3 ms·

> “what’s the difference between Consistency and Isolation again?” This is what bothers me most about this article. How can a person seemingly having advanced

> “what’s the difference between Consistency and Isolation again?” This is what bothers me most about this article. How can a person seemingly having advanced knowledge of relational databases cannot understand that Consistency is about dataset state and Isolation is about transactions. It's not that you get consistent results between transactions or even queries, but that if key is integral number, then `itoa()` works (given that bit length not too large) no matter what, period. If you want to support concurrent read and write operations on a database, then it's not much different from multithreaded programming - hard and impossible to get right without compromises.

amirouche 12y ago
The author makes this statement rhetorically, without explaining it more than all 4 ACID properties are tightly coupled. In the case of Consistency and Isolation, you might have a inconsistent databasse if Isolation allows phantom reads. Now that I think (again) about it, it seems to me consistency can only be a consequence of Atomicity and Isolation. Maybe not... because if Isolation allows to write things based on phatom reads then the database even if Atomic and with strong isolation it will be inconsistent. tl,dr: yes, the author was quick in that sentence, doesn't mean he is stupid. `itoa()` example is far fetched and non very instructive.
- friendzis 12y ago
  Sorry about that, had to finish up quickly. I will try to clarify my view, and sorry again if this wall of text is too long. I just wanted to stress that Consistency is about data itself, while Isolation is about dataset acquisition (the same goes for Atomicity). My view is that Consistency should only be mentioned when talking about data. It is much like method argument type checking in type-safe languages: if you get float passed in it WILL contain a valid number and you do not need any additional explicit checking for that. Consistency is data validation. Isolation, on the other hand, is a form of concurrency control. Once you have shared data between concurrent threads, care must be taken. SQL brings this to a new level by having only shared data between threads (transactions). Since performance is directly related to concurrency control techniques, some compromise must be taken. The article is just about those compromises. Phantom reads can be viewed as a manifestation of holding and releasing locks (semaphores) during thread execution. I believe that ACID properties are all orthogonal and databases should be examined where/when/how those properties are violated, e.g. Isolation is not always ensured because of the mentioned MVCC. Atomicity: data is treated in atomic datasets; Consistency: data validation; Isolation: data immutability during transaction; Durability: data retention (and as a consequence Consistency) over prolonged periods of time. But that is my own humble opinion.

amirouche 12y ago

The author makes this statement rhetorically, without explaining it more than all 4 ACID properties are tightly coupled. In the case of Consistency and Isolation, you might have a inconsistent databasse if Isolation allows phantom reads. Now that I think (again) about it, it seems to me consistency can only be a consequence of Atomicity and Isolation. Maybe not... because if Isolation allows to write things based on phatom reads then the database even if Atomic and with strong isolation it will be inconsistent. tl,dr: yes, the author was quick in that sentence, doesn't mean he is stupid. `itoa()` example is far fetched and non very instructive.

friendzis 12y ago
Sorry about that, had to finish up quickly. I will try to clarify my view, and sorry again if this wall of text is too long. I just wanted to stress that Consistency is about data itself, while Isolation is about dataset acquisition (the same goes for Atomicity). My view is that Consistency should only be mentioned when talking about data. It is much like method argument type checking in type-safe languages: if you get float passed in it WILL contain a valid number and you do not need any additional explicit checking for that. Consistency is data validation. Isolation, on the other hand, is a form of concurrency control. Once you have shared data between concurrent threads, care must be taken. SQL brings this to a new level by having only shared data between threads (transactions). Since performance is directly related to concurrency control techniques, some compromise must be taken. The article is just about those compromises. Phantom reads can be viewed as a manifestation of holding and releasing locks (semaphores) during thread execution. I believe that ACID properties are all orthogonal and databases should be examined where/when/how those properties are violated, e.g. Isolation is not always ensured because of the mentioned MVCC. Atomicity: data is treated in atomic datasets; Consistency: data validation; Isolation: data immutability during transaction; Durability: data retention (and as a consequence Consistency) over prolonged periods of time. But that is my own humble opinion.