hi Qing,

i'm glad you like the page and Zab.

yes, we are very familiar with Paxos. that page is meant to show a weakness of Paxos and a design point for Zab. it is not to say Paxos is not useful. Paxos is used in the real world in production systems. sometimes there are not order dependencies between messages, so Paxos is fine.

in cases where order is important, multiple messages are batched into a single operation and only one operation is outstanding at a time. (i believe that this is what Chubby does, for example.) this is the solution you allude to: wait for 27 to commit before 28 is issued.

for ZooKeeper we do have order dependencies and we wanted to have multiple operations in progress at various stages of the pipeline to allow us to lower latencies as well as increase our bandwidth utilization, which led us to Zab.


Qing Yan wrote:
    Anyone familer with Paxos protocol here?
    I was doing some comparision of ZAB vs Paxos... first of all, ZAB's FIFO
based protocol is really cool!

 http://wiki.apache.org/hadoop/ZooKeeper/PaxosRun mentioned the
inconsistency case for Paxos("the state change B depends upon A, but A was
not committed").
 In the "Paxos made simple" paper, author suggests fill the GAP (lost state
machine changes) with "NO OP" opeartion.

  Now I have some serious doubts how could Paxos be any useful in the real
world. yeah you do reach the consesus - albeit the content
is inconsistent/corrupted !?

  E.g. on the wiki page, why the Paxos state machine allow fire off 27,28
concurrently where there is actually depedency? Shouldn't you wait instance
27 to be committed before start 28?
  Did I miss something?

  Thanks for the enlight!



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