问题
I googled for load balancing but the only thing I can find is the working theory, which at the moment, is the "easy" part for me. But zero examples of how to implement one.
I have several questions pertaining load balancing:
I have a domain (example.com) and I behind it I have a load balancing server (lets call it A) which, according to the theory, will ask the client to close the connection with A, and connect to B, a sub-server and carry on the request with B. Will the client, in a web browser stop seeing "example.com/page.html" in the address bar and start seeing "B_ip_address/page.html" instead?
How can I implement a simple load balancer from scratch? My doubt targets the HTTP part. Is there some specific message or set of messages I need to send the client which will make him disconnect from me and connect to a sub-server?
What about lower level protocols than HTTP, such as TCP/IP, are there any standard packets to tell the client he just connected to a load balancer server and now he needs to connect to xxx.xxx.xxx.xxx to carry on the request?
What method is the most used? (1) client connects to load balancing server, and it asks the client to directly connect to one of the sub-servers, or (2) the load balancing server starts bridging all traffic from the client to the sub-server and vice-versa in a transparent fashion?
So question 2, 3 and 4 regard the load balancing protocol, and the 1st one the way a domain name can be connected to a load balancer and what are the underlying consequences.
回答1:
Your approach is a kind of static load balancing by redirect the calls to another server. All following calls may use this other server or are send to the load balancer again for redirect.
An implementation depends on the implementation of your system. A load balancer works best for independent requests with no session state. You need to sync the session state otherwise between the "end" servers. Or use a shared session store to provide the session state to all servers.
There exists a simple and transparent solution for HTTP server load balancing. You can use the load balancing module of an nginx server (http://nginx.org/en/docs/http/load_balancing.html). This can be used for HTTP and HTTPS requests. And it may be extended with extra servers dynamically if the load increases. You need to edit the nginx configuration and restart the server. This can be transparent to existing connections. And nginx does not cause problems with changing domain or host names.
Other protocols need some support by the client and the server. Load balancing may be transparent if a specialized device is between the client and server. Or the communication protocol needs to support connection redirects.
Edit: Load balancing can be implemented by DNS round robin too. Each DNS lookup call returns another IP address for the same domain name. The client choose an IP and connects to this server. Another client can use the next IP. The address bar name is the same all the time.
Example:
Non-authoritative answer:
Name: www.google.com
Addresses: 2a00:1450:4001:80f::1010
173.194.116.209
173.194.116.210
173.194.116.212
173.194.116.211
173.194.116.208
Non-authoritative answer:
Name: www.google.com
Addresses: 2a00:1450:4001:80f::1010
173.194.116.210
173.194.116.212
173.194.116.211
173.194.116.208
173.194.116.209
Non-authoritative answer:
Name: www.google.com
Addresses: 2a00:1450:4001:80f::1010
173.194.116.212
173.194.116.211
173.194.116.208
173.194.116.209
173.194.116.210
The IP address range rotates. Most HTTP load balancers work as transparent load balancer like nginx or other reverse proxy implementations. A redirecting load balancer is more a low tech implementation I think.
TCP/IP is not a protocol. It's the transport layer used to transfer data implementing a specific communication protocol. While TCP/IP itself is a protocol for the network components. But not the applications. You may check https://en.wikipedia.org/wiki/OSI_model .
来源：https://stackoverflow.com/questions/31684156/load-balancing-server-how-can-i-implement-it