If I understand the CAP Theorem correctly, availability means that the cluster continues to operate even if a node goes down.
I've seen a lot of people (http://blog.nahurst.com/tag/guide) list RDBMS as CA, but I do not understand how RBDMS is available, as if a node goes down, the cluster must go down to maintain consistency.
My only possible answer to this has been that most RDBMS are a single node, so there is no "non-failing" node. But, this seems to be a technicality, not true 'availability' and definitely not high availability.
Thank you.
First of all, let me clarify and state that the consistency in RDBMS is different than consistency in distributed systems. RDBMS (single system) applies consistency to transactional consistency, where as in distributed systems consistency means view from anywhere in the system (read from any node) is consistent. So RDMBS single node cannot be discussed with regards to CAP theorem. It is like comparing apple to orange.
RDBMS with master-slave can be compared to distributed systems. Here RDBMS can be configured to CA/CP or AP. MySQL for example, provides a way to configure the system in a way that if there is a quorum loss (not enough secondary available for commit log replication), the cluster is not available (CP system). MySQL also provides a configuration to allow the cluster to operate as long as master is available (CA system) with the potential of data loss. SQL Server AlwaysOn is an AP system, because commit log replication is asynchronous (even on sync replicas).
So RDBMS can be any of CA, CP or AP in a distributed world.
In CAP Theorem P is for Partition tolerance , which is the ability of system to handle partitions(partitions are isolated clusters - due to network failure or any other reason ..).
In a distributed network to handle a partition , system has to pick either Consistency or Availability.
In case of RDBMS there is no chance for partitions (assuming not distributed which is normal case) ,So Those will be always CA.
来源：https://stackoverflow.com/questions/29663645/why-are-rdbms-considered-available-ca-for-cap-theorem