Over the years, Tungsten Clustering has grown and matured. Early on, in response to our customers’ needs, we introduced the concept of the Passive Witness to allow for clusters with only two nodes. Normally, a cluster requires three nodes to create a voting quorum, which requires an odd number of members to avoid split-brain scenarios. It is the Witness which provides the tie-breaker in the event of a network partition. The original Passive Witness is just a pingable target that is checked by the managers to determine if the host is available. The witness host or hosts are used only as check to verify whether a failed host or failed network is the root cause.
As the product matured, it became obvious that the passive, pingable target was not ideal from a decision-making perspective. It was in response to that analysis that the Active Witness was created.
An Active Witness is an instance of Tungsten Manager running on a host that is otherwise not part of the dataservice. An active witness has full voting rights within the managers and can therefore make informed decisions about the dataservice state in the event of a failure. Active witnesses can only be a member of one cluster at a time.
All managers are active witnesses, and active witnesses are the recommended solution for deployments where network availability is less certain (i.e. cloud environments), and where you have two-node deployments.
As of Tungsten Clustering v6.1, the Passive Witness feature will be deprecated, and will be completely removed in the Tungsten Clustering v6.2 release.
For more information, please review our documentation on this subject here: http://docs.continuent.com/tungsten-clustering-5.3/deployment-host-types.html#deployment-host-types-witness