This page describes how ClustrixDB architecture was designed for Consistency, Fault Tolerance, and Availability.
Many distributed databases have embraced eventual consistency over strong consistency to achieve scalability. However, eventual consistency comes with a cost of increased complexity for the application developer, who must develop for anomalies that may arise with inconsistent data.ClustrixDB provides a consistency model that can scale using a combination of intelligent data distribution, multi-version concurrency control (MVCC), and Paxos. Our approach enables ClustrixDB to scale writes, scale reads in the presence of write workloads, and provide strong ACID semantics.
For an in-depth explanation of how ClustrixDB scales reads and writes, see Concurrency Control.
ClustrixDB takes the following approach to consistency:
ClustrixDB provides fault tolerance by maintaining multiple copies of data across the cluster. By default, ClustrixDB can accommodate a single node failure and automatically recover with no loss of data. The degree of fault tolerance (nResiliency) is configurable and ClustrixDB can be set up to handle multiple node failures and zone failure.
In order to understand ClustrixDB's availability modes and failure cases, it is necessary to understand our group membership protocol.
ClustrixDB uses a distributed group membership protocol. The protocol maintains two fundamental sets:
The cluster cannot form unless more than half the nodes in the static membership are able to communicate with each other (a quorum).
For example, if a six-node cluster experiences a network partition resulting in two sets of three nodes, ClustrixDB will be unable to form a cluster.
However, if more than half the nodes are able to communicate, ClustrixDB will form a cluster.
For performance reasons, MAX_FAILURES defaults to 1 to provide for the loss of one node or one zone.
In the above example, ClustrixDB formed a cluster because a quorum of nodes remained. However, such a cluster could offer only partial availability because the cluster may not have access to the complete dataset.
In the following example, ClustrixDB was configured to maintain two replicas. However, both nodes holding replicas for A are unable to participate in the cluster (due to some failure). When a transaction attempts to access data on slice A, the database will generate an error that will surface to the application.
ClustrixDB can provide availability even in the face of failure. In order to provide full availability, ClustrixDB requires that: