

我記得還有一個 Interval Tree Clocks 可以參考 (在「Interval Tree Clocks」這邊講的比較清楚),是針對節點的動態增刪而改善的演算法,但不確定有什麼比較有名的系統有用。

大多數應該都是用 Vector clock,畢竟是在 2007 年的「Dynamo: Amazon’s Highly Available Key-value Store」被發揚光大,而且也算是還不錯的演算法?

Let's Encrypt 的 Embed SCT 支援

翻到 Let's EncryptUpcoming Features 時看到:

Embed SCT receipts in certificates
ETA: February, 2018

對 Embed SCT 不熟,所以查了查這個功能。

這指的是在簽發 SSL certficiate 後,把資料丟給 Certificate Transparency (CT) 伺服器後,伺服器會提供 signed certificate timestamp (SCT);而這個資料放到 SSL certificate 內叫做 Embed SCT:(出自 CT 的 FAQ)

What is an SCT?
An SCT is a signed certificate timestamp. When a certificate authority or a server operator submits a certificate to a log, the log responds with an SCT. An SCT is essentially a promise that the log server will add the certificate to the log in a specific time. The time, known as the maximum merge delay (MMD), helps ensure that certificates are added to logs in a reasonable time. The SCT accompanies the certificate until the certificate is revoked. A TLS server must present the SCT to a TLS client (along with the SSL certificate) during the TLS handshake.

當使用 ECC 時會小於 100 bytes:

How big is an SCT?
SCTs are less than 100 bytes, assuming elliptic curve signatures are used.

這樣才能試著解釋前幾天提到要拔掉 HPKP 的事情:「Chromium 內提案移除 HPKP (HTTP Public Key Pinning)」,也就是為什麼他們是提 CT 解,而不是 DNS CAA 解...

不過我記得 CT server 可以自己架自己 submit 不是嗎?後來有另外規定一定要用第三方的嗎?這樣又很怪...

Facebook 在 MySQL 裡存時間的型態

MySQL at Facebook這邊說明提到了,Facebook 內部是使用 INT UNSIGNED 儲存時間:

Which gets us to the point that it is no different than storing INT (hello 2038?) or UNSIGNED INT (a bit later) or BIGINT (till the end of time) and possibly passing binary values in efficient protocols eventually.

If you got that far of this post, your likes in Facebook graph are stored with 'INT UNSIGNED' time field.

順道一提,INT 是 2038 年問題,INT UNSIGNED 是 2106 年問題。

而 Facebook 在 MySQL 上會選擇不使用 DATETIMETIMESTAMP 的原因其實跟技術搭不上太多關係,主因是因為 MySQL 根本沒打算修 XDDD

It is my favorite MySQL bug, simply because it forces any reasonable mind not to use TIMESTAMP, and MySQL is never going to fix it (nor will ever understand time). I lost my temper a bit on that bug:

我的猜測是已經爛成一團了,而且大家都有 workaround (呃,其實就是 Facebook 推薦用 INT UNSIGNED 的方法),再考慮到有一票現有程式,在上面狂用 side effect 讓執行結果正確,不如就不要修這種吃力不討好的東西了 XDDD

另外一方面 timezone 資訊其實常常變化,常常需要更新 MySQL 的 timezone database (而這對於維運來說不是什麼開心的事情):

There're few ways around that. One of them is side-load and maintain timezone data inside MySQL itself - it has support for internal timezone database and tracks obscure time shifts like ones for "Pacific War Time" and "Pacific Peace Time". That is operationally feasible (you have to remind yourself to update the database whenever time rules change, and they do change a lot, if you consider every timezone in the world), but has limited value.

這就是為什麼大家遇到 MySQL 時都會推薦用 INT UNSIGNED 了...

另外可以參考三年前的文章「MySQL 裡儲存時間的方式...」,裡面引用了 Baron Schwartz 的說明:

All date and time columns shall be INT UNSIGNED NOT NULL, and shall store a Unix timestamp in UTC.

其實這已經是個 best practice 了...

MySQL 裡儲存時間的方式...

這應該是 MySQL 的 best practice 之一,不知道為什麼 Baron Schwartz 又拿出來講:「A simple rule for sane timestamps in MySQL」。

MySQL 內可以儲存「日期與時間」的資料型態是 DATETIME 與 TIMESTAMP 兩種,不過 DATETIME 沒有時區觀念,而 TIMESTAMP 只能是 UTC (GMT+0)。

相較於隔壁棚 PostgreSQLDate/Time Types 就一種 TIMESTAMP,但支援 with timezone 與 without timezone 直接解決問題。

這使得 MySQL 上在儲存「日期與時間」以及處理的時候一直有種 WTF 的感覺...

就如同 Baron Schwartz 的建議,如果使用 MySQL,目前比較好的方法是用 INT UNSIGNED NOT NULL 儲存,把 timezone 的處理都放到應用程式端來處理,這樣產生的問題會比較少...

真的需要在 INSERT 或是 UPDATE 時更新欄位,可以用 trigger 處理,彈性反而比內建功能大不少。