Overview of Data Replication for VSAM

You can use Data Replication for VSAM to produce copies of your VSAM data sets and maintain current data in near-real time, typically on geographically dispersed sysplexes.

Purpose

IBM® InfoSphere® Data Replication for VSAM for z/OS® addresses your organizational requirements for reliable and available data:

  • High availability and disaster recovery (HADR)
  • Business intelligence
  • Redundancy
  • Data backup
High availability and disaster recovery (HADR)

Data center downtime is a significant interruption that affects productivity, revenue, and trust. For example, a global banking enterprise lost its secondary data centers in the aftermath of a terrorist attack because they were too close to the primary sites. The ATM network was down for days, at incalculable cost to the company and its customers. Extreme weather events, such as hurricanes or other natural disasters, can have similar consequences.

Data Replication for VSAM supports your data availability strategy by helping you to make sure the availability of your data from secondary or backup instances. It offers the following advantages for your HADR solution:

  • No geographic restrictions on the distance between the primary and secondary sites
  • Quicker recovery from VSAM failures compared to hardware-based solutions
Business intelligence

Data Replication for VSAM also supports scenarios that distribute your business intelligence workload to a secondary, read-only platform where analysts can run queries.

Redundancy

Use your synchronized replicas for active processing while you perform scheduled maintenance of your source data sets.

Data backup

Create synchronized backups of mission-critical data in near-real time.

How Data Replication for VSAM works

The illustration shows how Data Replication for VSAM maintains synchronized replicas that undergo rapid rates of change:

Figure 1. Classic data servers capture changes from VSAM log streams and send them immediately to target data sets
An illustration that depicts the basic flow of change data in Data Replication for IMS and Data Replication for VSAM .

The synchronized replicas are maintained in CICS regions.

A Classic source server uses a log reader to read VSAM log streams and retrieve change records that are captured for source VSAM data sets. Capture processing then packages these changes as change messages that describe insert, update, and delete operations on the data. The source server sends the change messages to a target server at a different site that applies the changes to a replica of the source data sets.

Data Replication for VSAM is unidirectional, which means that you send changes from one site to another instead of in both directions at once. You use Data Replication for VSAM with target data sets that you do not update actively so that you can maintain synchronization of your data between the source and target.

Technical capabilities

Data Replication for VSAM can capture changes from one or more VSAM log streams. It supports one VSAM log stream per subscription.

Data Replication for VSAM has the following key components and capabilities:

  • Subscriptions
  • Transactional consistency
  • Replicating historical changes
  • Monitoring and reporting
Subscriptions

You organize the data sets that support a given application by mapping them to their target data sets within a subscription, a unique combination of source data sets, memory caches, and communication paths. Because of its autonomous structure, you can start, stop, and maintain replication for a subscription independently of other subscriptions. Stopping replication for one subscription has no effect on the operations of others.

Transactional consistency

Data Replication for VSAM can manage transaction processing across multiple logical partitions and data sets. A subscription maintains the sequence of transactions as they occur at the source by applying changes to a given record in the correct order.

Replicating historical changes

Planned or unplanned outages can cause target data sets to fall behind current processing and to go out of synchronization. Data Replication for can automatically catch up with unprocessed changes that occurred in the past when replication is restarted.

Data Replication for VSAM maintains bookmark information that specifies where the log reader begins again in the event of an outage. The change data that the source server maintains in caches can reduce the time that it takes to catch up to current processing.

Replication of CICS journal log streams

You can define subscriptions that reference the name of a z/OS log stream that contains journal data created by CICS. Data Replication for VSAM is designed to create a copy of the source z/OS log stream at the log block level at the target.

These journal replication subscriptions do not support parallel apply operations, conflict detection, or transactional semantics. Instead, one of these subscriptions uses the GMT timestamp from the log block header to determine where to restart operations to prevent duplication of log blocks at the target.

Monitoring and reporting

You can review current and accurate metrics in the Classic Data Architect, a supplied graphical user interface. Using this tool, you can measure resource consumption, latency, throughput, and memory usage, enabling you to evaluate the replication process and optimize your environment.