IBM SC34-7012-01 manual Tier 5 - two-site, two-phase commit, Tier 6 - minimal to zero data loss

vCost is higher than for the tier 1 to 3 solutions, because you need dedicated hardware, software, and communication links.

Tier 5 - two-site, two-phase commit

A tier 5 solution is appropriate for a custom-designed recovery plan with special applications. Because these applications must be designed to use this solution, it cannot be implemented at most CICS sites.

Figure 23 summarizes the tier 5 solution.

Site One Site Two

VTAM

3745 3745

Channel extender

ESCON

39903990

Approach	Recovery
Shadow data synchronized	Recovery time in minutes
by remote two phase commit	Only inflight data lost
Reconnect network

Figure 23. Disaster recovery tier 5: two site, two-phase commit

Tier 5, remote two-phase commit, is an application-based solution to provide high currency of data at a remote site. This requires partially or fully dedicated hardware at the remote site to keep the vital data in image format and to perform the two-phase commit. The vital data at the remote site and the primary site is updated or backed out as a single unit of work (UOW). This ensures that the only vital data lost would be from transactions that are in process when the disaster occurs.

Other data required to run your vital application has to be sent to the secondary site as well. For example, current load libraries and documentation has to be kept up-to-date on the secondary site.

The benefits of tier 5 are fast recovery using vital data that is current. The drawbacks are:

vThe cost of maintaining and running two sites.

vThe solution is application program dependent. You must write your applications so that they write data both locally and remotely, by transmitting data to a partner application at the remote site that also writes the data.

vThis solution increases the time transactions take to respond. Your application program waits every time it needs to transmit data to the remote site.

Tier 6 - minimal to zero data loss

Tier 6 provides a very complete level of disaster recovery. You must assess whether the cost of achieving this level of disaster recovery is justified for your company.

Chapter 19. Disaster recovery 231