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The following gives you a brief introduction and advantages of load balancing, hoping to give you some help in practical application. Load balancing involves a lot of things, there are not many theories, and there are many books on the Internet. Today, we will use the accumulated experience in the industry to make an answer.

What is load balancing

Load balancing (Load Balance) means that it is distributed to multiple operating units for execution, such as Web cloud servers, FTP servers, enterprise critical application servers and other critical task servers, so as to complete work tasks together.

In the literal sense, it can be explained that N servers share the load evenly, and a server is not idle because of its high load and downtime. Then the premise of load balancing is that there must be multiple servers, that is, more than two servers.

Load balancing is based on the existing network structure, which provides a cheap, effective and transparent way to expand the bandwidth of network devices and servers, increase throughput, enhance network data processing capacity, and improve network flexibility and availability.

II. Advantages of load balancing

Reduce the pressure on the server, distribute the original access volume of one server to multiple servers, and improve the availability of the project, so that when a server dies, the project will not be paralyzed.

Layer 3 and layer 4 load balancing and layer 7 load balancing

Layer 4 load balancing works in the transport layer of the OSI model, and its main work is forwarding. After receiving the traffic from the client, it forwards the traffic to the application server by modifying the address information of the packet.

Layer 7 load balancer works in the application layer of the OSI model. Because it needs to analyze the application layer traffic, layer 7 load balancer also needs a complete TCP/IP protocol stack after receiving the traffic from the client. Layer-7 load balancer will establish a complete connection with the client and parse the request traffic of the application layer, then select an application server according to the scheduling algorithm, and establish another connection with the application server to send the request. Therefore, the main work of layer-7 load balancer is proxy. Layer 7 load balancer is also known as "content exchange", that is, the final choice of internal server is determined by the really meaningful application layer content in the message and the server selection method set by the load balancing device.

The advantages of seven-layer load balancing: this method can modify the request of the client and the response of the server in any sense, which greatly improves the flexibility of the application system in the network layer; high security.

Seven-layer load balancing mainly focuses on the widely used HTTP protocol, so its application scope is mainly many websites or internal information platforms and other systems developed based on Bamp S. Four-tier load balancing corresponds to other TCP applications, such as ERP based on Cramp S.

Fourth, the use of software for load balancing

Load balancing software Nginx, LVS, HaProxy and so on are the three most widely used load balancing software.

Fifth, load balancing algorithm

1. Round Robin: it is the easiest way to send requests in turn for all backend, and it is also the default allocation method.

2. Least Connections (least_conn): track the current number of active connections to backend. The minimum number of connections means that the backend has the lightest load. Assign the request to him. This way takes into account the weight weight information assigned to each upstream in the configuration.

3. Least Time (least_time): the request is assigned to the backend with the fastest response and the least number of active connections

4. IP Hash (ip_hash): calculate the hash value for the request source IP address, IPv4 will consider the first three octet,IPv6 will consider all the address bits, and then assign it to the backend through some mapping according to the resulting hash value

5. Generic Hash (hash): the hash value is calculated in the way of user-defined resources (such as URL) to complete the allocation. The optional consistent keyword supports the consistent hash feature.

6. Load balancing based on DNS

Usually, a domain name of a modern network server will be related to multiple hosts. When performing a DNS query, by default, the DNS server will return a list of IP addresses in a different order in the form of round-robin, so it naturally assigns customer requests to different hosts. However, this approach has inherent drawbacks: DNS does not check the accessibility of hosts and IP addresses, so the IP assigned to the client is not guaranteed to be available (Google 404); the parsing results of DNS are constantly cached on the client side and multiple intermediate DNS servers, so the allocation of backend is not so ideal.

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