When (not) to Use Greenlight

Greenlight is a global server load balancer (GSLB). It uses DNS to steer clients of an application to multiple servers running that application.

GSLB (either Greenlight or an alternative) can be a helpful tool to improve the availability of your applications. It’s worth considering using GSLB with your application if:

• You have multiple application servers that can function independently
  • One server can go down and the other(s) can keep running
  • Clients don’t encounter problems when their traffic shifts from Server A to Server B (e.g. no sticky sessions)
• Servers are not all in the same location
  • GSLB will still work, but local load balancers are probably a better fit (e.g. nginx or HAProxy)¹
  • Some environments will benefit from using both local load balancers and GSLB
• The limitations of DNS aren’t a problem

Footnotes

1. Local load balancers generally work by transparently guiding traffic to a number of backend servers without any involvement of the client; clients will always connect to the load balancer’s IP address. GSLB on the other hand uses DNS to steer clients to different IP addresses for each server. That means that if you want to steer traffic on the public internet (for example, to only use a backup internet connection during failover), GSLB can make sense even if the servers are in the same physical location. ↩