Why isn’t RDBMS Partition Tolerant in CAP Theorem and why is it Available?

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It is very easy to misunderstand the CAP properties, hence I’m providing some illustrations to make it easier.

Consistency: A query Q will produce the same answer A regardless the node that handles the request. In order to guarantee full consistency we need to ensure that all nodes agree on the same value at all times. Not to be confused with eventual consistency in which the network moves towards having all data consistent but there are periods of time in which it is not.

Availability: If the distributed system receives query Q it will always produce an answer for that query. This should not be confused with “high-availability”, this is not about having the capacity to process a higher troughput of queries, it is about not refusing to answer.

Partition Tolerance: The system continues to function despite the existence of a partition. This is not about having mechanisms to “fix” the partition, it is about tolerating the partition, i.e. continuing despite the partition.

Note that the following examples do not cover all possible scenarios. Consider the following caption:

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An example for CP:

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The system is partition tolerant because its nodes keep accepting requests despite the partition; it is consistent because the only nodes providing answers are those that maintain a connection to the master node that handles all the write requests; it is not available because the nodes in the other partition do not provide an answer to the queries they receive.

Examples for AP:

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Either because (respectively) we have the slave nodes replying to requests regardless whether they able to reach master or because the slave nodes in the other partition elect a new master, or because we have a masterless cluster, availability is achieved because all questions are getting an answer – consistency is dropped because both partitions are replying while potentially yielding different states.

Examples for CA:

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If we disconnect nodes when a partition occurs, we can ensure that we have at most one partition which ultimately means that the network is not partitioned anymore, or simply there is no service at all. This is the opposite of partition tolerance, because the system is avoiding the partition instead of functioning despite it. Consistency and availability holds in these partially or fully disconnected systems because all working nodes (if any) have the same state and all received queries (if any) will get an answer – shutdown nodes do not receive queries.

To answer the questions:

  1. Under default configurations, databases such as Cassandra and MongoDB are partition tolerant because they do not shutdown nodes to cope with partitions, whereas RDBMS such as MySQL do.

  2. Availability has very little to do with master/slave setup, e.g. Cassandra is masterless and very available because it doesn’t really matter which node dies. As for availability in a master/slave setup, there is no reason to stop responding to all queries when master is dead, but you may need to suspend write operations while electing a new one.

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