Comment by martius | Hacker News Reader

Comment by martius | Hacker News Reader

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Something always bugs me with CAP theorem explanations, and I am really confused by all that: usually, I read (or understand from those articles) that a system is partition tolerant when "the system is available and consistent while one server is down (=one server is out of reach)".

But, what I think is actually true is: Partition tolerant means that nodes still respond to requests while some other nodes are out of reach (one node being one or more instances of the DBMS in one cluster).

Am I right?

So, If that is true, having a RDBMS with master<=>master replication between two sites is Available and Partition tolerant, albeit inconsistent (the replication is not possible).

In the case of a master<=>master replication as a load-balancing/failover solution, it's about availability and consistency, but we should not even call that "distributed".

Still right?

I think, if you take the view espoused in http://codahale.com/you-cant-sacrifice-partition-tolerance/ , that P can be explained as the "the system is predictable across a partition."

Geography and whether it's a "failover solution" doesn't change that the system is distributed - anything with more than one node is fundamentally distributed, and I'd argue that even two processes communicating on one instance can be considered as a distributed system as well.

The system can choose which kind of predictable it wants to be: whether it sacrifices consistency in favor of being available (accepting reads and writes without knowing that reads are fresh or writes linearizable) or availability in favor of consistency (rejecting reads and writes that can't be confirmed).

In the case of "master<=>master replication" as it's configured in most relational databases, I think the system tries to be AP: conflicted writes or stale reads are possible because barring special options like 2-phase commit, the replicas generally lag each other (as anyone who's tried to reconcile a MySQL split-brain situation knows, this can be a tremendous pain).

It makes sense, thank you!

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