Primary, Secondary

In many high-availability and replication systems—such as DRBD—the roles "Primary" and "Secondary" designate the active and standby statuses of the participating nodes:

• Primary Node:

  • Active Role: The Primary node is the one actively handling I/O operations (reads and writes).

  • Data Updates: It is responsible for processing and committing changes, which are then replicated to the Secondary node.

  • Critical in Synchronous Replication: In synchronous setups, the Primary waits for confirmation that the Secondary has received the update before the write is considered complete, ensuring data consistency.

• Secondary Node:

  • Standby Role: The Secondary node remains in a passive or read-only state, continuously receiving and applying updates from the Primary node.

  • Data Backup: Its main purpose is to serve as a backup, maintaining an up-to-date copy of the data.

  • Failover Candidate: If the Primary node fails or is taken offline, the Secondary can be promoted to Primary, ensuring continued availability.

Example in DRBD

• In a typical two-node DRBD configuration, one node is designated as Primary (active, handling writes) while the other is Secondary (replicating data).

• Some setups (with a cluster file system) may allow a dual-primary configuration where both nodes operate in read-write mode, but this is less common and requires special coordination.

Key Benefits

• High Availability: By replicating data in real time from the Primary to the Secondary, the system ensures that if the Primary node fails, the Secondary can take over with minimal downtime.

• Data Consistency: Synchronous replication helps guarantee that both nodes have consistent data, which is crucial for applications requiring strong data integrity.

• Fault Tolerance: The presence of a Secondary node provides a safety net against hardware failures, network issues, or other disruptions affecting the Primary node.

In summary, the Primary node is the workhorse of a replicated system, actively managing and updating data, while the Secondary node is its mirror, ready to take over if needed. This division of roles is essential for maintaining continuous service availability and ensuring data durability in high-availability environments.