FINAL WORD
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APART FROM THE DIRECT COST OF DOWNTIME , WE ALSO SEE BUSINESS CONTINUITY , IN TERMS OF REPUTATION AND DATA LOSS , AS ANOTHER FACTOR ENCOURAGING BUSINESSES TO ENSURE HIGH AVAILABILITY IS IMPLEMENTED .
your ADC provides high availability for the applications whose traffic it manages . In the event that a server fails , the ADC can reroute traffic to another available server in the cluster . Key features that enable this function include :
Load balancing methods : There are several methods that can be used for server selection , including round robin , least connections , weighted round robin , weighted least connections , fastest response and more . Your ADC should offer all these options to allow the most suitable configuration for your environment and priorities .
Health monitoring : To ensure rapid failover with little or no downtime , server health should be continuously assessed based on a number of indicators , including :
While ensuring fault tolerance for servers is obviously critical , a high availability architecture must also consider the load balancing layer itself . If this becomes unable to perform its function effectively , the servers below run the risk of overflow , potentially compromising their own health as well as application performance and application availability . This makes redundancy just as important for the load balancer or ADC as for any other component in the data centre .
As with a high availability server cluster , there are several ways in which load balancers or ADCs can be deployed to provide high availability , including :
Active-standby : The most common configuration , the active-standby model includes a fully redundant instance of each ADC which is brought online only in the event that its primary node fails . Each active ADC can be configured differently , though each active-standby pair will share the same configuration .
Active-active : In this model , multiple similarly configured ADCs are deployed for routine use .
In the event that one node fails , its traffic is taken over by one or more of the remaining nodes and load balanced as needed to ensure consistent service . This approach assumes that there will be sufficient capacity available across the cluster for it to function even when one ADC is unavailable .
N + 1 : Providing redundancy at a lower cost than active-standby , an N + 1 configuration includes one or more extra ADCs that can be brought online in the event that any of the primary ADCs fails .
In each case , rapid failover enables fault tolerance and Disaster Recovery for the load balancing function so that application performance and application availability are not affected by the failure . Failover and traffic management is typically managed through a version of the Virtual Router Redundancy Protocol redundancy standard .
Key high availability features for load balancing or ADC
In addition to ensuring high availability for your ADC , you should also make sure that
• Time series of total bytes in and out from each server
• Time series of traffic rates ( in Mbps ) in and out from each server
• Percent of error traffic over range
• Number of good SSL connections
• Average application server latency by service
• Client-side latency SRTT , max , min , and average as a time series
• Custom health checks such as measuring the response time for specific SQL database queries
Why this is so critical ?
As enterprises become further dependent on the Internet to get business done , the threat of downtime can become a competitive disadvantage . Direct financial losses are substantial and a primary reason why businesses need to establish a high availability solution .
Apart from the direct cost of downtime , we also see Business Continuity , in terms of reputation and data loss , as another factor encouraging businesses to ensure high availability is implemented . Firstly , reputation will improve as the business and brand is known for its reliability versus its competitors . Secondly , reducing risk of data loss is essential as due to the severe penalties incurred under the terms of the GDPR . A highly available infrastructure also mitigates the negative impact of outages to revenue and productivity . •
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