The confusing CAP and ACID wording

Re:The confusing CAP and ACID wording by AlisdairO on Monday March 30, 2015 @12:52PM (#9290783)

First, I've made one sinful over-simplification in my own post, in conflating NoSQL systems with eventual consistency. While that's usually the case, it's absolutely not an intrinsic property: my apologies!

True SERIALIZABLE-level ACID does pretty much simply just work - and if you're using Postgres the performance hit really isn't too bad. Of course you're chucking away replication then, so whether it's suitable for your needs may vary rather!

Dynamo-based systems have 'tunable consistency' but that's almost always over one key: multi-key operations are usually inconsistent. That being the case, they're pretty much only 'easy to use' for applications with a very simple data model: my experience is that most applications of any real complexity will at some point want to do some kind of multi-key operation. That being the case, you're probably on the hook for a pretty expensive programmer.

I'm vaguely aware that this doesn't strictly apply to Cassandra, and it has a limited notion of transactions - last I checked they didn't work very well at all ( https://aphyr.com/posts/294-call-me-maybe-cassandra/ ), but that may well have changed

I do appreciate your blog post in general - I think there's an awful lot of oversimplifying of this stuff out there. Part of the problem is that high speed, concurrent, distributed data storage is a topic that is, at its heart, pretty damn complicated. Unfortunately,

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Re:The confusing CAP and ACID wording by nkeywal on Monday March 30, 2015 @03:34PM (#9291996)

Well, I've never used postgres, but from the documentation v9.4 it does not look that different from the others db engine: 'Read Committed is the default isolation'; 'applications using this level must be prepared to retry transactions due to serialization failures' (plus the one you mentioned already: 'Serializable transaction isolation level has not yet been added to Hot Standby replication targets')

Not exactly what I like to call 'simply works' ;-)

But I didn't want to say that a NoSQL database is always better than an traditional one. Just that Isolation is complex on traditional systems when dealing with volumes & concurrency. And, typically, transactions between tables or even rows are difficult/impossible for a distributed database, as these rows can be on different nodes (the 'multi-key operations' you mentioned)

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Re:The confusing CAP and ACID wording by AlisdairO on Monday March 30, 2015 @05:55PM (#9292889)

Postgres is quite different in one respect: it has a truly serializable snapshot isolation, at an acceptable performance cost (single digit percentage generally). Other DB systems are either not truly serializable, or have lock-based systems that are sometimes more difficult to work with for web apps.

> 'applications using this level must be prepared to retry transactions due to serialization failures'

True of any serializable system that supports concurrent access, AFAIK - not quite sure that's a fair criticism :-).

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> And, typically, transactions between tables or even rows are difficult/impossible for a distributed database

Depends what you mean by 'distributed', really - Oracle RAC is very much distributed, and supports normal transactional behaviour. On the other hand you won't get that working across a large geographical area.

I accept that understanding the impact of isolation levels can be complex - I'm just very much of the opinion that you'll take a lot more pain trying to maintain consistency in a typical NoSQL system.

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Re:The confusing CAP and ACID wording by nkeywal on Wednesday April 01, 2015 @10:34AM (#9303879)

I can agree with these facts. I just give them a different weight than you do: I don't like the incertitude of the 'serialization failures': it depends on a workload that can be difficult to predict, especially if you're a software vendor. YMMV :-) Thanks for the constructive feedback.

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Re:The confusing CAP and ACID wording by AlisdairO on Wednesday April 01, 2015 @11:01AM (#9304146)

Thanks for a good read/conversation!

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