Physical and logical replication

HCL OneDB™ replication technology is broadly classified into physical and logical replication.

With physical replication, logical log records get replicated to secondary server, and secondary server continuously apply logical log records as if server is in crash recovery mode. Physical replication requires database storage, hardware architecture, operating system and HCL OneDB server version to be same.

Logical replication does capture data from logical log records, but log records gets converted to a format understood by SQL engine. Logical replication does support heterogeneous hardware and software. Customers can pick and choose which tables to be replicated, and can replicate only few columns or few rows in a table.

HCL OneDB High Availability Data replication(HDR), Remote Standalone Secondary(RSS) and Shared Disk Secondary(SDS) servers fall under physical replication category.

Enterprise replication(ER) and flexible grid technology fall under logical replication category.

HDR, and SDS technologies support zero data loss (RPO=0) configuration.

RSS supports asynchronous replication across wide area networks and ideal deployment option to protect transactional database from natural disasters. RSS “delayed apply” configuration protects transactional database from human errors.

Shared disk technology(SDS) is ideal fit to scale-out transactional database to support spike in transactional volume during special events.

From CAP theorem perspective, HCL OneDB physical replication technologies support CP configuration. In case of network partitioning, secondary servers cannot accept write activity till new primary server was elected. With physical replication, primary server is the designated write server for transactional data.

HCL OneDB Enterprise Replication technology supports AP configuration. In case of network partitioning, applications can continue performing database write activity at all ER servers and ER rely on conflict resolution rules to resolve conflicting updates. ER technologies supports near-zero downtime maintenance operations like rolling application and schema upgrade, rolling server upgrade, and thus eliminate requirement for planned downtime for maintenance operations.