Zab agreement

ZAB agreement ： It is a distributed coordination service Zookeeper A specially designed message broadcasting protocol supporting crash recovery , The distributed data consistency is realized .
All client requests are written to Leader In progress , Then from leader Process synchronization to other nodes , be called Follower. In the process of cluster data synchronization , If appear Follower The node crashes or Leader When the process crashes , Need to pass through ZAB Protocol to ensure data consistency .
ZAB The protocol includes two basic modes ： Crash recovery and Message broadcast

1. Message broadcast
   All transaction requests of the cluster are made by Leader Node to handle . When Leader After receiving the request , Convert the requested transaction into a transaction Proposal, And will Proposal Distributed to all others in the cluster Follower.
   After the broadcast ,Leader wait for Follower Reply to . When more than half of Follower reply ,Leader Will be sent to the in the cluster again Follower radio broadcast Commit Information .Commit The message is to confirm the previous Proposal Submit .
   Leader Node writing is a two-step operation , The first step is to broadcast transaction operations , The second step is the broadcast submission operation .

2. Crash recovery
   a. Initialize cluster , When it started ;
   b. Leader The server is down
   c. Leader The server is disconnected from more than half of the nodes in the cluster

   At this point, a new round of elections will begin , Elected Leader With more than half Follower To synchronize , Make the data consistent . When synchronization is over , Exit recovery mode , Enter message broadcast mode .

Whole Zookeeper Cluster consistency guarantee , Is the switching between the above two states , When Leader When the service is normal , It's the normal message broadcast mode ; When Leader Is not available , Is to enter crash recovery mode , Crash recovery mode will synchronize data , After completion , Re enter message broadcast mode .