About Change Sequence Numbers (CSNs)

Replication is sequential, meaning that entries are replicated in an orderly way. Because replication is orderly, any change generated by a master is labeled by a change sequence number (CSN) that is unique for any change inside a multi-master topology. The CSN is a hexadecimal that appears in the logs as follows:

41e6ee93000e00640000

The first 8 hexa-digits represent the time when the change was generated in the master. The time is represented in seconds since January 1, 1970.

The next four digits are the sequence number, or the order in the current second in which the change occurred. For example, multiple changes occur in second 41e6ee93. The sequence number tells us at which point in this second the change occurred.

The next four digits specify the replica ID of the master that received the change in the first place.

The last four digits are always 0000.

CSNs are generated only when local traffic introduces a new change to a replica. So only masters that receive updates generate CSNs. Consumers always refer to masters, because all the updates they receive are through replication.

During troubleshooting, try to find the CSN or CSNs that are at the origin of the delay. To find the CSN associated with the delay, you need to use replica update vectors (RUVs). RUVs are described in the next section.