Understanding load balancing on the Global Traffic Manager
When the Global Traffic Manager receives a name resolution request, the system employs a load balancing mode to determine the best available virtual server. Once the Global Traffic Manager identifies the virtual server, it constructs a DNS answer and sends that answer back to the requesting client's local DNS server. The DNS answer, or resource record, can be either an A record that contains the IP address of the virtual server, or a CNAME record that contains the canonical name for a DNS zone.
Within the Global Traffic Manager, you have two categories of load balancing modes from which to select: static and dynamic. A static load balancing mode selects a virtual server based on a pre-defined pattern. A dynamic load balancing mode selects a virtual server based on current performance metrics.
The Global Traffic Manager provides tiered load balancing system. A tiered load balancing system is a load balancing system that occurs at more than one point during the resolution process. The tiers within the Global Traffic Manager are as follows:
- Wide IP-level load balancingWide IPs that contain two or more pools use a load balancing mode first to select a pool. Once the Global Traffic Manager selects a pool, the system then uses pool-level load balancing mode to choose a virtual server within the selected pool. If the Global Traffic Manager does not choose a virtual server in the first pool, it applies the load balancing mode to the next pool, either until it selects the best virtual server to respond to the request, or all the pools are tried.
- Pool-level load balancingA pool contains one or more virtual servers. After the Global Traffic Manager uses wide IP-level load balancing to select the best available pool, it uses a pool-level load balancing to select a virtual server within that pool. If the first virtual server within the pool is unavailable, the Global Traffic Manager selects the next best virtual server based on the load balancing mode assigned to that pool.
For each pool that you manage, the Global Traffic Manager supports three types of load balancing methods: preferred, alternate, and fallback. The preferred load balancing method is the load balancing mode that the system will attempt to use first. If the preferred method fails to provide a valid resource, the system uses the alternate load balancing method. Should the alternate load balancing method also fail to provide a valid resource, the system uses the fallback method.
One of the key differences between the alternate methods and the other two load balancing methods is that only static load balancing modes are available from the alternate load balancing list. This limitation exists because dynamic load balancing modes, by definition, rely on metrics collected from different resources. If the preferred load balancing mode does not return a valid resource, it is highly likely that the Global Traffic Manager was unable to acquire the proper metrics to perform the load balancing operation. By limiting the alternate load balancing options to static methods only, the Global Traffic Manager can better ensure that, should the preferred method prove unsuccessful, the alternate method will return a valid result.