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Membership services

ZooKeeper and friends can be seen as part of a long history of research into membership services, which goes back to the 1980s and has been important for building highly reliable systems, e.g., for air traffic control [110].

A membership service determines which nodes are currently active and live members of a cluster. As we saw throughout Chapter 8, due to unbounded network delays it’s not possible to reliably detect whether another node has failed. However, if you couple failure detection with consensus, nodes can come to an agreement about which nodes should be considered alive or not.

It could still happen that a node is incorrectly declared dead by consensus, even though it is actually alive. But it is nevertheless very useful for a system to have agreement on which nodes constitute the current membership. For example, choosing a leader could mean simply choosing the lowest-numbered among the current members, but this approach would not work if different nodes have divergent opinions on who the current members are.

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