Global Server Load Balancing manages traffic across multiple network entry points or data centres. For example, GSLB would distribute traffic across two or more data centres (active-active) or initiate automated data centre failover in a DR scenario (active-passive).
GSLB can also direct traffic based on geo-location information, providing the best user experience. For example, if I’m in Sydney connecting to my desktop located in Melbourne, I can provide a better user experience by directing my connection via a Sydney NetScaler and across my managed and high-speed network between my data centres.
Connecting directly to Melbourne would mean my connection traverses long distances over unmanaged networks – lowering my chances of a good experience.
How does GSLB work?
GSLB uses a combination of authoritative DNS (ADNS) and the status of the underlying services being monitored and load balanced in the data centre. The DNS response given is dictated by the state of the services in the data centre and how the GSLB is configured to react when those services are in a sub-optimal state.
A GSLB active-passive load balancing example
Access to a mobility service for Office 365 and DaaS is accessed by a business using https://myapps.mybusiness.com.au. The ADNS record for myapps.mybusiness.com.au is hosted on a pair of NetScalers (ns1.mybusiness.com.au and ns2.mybusiness.com.au), one in data centre A and one in data centre B.
The two NetScalers are configured to use GSLB and have a heartbeat between them for sharing the status of each data centre and its' related back-end services.
Data centre A is considered production and data centre B is for disaster recovery. The standard ADNS response for https://myapps.mybusiness.com.au is the IP address of data centre A (production).
A standard NetScaler Layer 4-7 Load Balancer in data centre A is load balancing two web servers responsible for facilitating authentication to the platform. The load balancer status is 100% up – i.e. both web servers are operational.
Data centre B has an identical setup with two web authentication servers and the backend services and is also fully operational.
Without notice, the web servers in data centre A experience a network issue preventing them from accessing their dependent back-end services and the NetScaler L4-7 Load Balancing probes establish this issue and determine the load balancer is in a down state – 0% up.
The GSLB services on the NetScalers detect the local Load Balancer in data centre A is now in a down state.
An ANDS request is received for https://myapps.mybusiness.com.au and the GSLB ADNS server responds with the IP address for data centre B and services continue to operate without intervention.
It’s important to remember that GSLB facilitates a ‘smart’ DNS response based on pre-configured and comprehensive criteria, which is then used by the recipient to direct traffic - GSLB does not route traffic.
I deployed a GSLB service for an Australian bank in 2015 that included multiple layers of NetScalers all responsible for load balancing services in areas of the deeper infrastructure. I used a pair of NetScalers for GSLB in each data centre to probe the NetScalers responsible for load balancing back-end services in the respective data centre. These NetScaler were also configured to probe other services that were not being load balanced at all.
The state of the probes, i.e. up or down state, was used to dictate if the site was operational, including if services deep in the architecture that weren’t being load balanced were fully operational.
Essentially, even if all primary layers of the service were operational but one small dependent service (that wasn’t being load balanced was offline, I could stipulate that GSLB failed over part of, or the entire service.
As you can see, the potential for GSLB is huge. It can be used to help with traffic management for whole services or individual components of services and can be used for external and internal traffic.
The next blog covers Content Switching that has some excellent potential use-cases…
By Darren Bennett, Partner Enablement Specialist (Citrix), rhipe