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+A cluster is a collection of loosely coupled cooperating computing elements
+which we refer to as nodes. Failures in clusters are not observed
+instantaneously or simultaneously by every node. Instead, failures occur
+asynchronously, and are observed stochastically and independently by the
+various nodes of the cluster. It is not even guaranteed that any particular
+failure will be observed in the same way by every node in the cluster, or
+even observed at all by every node.
+
+At any given point in time, a cluster is divided into zero or more
+subclusters. Each node has a view of subcluster membership, and each node
+acknowledges membership in no more than one of these subclusters. One of
+these subclusters may be designated the "distinguished" subcluster.
+Traditionally, this distinguished subcluster is said to "have quorum". How
+and why this subcluster is distinguished is implementation-dependent. How
+and why each node associates itself with a particular subcluster is
+implementation-dependent.
+
+Membership calculation is the process whereby each node associates itself
+with a particular subcluster, and obtains a (probabalistically current) view
+of subcluster membership. Quorum calculation is the process whereby the
+distinguished subcluster (if any) is selected.
+
+At any given point in time, any given node node can view the subcluster of
+which it is a member as either having stable membership or being in
+transition. Stable membership is defined to mean that no event has yet been
+observed by the node which would cause it to begin recalculation of
+subcluster membership and/or quorum. In transition means that a particular
+node has observed an event which may cause a membership change, but that the
+process of recomputing membership and/or quorum has not yet completed.
+Different nodes may have different views of the stable/transition state at
+any given point in time.
+
+Because failures occur asynchronously and are observed stochastically,
+membership provides only probabilistic (and not absolute) assurances of
+ability to communicate with any particular node. In a practical sense,
+membership is only probabilistically certain, never absolutely certain.
+
+The quorum calculation process is required to select no more than one
+distinguished subcluster at a time, but need not select any at all. In the
+presence of arbitrary failures, no quorum algorithm can provide an absolute
+guarantee of this property. Different quorum implementations provide
+different degrees of certainty for any given configuration and set of
+expected failures.
+
+Strongly connected subclusters are subclusters where each member of every
+subcluster can communicate with every member of its subcluster.
+
+Consensus subclusters are subclusters where during the time when every
+member of a subcluster views the subcluster membership as being stable (as
+defined above) each member of a subcluster has precisely the same view of
+subcluster membership as every other member of its subcluster.
+
+High-availability membership algorithms offer probabilistic guarantees that
+they only create strongly connected consensus subclusters.
+
+The lowest grade of membership algorithms only meet the letter of the law
+above, never select a distinguished subcluster, and make no guarantees as
+regards strong connectivity or consensus membership. Many traditional
+high-performance clustering applications tolerate such mechanisms, but most
+ high-availability applications require stronger guarantees.
+
+I have sometimes referred to such properties collectively as cluster quality
+of service (CQOS) properties. It is probably the case that they should be
+queryable by applications.

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