Availability Guide for Application Design

Data Protection and Recovery
Availability Guide for Application Design—525637-004
4-21
Transaction Playback
You can use queue files to help perform transaction playback to keep such an
application available. The simplified application shown in Figure 4-6 on page 4-21
shows this concept. Here, the point-of-sale devices use the application to check credit
card references and debit the account. By accepting a small reduction in the speed of
the provided function to use the queue file, the application can remain available even if
the server system becomes unavailable.
The queue file is reliable because requests can be enqueued with transaction
protection.
Note that the order in which credit checks are performed is considered to be
unimportant. This technique does not work if the sequence of requests matters.
Figure 4-6. Queue Files and Transaction Playback
Customer
Data
Audit Trail
Server
Queue File
Audit Trail
Local Data
Point-of-Sale
Device
Point-of-Sale
Device
Point-of-Sale
Device
Remote
Server
System
Local
Switch
Client
Dequeue
Normal
Information
Flow
Alternate
Information
Flow
VST306.vdd