Method or apparatus for selecting a cluster in a group of nodes

1. A method of selecting a cluster in a group of nodes, said method comprising the steps of:

a) assigning a token to a first node in the group of nodes;

b) identifying subgroups, from among the group of nodes, that are interconnected; and

c) if two largest subgroups, from among the identified subgroups, comprise equal numbers of nodes, then selecting as the cluster a subgroup containing the node to which said token is assigned.

2. A method according to claim 1 in which said token is passed from one node to another.

3. A method according to claim 2 in which said token is passed between nodes at predetermined time intervals and/or random time interval.

4. A method according to any of claim 2 in which said passing of said token can be suspended.

5. A method according to claim 2 in which said passing of said token between nodes is carried out atomically so that said token is not lost and remains unique.

6. A method according to claim 2 in which said passing of said token between nodes is carried out using a three phase commit protocol.

7. A method according to claim 1 in which said first node is the first node assigned to the cluster during cluster configuration.

8. A method according to claim 1 in which if a node holding said token is required to shut down then prior to said shut down said token is passed to another node.

9. A method according to claim 1 in which if a node holding said token crashes then step c) is suspended until said node reboots.

10. A method according to claim 1 in which each node is assigned one or more votes and in step c) if two subgroups hold the largest numbers of votes then selecting as the cluster the subgroup containing the node to which said token is assigned.

11. A method according to claim 10 in which said token counts for one or more votes.

12. A method according to claim 10 , in which the number of votes or nodes held by subgroups is treated as being equal if the difference between them falls within a predetermined limit.

13. The method according to claim 1 , wherein the token is a global variable.

14. An apparatus for selecting a cluster in a group of nodes, wherein the apparatus is a part of a computer system, the apparatus comprising:

a) a token assigned to a first node in the group of nodes;

b) communication means for identifying subgroups of nodes in the group of nodes that are interconnected; and

c) selecting means, operable if two subgroups are largest of the identified subgroups and the two subgroups comprise equal numbers of nodes, to select as the cluster the subgroup containing the node to which said token is assigned.

15. The apparatus of claim 14 , wherein said token is between nodes at predetermined time intervals and/or random time interval.

16. The apparatus of claim 15 , wherein said passing of said token between nodes is carried out atomically so that said token is not lost and remains unique.

17. The apparatus of claim 14 , said first node is the first node assigned to the cluster during cluster configuration.

18. The apparatus of claim 14 , wherein if a node holding said token is required to shut down then prior to said shut down said token is passed to another node.

19. The apparatus of claim 14 , wherein if a node holding said token crashes then operation of said selecting means is suspended until said node reboots.

20. The apparatus of claim 14 , wherein each node is assigned one or more votes and said selecting means is operable if two subgroups hold the largest numbers of votes to select as the cluster the subgroup containing the node to which said token is assigned.

21. The apparatus of claim 20 , wherein a number of votes or nodes held by subgroups is treated as being equal if a difference between them falls within a predetermined limit.

22. The apparatus according to claim 14 , wherein the token is a global variable.

23. A method of operating a node in a cluster, said method comprising the steps of:

a) determining a number of nodes connected to form a connected group of nodes;

b) if said connected group of nodes comprises more than half of a total number of nodes in the cluster, then forming the cluster from said group of nodes; or

c) if said connected group comprises half of said total number of nodes, then forming said cluster if said connected group includes a node to which a token is assigned;

d) wherein a number of nodes held by subgroups is treated as being equal if a difference between them falls within a predetermined limit;

wherein the token is a global variable.