Load Balancing Methods for DNS Traffic Control

You can define the following load balancing methods:

Based on the load balancing method defined for an LBDN, the DNS Traffic Control selects an available pool. Based on the method selected for a pool, it selects an available server.

The following is a description of the load balancing methods with examples for pools or LBDNs:

• Using the All Available method, the appliance responds to the query with all the available servers in the DTC pool for the appropriate record type. The responses are returned in the same order in which the servers are listed in the DTC pool, eliminating the unavailable servers.
  Consider the following example for all available records load balancing method with an LBDN x.abc.com:

  Pool= Pool_1; load balancing method= all available records; health check= HTTPS
  10.10.1.1; Availability = up
  2001:db8:a22:a00::29; Availability = up
  10.10.2.2; Availability = down
  2001:db8:a22:a00::32; Availability = up
  10.10.3.3; Availability = up

• Using the Global Availability method, the appliance creates the response to the query, so that the clients are directed to the first available pool and server.
  Consider the following example for global availability load balancing method with an LBDN x.abc.com:

  • Pool= Pool_1; load balancing method= global availability; health check= HTTPS 10.10.1.1
    10.10.2.2
    10.10.3.3

  • Pool= Pool_2; load balancing method = global availability; health check = HTTPS 10.10.4.4
    10.10.5.5
    10.10.5.5

• Using the Source IP Hash method, NIOS matches an IP address from an incoming query with the health statuses of pools and servers to address the responses by the client. When multiple pools or servers are configured, NIOS uses the source IP hash load balancer method, a load-balancing pattern in which requests are distributed based on the hash value of an IP address from an incoming query and the health status of the pool or server. With the source IP hash load balancing method, clients have their own pool or server and are always associated with the same pool or server for the same query as long as the pool or server is green; if the health status of the pool or server turns red, NIOS switches the clients to the working pool or server and switches back when the health restores to green.
  
  

  Consider the following example for the source IP hash load balancing method. The DNS Traffic Control pool has two servers and uses the source IP hash load balancer. When the health statuses are green for both the servers, depending on the client IP address and DNS Traffic Control health status, the pool returns the DNS record from the first DNS Traffic Control server for the first client and returns the DNS record from the second DNS Traffic Control Server for the second client. This continues as long as the health statuses are green; if the health status turns red for a DNS Traffic Control server, the pool switches the client to the working server and switches it back when the health status restores to green.
  
  

  For information on the Limitations and Warnings of Source IP Hash Load Balancing Method, see the following section, Limitations of Source IP Hash Load Balancing Method.

• Using the Ratio: Fixed method, NIOS adjusts the response to the query so that the clients are directed to servers in a pool or among pools. When multiple pools or servers are configured, the appliance uses the weighted round robin method, a load-balancing pattern in which requests are distributed among several pools or servers based on a weight assigned to each pool or server. Note that the system considers the weight of available servers only. Consider the following example for ratio load balancing method with an LBDN x.abc.com, load balancing method = Ratio and two linked pools: Pool_1 with weight = 70 and Pool_2 with weight = 30.

  • Pool = Pool_1; load balancing method = ratio; health check = HTTPS 10.10.1.1; Weight = 50

    10.10.2.2; Weight = 2

    10.10.3.3; Weight = 25

  • Pool = Pool_2; load balancing method = ratio; health check = HTTPS 10.10.4.4; Weight = 50

    10.10.5.5; Weight = 25

    10.10.5.5; Weight = 25

• Using the Ratio: Dynamic method, the appliance weights the DTC servers dynamically based on round trip delay or SNMP health monitor data. You can use one of the following options:

  • Round trip delay: Based on the round trip delay from the DTC member that received a client’s DNS request, the system sends clients to the server with the minimal latency time, i.e. the closest one. You need a pre-configured health monitor for this load balancing method.
    For example:

    • Server A latency = 25 ms

    • Server C latency = 18 ms

    • Server D latency = 50 ms
      In this case, the traffic distribution is as follows:

    • Server A = 0%

    • Server C= 100%

    • Server D = 0%

• SNMP: Based on data from the SNMP monitor associated to the server, for example, CPU or memory utilization, the system sends clients to the server with the lowest load. For this load balancing method, you need a pre-configured SNMP health monitor with a required metric to be tracked. The metric is set through an object identifier (OID) in the monitor properties. This method supports only OIDs for which the server can return an integer value.
  The value of the monitored metric defines how the traffic is directed. By default, the servers with the highest metric values receive the client requests. There may be cases when your selected metric reflects server availability in the opposite way, that is, the lowest metric values indicate available servers. For such cases, you can invert the value of the OID, that is, of the monitored metric, and have the traffic directed to the lowest-rated servers.
  You can select to weight servers by either priority or ratio. In case of priority, traffic is directed towards the servers that report the best metric values, other servers being bypassed. In case of ratio, traffic is distributed across all servers based on the values of the monitored metric for each of them. If a health check for a server is failed, the server is excluded from the load balancing.
  Consider the following example where the CPU utilization metric is used for server monitoring:

  • Server A CPU utilization = 90%

  • Server C CPU utilization = 50%

  • Server D CPU utilization = 10%
    With normal dynamic weights, the distribution is as follows:

  • Server A 60% (calculated as 90 / (10 + 50 +90) = 90/150 = 0.6)

  • Server C 33% (calculated as 50 / (10 + 50 +90) = 50/150 ~ 0.33)

  • Server D 7% (calculated as 10 / (10 + 50 +90) = 10/150 ~ 0.066)
    This means that the most loaded server will receive most requests than the less loaded one. For this case, the metric should be inverted to reflect server availability appropriately:

  • Server A 8% (calculated as 1/90 / (1/90 + 1/50 + 1/10) = 0.011/0.131 ~ 0.08)

  • Server C 15% (calculated as /50 / (1/90 + 1/50 + 1/10) = 0.02/0.131 ~ 0.15)

  • Server D 77% (calculated as 1/10 / (1/90 + 1/50 + 1/10) = 0.1/0.131 ~ 0.77)

• Using the Round Robin method, the appliance returns servers sequentially and cyclically. Consider the following example for round robin load balancing method with an LBDN x.abc.com:

  Pool = Pool_1; load balancing method = Round Robin; health check = HTTPS 10.10.1.1;
  10.10.2.2;
  10.10.3.3;

• Using the Topology method, the appliance applies a predefined geographic mapping method and evaluates user-defined geography, subnet, or extensible attribute rules sequentially. Geographical locations for the geography rules are provided through an external topology database. The appliance supports the MaxMind GeoIP2 City or Country database and the MaxMind GeoLite2 City or Country database. For more information, see the following section, Configuring Topology Rules and Rulesets.

Limitations of Source IP Hash Load Balancing Method

• The source IP hash load balancing method does not balance clients evenly between DNS Traffic Control pools and DNS Traffic Control servers because the load is divided based on the IP addresses of the clients and the health statuses of the DNS Traffic Control servers.

• For the source IP hash load balancing method with the Auto Consolidated Monitor option enabled, when the health status of a DNS Traffic Control server changes and simultaneously two DNS Traffic Control Grid members receive the same DNS requests from a single client, the response from the Grid members may vary as the DNS Traffic Control consolidated monitors need a little time to share health results across DNS Traffic Control Grid members.