Why is my aol.com out of order?

When describing the ZooKeeper atomic broadcast, why are both "Total Order" and "Causal Order" mentioned?

• When describing the ZooKeeper atomic broadcast, it is mentioned that it satisfies "Total Order" and "Causal Order." But in my understanding, "Total Order" encompasses "Partial Order." So why does the documentation mention both? What am I missing in my understanding? Links I am referring: http://en.wikipedia.org/wiki/Total_order http://en.wikipedia.org/wiki/Partially_ordered_set http://zookeeper.apache.org/doc/r3.3.2/zookeeperInternals.html

• Answer:

  I think I asked the question too soon. It was right in front of me. The first mistake I did was that I confused Causal ordering with http://en.wikipedia.org/wiki/Partially_ordered_set. The concept is similar but they are not the same. There is a beautiful paper on ZAB, "http://labs.yahoo.com/publication/a-simple-totally-ordered-broadcast-protocol/. Pasting relevant snippet here: Reliable delivery: If a message, m, is delivered by one server, then it will be eventually delivered by all correct servers. Total order: If message a is delivered before message b by one server, then every server that delivers a and b delivers a before b. Causal order: If message a causally precedes message b and both messages are delivered, then a must be ordered before b. …. 1. The reliability and total order guarantees ensure that all of the replicas have a consistent state; 2. The causal order ensures that the replicas have state correct from the perspective of the application using Zab; So what it essentially means Zab needs Total order to ensure atomic broadcast. Causal order is needed for efficient working of Zookeeper. Causal order means if message a cause message b, then there is no way b should occur before a. This concept is necessary in Zookeer for leader's write request. References: If this is still confusing, then along with paper mentioned above following two are very good articles which will help to clear your understanding: A nice explanation on causal ordering: http://scattered-thoughts.net/blog/2012/08/16/causal-ordering/ A nice explanation of causal ordering in context of Zookeeper: http://work.tinou.com/2011/03/causal-atomic-broadcast.html