High Availability Service Clusters

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Clusters of servers offering the same service in active/standby or active/active setup are a common high availability solution. They work very well as long as:

  • The clustering software can reliably detect partitions – there should always be an odd number of cluster members, or extra witness nodes
  • The cluster is not stretched across unreliable infrastructure.

You should also understand how these solutions work:

  • Node or service failure in an active/standby setup might cause significant downtime while the service is restarted on another cluster member.
  • No clustering solution will protect you against operator mistakes.

Once you grasp these fundamentals, it’s perfectly possible to design and deploy well-functioning clusters including network services clusters:

Not surprisingly, solutions created by networking vendors (including multi-chassis link aggregation clusters) ignore all of the above. This is what I had to say about the sad state of affairs:

SDN controllers are no exception:

One of the worst examples of the services clusters are firewall clusters. They are almost always implemented with two nodes without a witness, and often stretched across multiple data centers.

For even more information, watch: