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1. DefaultEnsemblePlacementPolicy: Changes will be made to newEnsemble() and replaceBookie() interfaces to select the bookies based on their free disk space based weight when the configuration suggests that weight based placement is to be honored. 
2. RackawareEnsemblePlacementPolicy: Enhancements will be made to selectRandomFromRack() and selectRandomInternal() to take into account the weights of the bookies. No changes will be made to the rack selection logic. But once a rack has been selected the bookie selection within that rack will be based on disk weight. 
3. RegionAwareEnsemblePlacementPolicy: No changes are needed to the region selection logic. In this policy  once a region is selected, the bookie selection is done using RackawareEnsemblePlacementPolicy. 

1.4.2 BookieInfo

The following protocol message is being added to retrieve the BookieInfo details. This message can be extended to retrieve other information such as network bandwidth in the future.

message GetBookieInfoRequest {
    enum Flags {
        TOTAL_DISK_CAPACITY = 0x01;
        FREE_DISK_SPACE = 0x02;
     }
     // bitwise OR of Flags
     optional int64 requested = 1;
}

message GetBookieInfoResponse {
    required StatusCode status = 1;
    optional int64 totalDiskCapacity = 2;
    optional int64 freeDiskSpace = 3;
}

1.4.3 BookieInfoReader

An instance of BookieInfoReader is created to read periodically the bookie information such as free disk space, bookie load etc. This singleton is instantiated once per Bookkeeper object. It uses a SingleThreadedExecutor to query the bookie info from all the bookies. It then communicates the updated information to the ensemble policies via the updateBookieInfo() interface described above. Changes in the number of bookies in the cluster also triggers this operation. The event is triggered from BookieWatcher singleton.

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