Sybase 12.4.2 - Manual (Page 311)

CHAPTER 8 Transactions and Versioning
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Concurrency and IQ Multiplex
IQ Multiplex extends Adaptive Server IQ to allow concurrent processing of
read transactions on multiple Adaptive Server IQ servers. IQ Multiplex extends
snapshot versioning to maintain the consistency of the database across multiple
servers. For more information about using IQ Multiplex, see Adaptive Server
IQ Multiplex User’s Guide.
Concurrency for backups
Adaptive Server IQ also uses transaction processing and snapshot versioning
to allow you to back up your database concurrently with read and write
operations. Restore operations, however, require exclusive access, because
they write to the database. See Chapter 11, “Backup and Data Recovery” for
more information on concurrency issues for backup and restore operations.
Why concurrency benefits you
A data warehouse is a common repository of information shared by a large
number of people. These people may need frequent access to the information.
To avoid impeding their work, the database server must be able to process
many transactions at the same time.
Moreover, many sites also require frequent updates to the database. In high
availability sites, the DBA cannot postpone insertions and deletions to a time
when exclusive access is possible. Similarly, it is important to be able to back
up the database on a regular basis, without disrupting the activities of other
users.
Adaptive Server IQ's approach to concurrency gives query users immediate
access to information, and allows you to ensure the safety and accuracy of the
information they receive.
Introduction to versioning
Adaptive Server IQ uses snapshot versioning to allow transactions to operate
concurrently.
You can think of snapshot versioning as you would a snapshot you take with a
camera. When you photograph a snapshot of an object or scene, you get an
image of it as it appears at a given moment in time. Likewise, when IQ takes a
snapshot of an object in your database, it retains an image of that object at a
given instant in time.