Читать книгу Big Data - Seifedine Kadry - Страница 54

Load‐balancing clusters are designed to distribute workloads across different cluster nodes to share the service load among the nodes. If a node in a load‐balancing cluster goes down, the load from that node is switched over to another node. This is achieved by having identical copies of data across all the nodes, so the remaining nodes can share the increase in load. The main objective of load balancing is to optimize the use of resources, minimize response time, maximize throughput, and avoid overload on any one of the resources. The resources are used efficiently in this kind of cluster algorithm as there is a good amount of control over the way in which the requests are routed. This kind of routing is essential when the cluster is composed of machines that are not equally efficient; in that case, low‐performance machines are assigned a lesser share of work. Instead of having a single, very expensive and very powerful server, load balancing can be used to share the load across several inexpensive, low performing systems for better scalability.

Round robin load balancing, weight‐based load balancing, random load balancing, and server affinity load balancing are examples of load balancing. Round robin load balancing chooses server from the top server in the list in sequential order until the last server in the list is chosen. Once the last server is chosen it resets back to the top. The weight‐based load balancing algorithm takes into account the previously assigned weight for each server. The weight field will be assigned a numerical value between 1 and 100, which determines the proportion of the load the server can bear with respect to other servers. If the servers bear equal weight, an equal proportion of the load is distributed among the servers. Random load balancing routes requests to servers at random. Random load balancing is suitable only for homogenous clusters, where the machines are similarly configured. A random routing of requests does not allow for differences among the machines in their processing power. Server affinity load balancing is the ability of the load balancer to remember the server where the client initiated the request and to route the subsequent requests to the same server.