Wednesday, October 11, 2017

HBase, Cassandra and CAP

Which is best - Cassandra or HBase? Well, it depends what you're trying to do.

The oft-quoted comparisons generally involve Eric Brewer's CAP-theorem (the formal proof by Gilbert and Lynch can be found here and an updated review here) and you can see pictures like this dotted around the web:

From "Cassandra: the Definitive Guide" quoted here
(where the RDMBS offerings imply Two-Phased Commit).

Most developers have some awareness of CAP but there are a lot of misconceptions - typically that you can have at most two elements of Consistency, Availability or Partition tolerance - but not all three. "The 2 of 3 formulation was always misleading because it tended to oversimplify the tensions among properties" (from an article by Brewer himself found here).
Allowing at least one node to update state will cause the nodes to become inconsistent, thus forfeiting C. Likewise, if the choice is to preserve consistency, one side of the partition must act as if it is unavailable, thus forfeiting A. Only when nodes communicate is it possible to preserve both consistency and availability, thereby forfeiting P. The general belief is that for wide-area systems, designers cannot forfeit P and therefore have a difficult choice between C and A.
Here are some common misconceptions corrected.

  • The C in CAP "has got nothing to do with the C in ACID, even though that C also stands for 'consistency'" [1]. "Gilbert and Lynch use the word “atomic” instead of consistent in their proof, which makes more sense" [2]. The C in ACID has several similar but not mutually exclusive meanings which include not violating any database constraints, linearizability (see below), ensuring the data is consistent (eg, domain-specific rules such as taking X out of one bank account and into another means the total money overall does not change).
  • "The CAP model says nothing about transactions that touch multiple objects. They are simply out of scope for the theorem" [2].

In "Please stop calling databases CP or AP", Martin Kleppmann says:

"But if you’re using some other notion of consistency or availability, you can’t expect the CAP theorem to still apply. Of course, that doesn’t mean you can suddenly do impossible things, just by redefining some words! It just means that you can’t turn to the CAP theorem for guidance, and you cannot use the CAP theorem to justify your point of view.

"If the CAP theorem doesn’t apply, that means you have to think through the trade-offs yourself. You can reason about consistency and availability using your own definitions of those words, and you’re welcome to prove your own theorem. But please don’t call it CAP theorem, because that name is already taken."

In the CAP theorem:
  • Consistent means Linearizability,  think of visibility in terms of variables in the JVM. "If operation B started after operation A successfully completed, then operation B must see the the system in the same state as it was on completion of operation A, or a newer state." [1] A master that asynchronously replicates to slaves is an example of a non-linearizable system. Note: some systems (eg a DB that uses MVCC) are intentionally non-linearizable.
  • Availability means “every request received by a non-failing [database] node in the system must result in a [non-error] response” [from the proof]. Note, the response can take an arbitrary amount of time which might violate some people's notion of availability...
  • "Partition Tolerance (terribly mis-named) basically means that you’re communicating over an asynchronous network that may delay or drop messages." Which is basically the internet.
Using these definitions, Cassandra is not AP for quorum read/writes. If there is a partition split, the nodes on the wrong side of the split cannot reach a consensus and therefore are not available. (Interestingly, Brewer notes "as some researchers correctly point out, exactly what it means to forfeit P is unclear.")

So, although Cassandra and HBase both have their pros and cons, mentioning the CAP theory is orthogonal.

(Aside: although Elastic Search is a search engine not a database, there is an interesting chat about how CAP applies to it here.)

A solution?

There's a cheekily entitled blog post (it's titled "How to beat the CAP theorem" but in it he says "You can't avoid the CAP theorem, but you can isolate its complexity and prevent it from sabotaging your ability to reason about your systems"). This isolation is via immutability.

In this post, author Nathan Marz talks of DBs "choosing availability over consistency":
The best consistency guarantee these systems can provide is eventual consistency. If you use an eventually consistent database, then sometimes you'll read a different result than you just wrote. Sometimes multiple readers reading the same key at the same time will get different results... It is up to you to repair the value once you detect that the values have diverged. This requires tracing back the history using vector clocks and merging the updates together (called read repair)
Marz' proposal to use immutable data leads us to this conclusion:
If you choose consistency over availability, then not much changes from before. Sometimes you won't be able to read or write data because you traded off availability... Things get much more interesting when you choose availability over consistency. In this case, the system is eventually consistent without any of the complexities of eventual consistency. Since the system is highly available, you can always write new data and compute queries. In failure scenarios, queries will return results that don't incorporate previously written data. Eventually that data will be consistent and queries will incorporate that data into their computations.
This mitigates Brewer's proposal that revolves around reconciliation during a partition recovery phase. Marz' proposes no reconciliation, just a deferred sharing of the data.

[1] Martin Kleppmann's blog.
[2] Julian Browne's blog.

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