How GSLB Works
An conceptual overview of how GSLB works.
Summary
GSLB directs DNS requests to the best-performing GSLB site in a distributed Internet environment. The Citrix NetScaler implementation of GSLB is DNS-based.
GSLB enables distribution of traffic across multiple sites, manages disaster recovery, and ensures that applications are consistently accessible.
When a client sends a DNS request, the system determines the best-performing site and returns its IP to the client. In the process of ascertaining the best- performing site, the system performs these intelligent decisions:
- Directs client requests to the geographically closest GSLB site (geographic and network proximity-based traffic redirection)
- Directs client requests to surviving data centers when an outage occurs
- Directs client requests to alternate data centers, when a pre-defined traffic load limit is reached
- Directs client requests to be distributed among multiple data centers (assigns each user to the GSLB site with lowest latency)
The system performs these intelligent decisions using the Metric Exchange Protocol (MEP), GSLB policies, and GSLB methods supported by the system.
GSLB methods are algorithms that control how the system load-balances client requests across distributed data centers. The system provides support for creating policies for distributing or redirecting client request. GSLB policies direct the traffic to a pre-defined target site.
Multiple sites exchange metrics with each other using the Metric Exchange Protocol (MEP). The system uses this protocol to exchange load, network, and persistence information between GSLB sites. The system also uses this information to perform load balancing between GSLB sites.
A typical GSLB deployment contains the entities described in the following figure. 
GSLB Entity Model
To configure GSLB, you must configure a GSLB site. As shown in the figure, a GSLB site is the logical collection of GSLB vserver, GSLB service, LB vserver, service, domain, and ADNS service. It is the central entity in a GSLB deployment, and is represented by a name and an IP address.
To create a GSLB site, you must configure load balancing on the system. You must create GSLB vservers and GSLB services for each site. You must bind GSLB services to GSLB vservers. You must then create an ADNS service that provides the IP address of the best performing site to the client's request.
A GSLB vserver is an entity that performs load balancing for the domains bound to it by returning the IP address of the best GSLB service. A GSLB service is a representation of the load balancing/content switching vserver. An LB vserver load balances incoming traffic by identifying the best server, then directs traffic to the corresponding service. It can also load-balance external DNS name servers. Services are entities that represent the servers. The domain is the domain name for which the system is the authoritative DNS server. By creating an ADNS service, the system can be configured as an authoritative DNS server.
Comments (10)
Apr 29, 2008
Anonymous says:
Missing an image....Missing an image....
Apr 29, 2008
Anonymous says:
Also, this is pretty high-level. It doesn't really say how GSLB works.Also, this is pretty high-level. It doesn't really say how GSLB works.
Apr 30, 2008
Vishal Ganeriwala says:
Thanks for pointing out the missing image. I will coordinate with NetScaler team...Thanks for pointing out the missing image. I will coordinate with NetScaler team to update this page.
Vishal
Apr 30, 2008
Gaurav Joshua Vaz says:
Has been fixed! will replace this with a better image soon.Has been fixed! will replace this with a better image soon.
Apr 30, 2008
Megha Shaseendran says:
Changed the image.Changed the image.
May 07, 2008
Anonymous says:
Here's a question, I understand the whole TTL for a DNS record out there on DNS ...Here's a question, I understand the whole TTL for a DNS record out there on DNS servers etc, but what about browser caches, eg IE, NetScape.
My understanding of this is IE will cache a record (outside of the host machine for 30mins) or until the browser is restarted. Check out the following link for more information. I'd be interested to hear comments on how GSLB within the NetScaler may or may not work around this. As the article is a bit older...http://www.tenereillo.com/GSLBPageOfShame.htm
Jun 11, 2008
Anonymous says:
Even I was reading about failure of DNS based GSLB techniques on the same webpag...Even I was reading about failure of DNS based GSLB techniques on the same webpage
Jun 13, 2008
Craig Ellrod says:
The NetScaler has a "Multiple IP Response" option in it's GSLB configuration. W...The NetScaler has a "Multiple IP Response" option in it's GSLB configuration. With the Multiple IP Response option, when the DNS request comes to a GSLB domain, and the GSLB VIP is up or it has a back-up GSLB VIP that is up, then the best service is selected. If MIR is enabled, the system puts the best service as the first record in the response and then puts the rest of the active services as subsequent records.
Jul 18, 2008
Surya ARBY says:
what about client's DNS servers making changes about MIR ? In fact I hea...what about client's DNS servers making changes about MIR ?
In fact I heard that BIND and all other popular DNS servers changes the order of the multiples IP addresses sent in DNS responses. So this is not the best service which is received first by some clients, as I understand, with MIR, many DNS servers use round-robin to distribute the multiple IP addresses seen in DNS answers sent by NetScaler.
Jun 19, 2008
Anonymous says:
Also be aware that, although most modern software understands MIR, some le...Also be aware that, although most modern software understands MIR, some legacy apps may not. As an example try to use this command on Linux and Windows and note the difference "nslookup www.google.com"
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