Databases Reference
In-Depth Information
What Is High Availability?
Before we can begin a discussion of how to ensure a high level or maximum level of
availability, you need to understand the exact meaning of the term
availability
.
Availability can mean different things for different organizations. For this discussion,
we'll consider a system to be available when it is
up
(meaning that the Oracle database
can be accessed by users) and
working
(meaning that the database is delivering the
expected functionality to business users at the expected performance). In other words,
the system is exceeding service level agreements (SLAs).
When the database is unavailable, we refer to that as
downtime
. Of course, the goal is
to eliminate unplanned downtime caused by server, storage, network, and/or software
failures, and human error, a common source of problems. You will also want to minimize
planned downtime during system and Oracle database changes, structural data changes,
and application changes.
Most businesses depend on data being available when needed to make important deci‐
sions. Accessibility to data via web-based solutions means that database failures can
have a broad impact. Failures of systems accessed by a wider community outside of
company boundaries are, unfortunately, immediately and widely visible and can seri‐
ously impact a company's financial health and image. Consider a web-based customer
service application provided by a package shipping company that enables customers to
perform package tracking. As these customers come to depend on such service, any
interruption in that service could cause these same customers to move to competitors.
Taking this a step further, consider complexities in accessing data that resides in multiple
systems. Integrating multiple systems can increase chances of failures and could cause
access to an entire supply chain to be unavailable.
To implement Oracle databases that are highly available, you must design an infra‐
structure that can mitigate unplanned and planned downtime. You must embrace tech‐
niques that enable rapid recovery from disasters, such as implementing appropriate
backup solutions. The goal of all of this is to ensure business continuity during situations
that range from routine maintenance to dire circumstances. In some organizations, the
requirements are 24/7/365. In others, less stringent business requirements might drive
different solutions.
Measuring High Availability
To provide some perspective on what might be described as high availability, consider
Table 11-1
, which translates the percentage of availability into days, minutes, and hours
of annual downtime based on a 365-day year.
