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v Cost 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.
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:
v The cost of maintaining and running two sites.
v The 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.
v This 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.
Approach Recovery
Recovery time in minutes
Only inflight data lost
Site TwoSite One
VTAM
Channel extender
37453745
3990
3990
ESCON
Shadow data synchronized
by remote two phase commit
Reconnect network
Figure 23. Disaster recovery tier 5: two site, two-phase commit
Chapter 19. Disaster recovery 231