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:
  • Changes made in the current replica. The number of operations that originated at the current replica.
  • Changes at sibling replicas that have been imported to the current replica. When syncreplica writes an operation from an update packet to the current replica, it increments the epoch number that records the number of operations originating at the sibling replica that have been imported at the current replica.
  • Estimates of the states of other replicas. For each other replica, an estimate of its own changes and other replicas’ changes. The current replica keeps track of the operations it has sent to other replicas, and assumes that these operations are imported successfully.
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:
  • When work occurs in the boston_hub replica, its own epoch number is incremented.
  • When the boston_hub replica receives an update from sanfran_hub, it revises its own row (boston_hub) and the sanfran_hub row in its epoch number matrix.
  • When the boston_hub replica generates an update packet to be sent to sanfran_hub, it revises the sanfran_hub row in its epoch number matrix.

    A syncreplica –export command updates epoch numbers immediately. It does not wait for acknowledgment from the importing replica that the packet has been received and applied correctly. During normal MultiSite processing, manual intervention is not necessary to maintain the accuracy of the epoch number matrices for the various replicas. However, failure to apply a packet may require manual intervention., as described in Lost update packet

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: 
multitool lsepoch 
For VOB replica "/vobs/dev":
Oplog IDs for row "boston_hub" (@ minuteman):
 oid:87f6265f.72d911d4.a5cd.00:01:80:c0:4b:e7=950    (boston_hub) 
 oid:0eaa6fc3.737d11d4.adbe.00:01:80:c0:4b:e7=504    (sanfran_hub) 
Oplog IDs for row "sanfran_hub" (@ goldengate):
 oid:87f6265f.72d911d4.a5cd.00:01:80:c0:4b:e7=912    (boston_hub) 
 oid:0eaa6fc3.737d11d4.adbe.00:01:80:c0:4b:e7=504    (sanfran_hub) 

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: 
    multitool lsepoch 
For VOB replica "/vobs/dev":
Oplog IDs for row "boston_hub" (@ minuteman):
 oid:87f6265f.72d911d4.a5cd.00:01:80:c0:4b:e7=950    (boston_hub) 
 oid:0eaa6fc3.737d11d4.adbe.00:01:80:c0:4b:e7=504    (sanfran_hub) 
Oplog IDs for row "sanfran_hub" (@ goldengate):
 oid:87f6265f.72d911d4.a5cd.00:01:80:c0:4b:e7=950    (boston_hub) 
 oid:0eaa6fc3.737d11d4.adbe.00:01:80:c0:4b:e7=504    (sanfran_hub) 

    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