C H A P T E R 2 | Intel® NetStructure™ 7190 |
Partial Failures
load potential of the servers underneath the load balancers it monitors. Therefore it allows the next best thing. For example, it allows the user to declare that if Server A is within 2400 connections (80% of maximum) of Server B, consider referrals to Server A even if Server B is the lowest. The factor setting would be 2400 for the service associated with Server A and 0 for the service associated with Server B.
Flash DNS
Network topology can greatly affect the overall transfer rate between a site and a client. A solution is to replicate sites on multiple networks (which are sometimes also distributed geographically). Without a 7190, it can be cumbersome to provide users with a list of mirror sites and then let them choose which one appears to be best for them.
Flash DNS addresses this problem by letting a 7190 at each network site send out responses to a DNS query for the site host name. If all responses are sent at the same time then the site that has the least network latency connecting to the user should get its response to the user first. The user ignores all the other responses.
Since many enterprise clients are behind firewalls that perform Network Address Translation (NAT), by default, the 7190 performs source IP using the IP address of the 7190 that originally received the DNS request. This feature can be disabled.
The 7190 maintains the current metric state of each service on a per host basis. A “service” is an IP address and port at a site. The maintained state has four levels relating to the amount of information that the 7190 has about each service.
Each load balancing method handles partial failures in the same basic way. Each balancing method always tries to find at least one service with a metric status of “OK” with which to balance. If no services are available at this highest metric state, and it is not using a metric balancing method, then it looks for at least one service with a metric state of “No Services”. If none are found then it looks for at least one service with a metric state of “No Metrics”. Finally, if no services have been found at a high enough metric state level to balance normally, it performs a round robin selection among all the enabled services.
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