System Availability and Business Impact

The availability of individual systems in the hierarchy of systems and related services supported by these systems is a key factor in reliable Business Service delivery. As seen in the diagrams in topics 'SLA, OLA and UC Service Classes' and 'Business Impact Analysis and Continuous Business Criticality Improvements', systems and services are functionally inter-related and thus it is necessary to consider the business impact of availability disruptions of both services and systems.

For this reason, availability-related attributes are provided in the 'Business Impact' tab of the System editor:

• Availability

  'Availability' expresses availability of the system, i.e. the likelihood that the given system will be available. System availability is expressed in percentage values. For instance, a system with availability 100% is guaranteed to be always available, a system with availability 50% may ba available only half of the time.

• Availability Calculation Method

  When a system depends on a number of 'incoming' systems, availabilities of individual incoming systems determine the availability of the parent 'outgoing' system. Several methods of determining the resulting availability exist. Attribute 'Availability Calculation Method' specified for the parent system determines which method calculating its availability from incoming systems availabilities will be used. (See below for details.)

• Availability State

  'Availability State' expresses availability in discrete levels with descriptive names ('Running', 'Malfunction', 'Outage'). While 'Availability' (expressed in percentage numbers) is useful for further calculations, 'Availability State' enables easy display and identification of the availability status, which may be especially important in critical situations.

Availability Calculation Method

The following availability calculation methods can be used for a system comprising of two or more incoming systems:

• Best

  Parent system availability equals to the best value of incoming systems availabilities. For instance, if a system comprises of three incoming systems with availabilities 70,80 and 90%, parent system availability will be 90%. This calculation method is likely to be used with incoming systems which can substitute one another.

• Worst

  Parent system availability equals to the worst value of incoming systems availabilities. In the example with incoming availabilities 70,80 and 90%, parent system availability would be 70%. This calculation method is likely to be used with incoming systems with a 'linear' functional structure where the 'weakest link' determines the outcome.

• Weighted

  Parent system availability equals to the weighted average of incoming systems availabilities. For this method to be applicable, weights of individual incoming systems within the parent system must be specified. The weight of an incoming system expresses how crucial the incoming system is for the parent system availability. The weight is determined by attribute 'Aggregation Weight' of the system-incoming system linking object. This linking object can be opened for editing by calling the 'Assign Option' action in the 'Incoming Systems' reference catalog.

  For instance: A parent system 'X' comprises of three incoming systems 'A', 'B' and 'C':

  'A' - Availability = 70%, Weight = 10%

  'B' - Availability = 80%, Weight = 20%

  'C' - Availability = 90%, Weight = 70%

  If 'Weighted' availability calculation method is used, then overall availability of the parent system 'X' is:

  AvX = (0.10)(70) + (0.20)(80) + (0.70)(90) = 86

The availability of a parent system resulting from a multi-tier structure of incoming systems will be the result of availabilities gradually calculated in a bottom-to-top direction.

Availability Simulation

The 'Availability Simulation' functionality makes it possible to simulate disruptions to a system's availability and evaluate their impact on outgoing (parent) systems. Availability simulation is offered if a system is opened for editing from catalog 'System Availability Simulation'. The System editor contains the following additional fields:

• Availability Simulation

  Use this field to enter the "simulated" availability value. For instance, simulate system outage by entering 'Availability Simulation' = 0

• Availability State Simulation

  The field displays the 'state' equivalent of the simulated value.

Note that while 'Availability' value of a system is calculated from its incoming systems availabilities and thus cannot be entered manually, the 'Availability Simulation' field allows for arbitrary values to be entered.

Using Availability Simulation

1. Open the 'System Availability Simulation' catalog
2. From the catalog, open the system whose impact on other outgoing systems is to be simulated
3. Simulate limited system availability by entering a corresponding value in the 'Availability Simulation' field.
4. Observe value changes in the 'Simulation...' columns of the 'Outgoing Systems' reference catalog at the bottom of the editor.

For more information on business impact analysis, please see topic 'Business Impact Analysis and Continuous Business Criticality Improvements'.