Epoch numbers

An epoch number is the total number of operations that originated at a particular replica.

In Figure 2 from Operation tracking for each replica, the epoch number for boston_hub is 950.

The MultiSite synchronization scheme attempts to minimize the amount of data transmitted among replicas. Each replica keeps track of these epoch numbers:
Table 1 shows how these epoch numbers fall into an epoch number matrix. Each replica maintains its own such matrix, revising its rows as work occurs locally and as it exchanges update packets with other replicas:
Table 1. Two-row epoch number matrix at replica boston_hub
  Operations originated at boston_hub Operations originated at sanfran_hub
boston_hub’s record of its own state 950 504
boston_hub’s estimate of sanfran_hub’s state 912 504
The contents of this matrix are reported by the lsepoch command at the boston_hub replica:
multiutil lsepoch -clan telecomm -site boston_hub -family PRODA -user bostonadmin -password secret 
Multiutil: Estimates of the epochs from each site replayed at site ’boston_hub’ (@minuteman):
boston_hub: 950
sanfran_hub: 504
Multiutil: Estimates of the epochs from each site replayed at site ’sanfran_hub’ (@goldengate):
boston_hub: 912
sanfran_hub: 504
A syncreplica –export command entered at boston_hub uses this matrix as follows to generate an update destined for sanfran_hub:
  1. At the boston_hub replica, the number of local operations is 950 (the number in upper left corner of matrix), and the estimate is that the sanfran_hub replica has imported all operations through oplog ID 912 (the number in lower left corner).
  2. The update packet that the boston_hub replica sends to the sanfran_hub replica includes boston_hub oplog entries 913-950. After the Boston administrator invokes syncreplica –export, the sanfran_hub row is updated:
    multiutil lsepoch -clan telecomm -site boston_hub -family PRODA -user lexadmin -password secret 
    Multiutil: Estimates of the epochs from each site replayed at site ’boston_hub’ (@minuteman):
    boston_hub: 950
    sanfran_hub: 504
    Multiutil: Estimates of the epochs from each site replayed at site ’sanfran_hub’ (@goldengate):
    boston_hub: 950
    sanfran_hub: 504

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