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A load balancer is a device that acts as a reverse proxy and distributes network or application traffic across a number of servers. It helps scale horizontally across an ever-increasing number of servers.

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The benefits of a Load Balancer

  • Reduced the work-load on an individual server.
  • Large amount of work done in same time due to concurrency.
  • Increased performance of your application because of faster response.
  • No single point of failure. In a load balanced environment, if a server crashes the application is still up and served by the other servers in the cluster.
  • When appropriate load balancing algorithm is used, it brings optimal and efficient utilization of the resources, as it eliminates the scenario of some server’s resources are getting used than others.
  • Scalability: We can increase or decrease the number of servers on the fly without bringing down the application.
  • Load balancing increases the reliability of your enterprise application.
  • Increased security as the physical servers and IPs are abstract in certain cases.

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How does it work

  • Define IP or DNS name for LB: Administrators define one IP address and/or DNS name for a given application, task, or website, to which all requests will come. This IP address or DNS name is the load balancing server.
  • Add backend pool for LB: The administrator will then enter into the load balancing server the IP addresses of all the actual servers that will be sharing the workload for a given application or task. This pool of available servers is only accessible internally, via the load balancer.
  • Deploy LB: Finally, your load balancer needs to be deployed — either as a proxy, which sits between your app servers and your users worldwide and accepts all traffic, or as a gateway, which assigns a user to a server once and leaves the interaction alone thereafter.
  • Redirect requests: Once the load balancing system is in place, all requests to the application come to the load balancer and are redirected according to the administrator’s preferred algorithm.

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Type of Load Balancer

  • Network Load Balancing: Network load balancing, as its name suggests, leverages network layer information to decide where to send network traffic. This is accomplished through layer 4 load balancing, which is designed to handle all forms of TCP/UDP traffic. Network load balancing is considered the fastest of all the load balancing solutions, but it tends to fall short when it comes to balancing the distribution of traffic across servers.
  • HTTP(S) Load Balancing: HTTP(S) load balancing is one of the oldest forms of load balancing. This form of load balancing relies on layer 7, which means it operates in the application layer. HTTP load balancing is often dubbed the most flexible type of load balancing because it allows you to form distribution decisions based on any information that comes with an HTTP address.
  • Internal Load Balancing: Internal load balancing is nearly identical to network load balancing but can be leveraged to balance internal infrastructure.

When talking about types of load balancers, it’s also important to note there are hardware load balancers, software load balancers, and virtual load balancers.

  • Hardware Load Balancer: A hardware load balancer, as the name implies, relies on physical, on-premises hardware to distribute application and network traffic. These devices can handle a large volume of traffic but often carry a hefty price tag and are fairly limited in terms of flexibility.
  • Software Load Balancer: A software load balancer comes in two forms—commercial or open-source—and must be installed prior to use. Like cloud-based balancers, these tend to be more affordable than hardware solutions.
  • Virtual Load Balancer: A virtual load balancer differs from software load balancers because it deploys the software of a hardware load balancing device on a virtual machine.

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Load Balancer Locations

  • Between user and web servers (User => Web Servers)
  • Between web servers and an internal platform layer (application servers, cache servers) (Webservers => App or Cache servers)
  • Between internal platform layer and database (App or Cache servers => Database servers)

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There is a variety of load balancing methods, which use different algorithms best suited for a particular situation.

Round-robin load balancing is one of the simplest and most used load balancing algorithms. Client requests are distributed to application servers in rotation. For example, if you have three application servers: the first client request to the first application server in the list, the second client request to the second application server, the third client request to the third application server, the fourth to the first application server and so on. This load balancing algorithm does not take into consideration the characteristics of the application servers i.e. it assumes that all application servers are the same with the same availability, computing and load handling characteristics.

Weighted Round Robin builds on the simple Round-robin load balancing algorithm to account for differing application server characteristics. The administrator assigns a weight to each application server based on criteria of their choosing to demonstrate the application servers traffic-handling capability.

If application server #1 is twice as powerful as application server #2 (and application server #3), application server #1 is provisioned with a higher weight and application server #2 and #3 get the same weight. If there five (5) sequential client requests, the first two (2) go to application server #1, the third (3) goes to application server #2, the fourth (4) to application server #3 and the fifth (5) to application server #1.

Least Connection load balancing is a dynamic load balancing algorithm where client requests are distributed to the application server with the least number of active connections at the time the client request is received. In cases where application servers have similar specifications, an application server may be overloaded due to longer lived connections; this algorithm takes the active connection load into consideration.

Weighted Least Connection builds on the Least Connection load balancing algorithm to account for differing application server characteristics. The administrator assigns a weight to each application server based on criteria of their choosing to demonstrate the application servers traffic-handling capability. The LoadMaster is making the load balancing criteria based on active connections and application server weighting.

Resource Based (Adaptive) is a load balancing algorithm requires an agent to be installed on the application server that reports on its current load to the load balancer. The installed agent monitors the application servers availability status and resources. The load balancer queries the output from the agent to aid in load balancing decisions.

SDN Adaptive is a load balancing algorithm that combines knowledge from Layers 2, 3, 4 and 7 and input from an SDN Controller to make more optimized traffic distribution decisions. This allows information about the status of the servers, the status of the applications running on them, the health of the network infrastructure, and the level of congestion on the network to all play a part in the load balancing decision making.

Fixed Weighting is a load balancing algorithm where the administrator assigns a weight to each application server based on criteria of their choosing to demonstrate the application servers traffic-handling capability. The application server with the highest weigh will receive all of the traffic. If the application server with the highest weight fails, all traffic will be directed to the next highest weight application server.

Weighted Response Time is a load balancing algorithm where the response times of the application servers determines which application server receives the next request. The application server response time to a health check is used to calculate the application server weights. The application server that is responding the fastest receives the next request.

Source IP hash load balancing algorithm that combines source and destination IP addresses of the client and server to generate a unique hash key. The key is used to allocate the client to a particular server. As the key can be regenerated if the session is broken, the client request is directed to the same server it was using previously. This is useful if it’s important that a client should connect to a session that is still active after a disconnection.

URL Hash is a load balancing algorithm to distribute writes evenly across multiple sites and sends all reads to the site owning the object.

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