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) (exhypothesi.com)」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?

用 Automerge 處理 CRDT 問題

上個月看到 Automerge 出了 2.0 版的消息:「Automerge 2.0」,Automerge 這個套件可以幫你處理複雜的 CRDT 結構 (Conflict-free replicated data type)。

可以看到 Automerge 在 2.0 之後的效能改善不少,可以跟 yjs 比較了:

所以練了一下手測界面怎麼用,另外也看一下 conflict 時的處理方式。

這邊先產生 hello, world.,然後做了三個操作,第一個是把開頭的 h 改成 H;第二個是把 world 改成 test;第三個是把 world 改成 example

(() => {
  const Automerge = require('@automerge/automerge');

  let doc1 = Automerge.init();

  doc1 = Automerge.change(doc1, 'Init', doc => {
    doc.text = new Automerge.Text();
    doc.text.insertAt(0, 'h', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd', '.');
  });
  
  let doc2 = Automerge.clone(doc1);
  let doc3 = Automerge.clone(doc1);

  doc1 = Automerge.change(doc1, 'Capitalize', doc => {
    doc.text.deleteAt(0);
    doc.text.insertAt(0, 'H');
  });
  doc2 = Automerge.change(doc2, 'world => test', doc => {
    delete doc.text.deleteAt(7, 5);
    doc.text.insertAt(7, 'test');
  });
  doc3 = Automerge.change(doc3, 'world => example', doc => {
    delete doc.text.deleteAt(7, 5);
    doc.text.insertAt(7, 'example');
  });

  let finalDoc = Automerge.merge(doc1, doc2);
  finalDoc = Automerge.merge(finalDoc, doc3);
  console.log(finalDoc);
})();

這樣最後會產生出 Hello, testexample.

{
  text: Text {
    elems: [
      'H', 'e', 'l', 'l', 'o',
      ',', ' ', 't', 'e', 's',
      't', 'e', 'x', 'a', 'm',
      'p', 'l', 'e', '.'
    ]
  }
}

這結果看起來還行。

只能說以前要是有這些 library 就好了,當初在 KKBOX 做雲端歌單自己搞半天...

ScyllaDB 1.7 要支援 Cassandra 的 Counter 了

Counter 是 Cassandra 裡一個特別的資料型態,總算是要 porting 進 ScyllaDB 了:「Scylla 1.7 introduces experimental support for counters」。

由於資料結構的特殊性,在文章裡有提到想像得到的限制:

  • once deleted, counter column value cannot be used again—this is a consequence of the fact that counters can only be incremented or decremented, they cannot be set to any specific value
  • a table cannot contain both counter and regular columns—without this limitation it wouldn’t be possible to provide proper atomicity and isolation guarantees for updates that modify both counters and regular columns in a single row
  • counter columns cannot be members of primary key
  • updates are not idempotent – in case of a write failure the client cannot safely retry the request
    counter columns cannot have time-to-live set

跟 Cassandra 一樣是透過 CRDT 實做的,行為上會是 eventually consistency:

Scylla implements counters using state-based conflict-free replicated data types (CRDT), which allow atomic operations, like increment or decrement, to be performed locally without the need for synchronization between nodes.

在「Scylla Status and Roadmap」這邊可以看到其他會在 1.7 出現的功能。