Explanation of nginx load balancing and reverse proxy

Load Balancing

Load balancing is a process where multiple servers are symmetrically combined into a server set. Each server has an equal status and can provide services independently without the assistance of other servers. Through a certain load sharing technology, the requests sent from the outside are evenly distributed to a server in the symmetrical structure, and the server that receives the request responds to the user's request independently. Load balancing can evenly distribute service requests to the server array, quickly obtain data, and solve the problem of large numbers of concurrent access services.

Load balancing classification

1. DNS load balancing

On the DNS server, configure multiple A records. The servers corresponding to these A records form a cluster.

advantage :

• 1. Easy to use: Load balancing work is handled by the DNS server, saving the trouble of load balancing server maintenance
• 2. Improve performance: It can support address-based domain name resolution, which can be resolved into the server address closest to the user, which can speed up access and improve performance.

shortcoming:

• 1. Poor availability: DNS resolution is multi-level resolution. After adding/modifying DNS, the resolution time is long. During the resolution process, users will fail to access the website.
• 2. Low scalability: The control of DNS load balancing is in the hands of the domain name provider, and it is impossible to make further improvements and expansions.
• 3. Poor maintainability: It cannot reflect the current running status of the server; it supports few algorithms; it cannot distinguish the differences between servers (it cannot judge the load based on the status of the system and the server)

2. IP load balancing

At the network layer, load balancing is performed by modifying the request target address. After the user request data packet arrives at the load balancing server, the load balancing server obtains the network data packet in the operating system kernel process, obtains a real server address based on the load balancing algorithm, and then modifies the requested target address to the obtained real IP address.
After the real server completes the processing, the corresponding data packet returns to the load balancing server. The load balancing server then modifies the source address of the data packet to its own IP address and sends it to the user's browser.

IP load balancing, the real physical server returns to the load balancing server. There are two ways:

• (1) The load balancing server modifies the source address while modifying the destination IP address. Set the source address of the data packet to its own disk, which is called source address translation (SNAT).
• (2) The load balancing server also serves as the gateway server of the real physical server cluster.

advantage :

• (1) Data distribution is completed in the kernel process, which has better performance than distribution at the application layer;

shortcoming :

• (1) All request responses need to go through the load balancing server, and the maximum throughput of the cluster is limited by the network card bandwidth of the load balancing server;

3. Link layer load balancing

Modify the MAC address at the data link layer of the communication protocol to perform load balancing.
Data distribution does not modify the IP address, which means modifying the target's MAC address and configuring the virtual IP addresses of all machines in the real physical server cluster to be consistent with the IP address of the load balancing server, so as to achieve the purpose of data distribution without modifying the source address and destination address of the data packet.

The actual processing server IP is consistent with the data request destination IP, and does not need to be translated by the load balancing server. The response data packet can be returned directly to the user's browser, preventing the load balancing server network card bandwidth from becoming a bottleneck. Also called direct routing mode (DR mode). As shown below:

Advantages : good performance
Disadvantages : Complex configuration

4. Hybrid load balancing

Due to the differences in hardware equipment, scale, and services provided by multiple server clusters, you can consider using the most appropriate load balancing method for each server cluster, and then load balance or cluster them again among so many server clusters to provide services to the outside world as a whole (treating multiple server clusters as a new server cluster) to achieve optimal performance.

Method 1, as shown below:

The above mode is suitable for scenarios with dynamic and static separation. The reverse proxy server (cluster) can play the role of caching and dynamic request distribution. When static resources are cached in the proxy server, they are directly returned to the browser. If it is a dynamic page, the subsequent application load balancing is requested.
Method 2, as shown below:

The above scenarios are suitable for dynamic request scenarios.

Load Balancing Algorithm

Common load balancing algorithms include polling, random, least link, source address hashing, weighted, etc.

1 Polling

Distribute all requests to each server in turn, which is suitable for scenarios with the same server hardware.

Advantages : The number of server requests is the same;

Disadvantages : The server pressure is different, which is not suitable for situations with different server configurations;

2 Random

Requests are randomly assigned to servers.
Advantages : Easy to use;

Disadvantages : Not suitable for scenarios with different machine configurations;

3 Minimum links

Dispatches requests to the server with the fewest connections (the server currently handling the fewest requests).

Advantages : Dynamic allocation based on the current request processing status of the server;

Disadvantages : The algorithm is relatively complex to implement and requires monitoring the number of server request connections;

4 Hash (source address hash)

Perform hash calculation based on the IP address to obtain the IP address.

Advantages : forward requests from the same IP address to the same server during the same session; achieve session stickiness.

Disadvantages : If the target server goes down, the session will be lost;

5 Weighted

Based on polling, random, least link, Hash' and other algorithms, load server distribution is performed in a weighted manner.

Advantages : Adjust the number of requests to the forwarding server according to the weight;

Disadvantages : relatively complicated to use;

Reverse Proxy

Reverse proxy means that the proxy server accepts connection requests on the Internet, then forwards the requests to the server on the internal network, and returns the results obtained on the server to the client requesting the connection on the Internet. At this time, the proxy server appears to the outside world as a server.

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