Consistency in distributed system …
在分布式中，我們一直談論一致性的問題。也為了一致性想出了多種的架構及分法。那麼我們所追求的一致性，到底可以分幾種呢。
In distributed system, we usually mention about consistency. For it, we figured out many approaches and architectures. How many types of consistency we are desired?
Linearizability …
- 每個寫入順序完全一樣。 Total ordering of writes.
- 讀取時，返回最後完成結果。 When read data, it always returns last completed result.
- 一致性最高，但是花費的效能也最高。 The strongest consistency, but the cost of efficiency is highest.
Sequential consistency …
- 所以執行序看到的執行結果相同。 The result shows in any process is the same.
- 讀取時，不保證回傳最終的完成結果。 There is no guarantee that it returns last completed result when read data.
Example 1
這時候，所有的執行序看到一樣的結果，這是 sequential consistency 的情況。
In this time, all processes see the same result. It is sequential consistency.
Example 2
這次的結果，執行序 3 跟 執行序 4 看到不一樣的結果，沒有達成 sequential consistency。
This time, process 3 and process 4 see different result. It is not sequential consistency.
Causal consistency …
- 結果顯示順序是按照因果排序。The results order by causally.
- 如果動作 a 比動作 b 執行更早，則看到 b 時，一定會看到 a。If the action a happened before action b, then users would see action a if they see action b.
Example 1
如果你看到一則評論，一定會看到評論的文章。
If you see a comment, you must see the post which the comment is commented.
Example 2
執行序 4 符合 causal consistency，但不符合 sequential consistency。
Process 4 is causal consistency, but it is not sequential consistency.
-MsHe
MsHe Channel 