How can MySQL Cluster 7.3 achieve 99,999% Availability? Antithesis to CAP Theorem

Question

According to the "Guide to Scaling Web Databases with MySQL Cluster", MySQL Cluster 7.3 can acchieve 99,999% availability while using synchronous update replication. This would be a antithesis to the CAP Theorem since it states that perfect availability (99,999% can be seen as this, no?) and consistency is not acchievable in distributed systems.

How would the cluster react for an update, if the datanode which is responsible for the replica, is not reachable? For a synchronous update replication it must block, which would affect availability.

The Guide states:

• The data within a data node is synchronously replicated to all nodes within the Node Group. If a data node fails, then there is always at least one other data node storing the same information.
• In the event of a data node failure, the MySQL Server or application node can use any other data node in the node group to execute transactions. The application simply retries the transaction and the remaining data nodes will successfully satisfy the request.

But how can this work if a Node Group consists of two Nodes and one crashes (example here)? There would be no Node to replicate a Update to what, as far as I understand, would make the update fail while using synchronous update replication?! Is the Replication just suspended for the time there does not exist a Node to write a replica to?

Answer 1

On master-master replication if the connection among the hosts are down, then if you try to alter data in any database of any host then certainly to achieve this kind availability the consistency is getting broken. Because now the hosts are not synched and so the data is not consistent. Please look at the below cases:

Case 1: Getting A and C but not P

For example if I don’t replicate a database then the whole database is inside a single host. So here we are getting Consistency and Availability but not Partition tolerance.

Case 2: Getting C and P but not A

For example if I replicate a database(master-master) and keep each one in two hosts. Part P1 is in host H1 and part P2 is in host H2. Now to get partition tolerance I can cut the connection of H1 and H2. Now to get the consistency I shall not allow anyone to change any of P1 and P2. And eventually we are losing Availability.

Case 3: Getting A and P but not C

For example if I replicate a database(master-master) and keep each one in two hosts. Part P1 is in host H1 and part P2 is in host H2. Now to get partition tolerance I can cut the connection of H1 and H2. Now to get the availability I shall allow anyone to change any of P1 and P2. And eventually we are losing Consistency.