What is High Availability

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What is High Availability?
When do you need it? How do you create it ? How
do you measure it?
Lee Hampton-Whitehead
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What is High Availability?

• High availability is a system design protocol
  and associated implementation that ensures a
  certain absolute degree of operational continuity
  during a given measurement period.
• Source http//en.wikipedia.org/wiki/High_availabil
  ity

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What is High Availability?

• Minimum service / application outages
• Uptime is not the same as availability
• Planned outage considerations
• Quality of Service

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When do you need it?

• Any system failure that may impact business
  function is a candidate for a HA solution
• Internal Own Use systems (Email or Resource
  management)
• Customer Advisor (Helpdesk or CRM)
• External Customer Facing (Online Shopping or
  banking)

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When do you need it?

• The cost of failure
• Financial
• Loss of orders
• Share Price
• Contractual penalty clauses
• Brand Value
• Customer satisfaction and loyalty
• Other considerations
• Regulatory requirements
• Safety Implications

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When do you need it?

• Business impact analysis is required to assess
• The cost of service outage
• Total Cost of Ownership (TCO)
• Return on Investment (ROI)
• Budget spend

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When do you need it?

• Risk analysis factors
• Time to recovery
• Data impact of outage or recovery
• Associated hardware software and resource costs
• Probability of event
• Implications on the business
• Complexity risks
• Third-party solutions
• Pros Cons

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Design Sizing Criteria

• Number of transactions
• Size of transactions
• Secondary load considerations

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Service Level Agreement

• Agreed availability criteria
• Measurement techniques
• Penalty clauses
• Capacity, Performance and usage
• Help desk support
• Contingency
• Costing

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Costs

• The cost implications of most availability
  solutions include, but are not limited to, the
  following
• Hardware
• Software
• Network infrastructure
• Training
• Serviceability
• Operational costs

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Costs
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Redundant Infrastructure
How do you create it ?

• Effective removal of all SPOF (Single Point Of
  Failure)
• Power
• Network (including LAN, WAN, DNS etc)
• Server hardware
• Storage and associated interconnect
• Air conditioning

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How do you create it ?
Web Servers
Corporate Network
Users
Load Balancers
SQL Servers
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Load Balancing
How do you create it ?

• Typically used for WEB based applications
• User requests are distributed across a farm of
  servers
• The Load Balancing technology will redistribute
  the application traffic in the event of a server
  failure
• All servers / hardware actively participate in
  delivering the application

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Failover Cluster
How do you create it ?

• Typically used for the database components of an
  application
• Can be used for File Serving, Print Serving and
  Storage solutions where a load balanced may not
  be suitable
• A Typical failover cluster deployment
  (Active-Passive) may result in under utilised
  hardware

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Data Replication / Mirroring
How do you create it ?

• Data is central to any business application
• Log Shipping
• Transactional replication
• Database mirroring

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How do you create it ?
Corporate Infrastructure

• Basic Requirements
• IP Routed Network
• DHCP
• DNS
• AD

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Proactive System Monitoring
How do you create it ?

• Indentify and correct minor / non service
  affecting problems before they become major
  outages
• Baseline Server utilisation values
• Application / Service monitoring should be
  representative of typical user activities
• HA Testing

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Application Architecture
How do you create it ?

• Poorly written code can destabilise any HA
  implementation
• The Application should be able cope with the loss
  of transitory data
• Data Consistency , Use of Transaction

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IT Processes
How do you create it ?

• Restricted / managed system access
• Software development lifecycle
• Infrastructure life cycle
• Effective service wrap

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Unapproved changes
How do you create it ?
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Availability Comparison

• 9536.5 hours/month or 18.25 days/year
• 99.9 (Three nines) 43.8 minutes/month
  or 8.76 hours/year
• 99.99 (Four nines)4.38 minutes/month or
  52.6 minutes/year
• 99.999 (Five nines") 26.2 seconds/month or
  5.26 minutes/year

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How do you measure it?
SLA

• The Service level agreement will define a good,
  poor or failed service in terms
• Service times
• Response times
• Fault resolution times
• The SLA will also define how these values are to
  be measured

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How do you measure it?
Measurement considerations

• Users Perspective
• Monitor every component
• Synthetic transactions
• Simplicity

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Availability Report
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Summary

• Indentify Business requirements
• Risk Assessment
• Decide on appropriate solution
• Indentify and address any SPOF
• Application design is important to ensure high
  availability
• Implement and Follow Procedure
• Consider the user story
• Rigorous testing

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Q A