The term latency refers to the time difference between when a transaction is completed at the primary database and when it is completed at the replicate database. A heartbeat is used to establish a data flow between the primary and replicate databases to measure system latency. In a replication system that already has data movement between primary and replicate databases, a heartbeat is not required to measure latency.
You can start heartbeats and latency graphs from the Sybase Central desktop (RSM Client) by right-clicking a connection listed under a Replication Server, or by right-clicking a database entry listed under a data server and selecting Heartbeats or Latency.
The heartbeat functionality in RSM Server consists of creating a table in the primary database, defining a replication definition with customized function strings on the table, and creating a subscription to the replication definition. Once the subscription is marked as valid, RSM Server populates the table and updates it according to the heartbeat interval specified (every 60 seconds, by default).
Each time the table is updated, data is replicated to the replicate database. The function strings defined on the replication definition change all inserts, updates, and deletes into commits, which become null operations on the replicate database. Since no data is actually inserted on the replicate there is no need for a replicate heartbeat table.
Latency is calculated by reading the rs_lastcommit table on the replicate database. Although a heartbeat does not insert data into a replicate database, it does cause the rs_lastcommit table to be updated, allowing latency from the primary database to the replicate database to be determined.
You can modify, stop, or remove an RSM Server heartbeat process after it is started. To modify the heartbeat, change the Heartbeat interval and select Apply.
The Stop button instructs RSM Server to delete the appropriate row in the heartbeat table, and stop the heartbeat thread that updates the table. Remove performs the same action as Stop, but it also removes the replication definition and subscription that were created for the heartbeat table. The table itself is not deleted.
