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I know how load balancers work for http requests. A client opens a connection with the LB, LB forwards the request to the backend servers, LB gets a response and from the same connection it sends the response to the client and closes the connection. I want to know the internal details of load balancer for websockets. How the connections are maintained and how the response is sent to the client. I read many questions on stackoverflow but none of them gave a clear picture of the internal implementation of LB

ffff
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Implementations of load balancers have great variety. There are load balancers that support websockets, like F5's BIG-IP (https://support.f5.com/kb/en-us/solutions/public/14000/700/sol14754.html), and LB's that I don't think support websocekts, like AWS ELB (there is a thread where somebody says they could make it with ELB but I suppose they added some other component behind ELB: How do you get Amazon's ELB with HTTPS/SSL to work with Web Sockets?.

Load Balancer's not only act as terminators of HTTP connections, they can terminate also HTTPS, SSL, and TCP connections. They can implement stickiness based on different parameters, like cookies, origin IP, etc. (like F5) In the case of ELB's they use only cookies, and it could be application generated cookies or LB generated cookies (both only with HTTP or HTTPS). Also stickiness can be kept for certain defined time, sometimes configurable.

Now, in order to forward data corresponding to websockets, they need to terminate, and forward, connections at level of SSL or TCP (not HTTP or HTTPS). Unless they understand websocket protocol (I don't know if any does it). Additionally, they need to keep stickiness to the server with which the connetion was opened. This is not possible with ELB but yes with more complex LB's like BIG-IP.

Community
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rodolk
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the LB just route the connection to a server behind it.

so as long you keep the connection open you will keep being connected to the same server and do not communicate with the LB again. depending on the client, on reconnection you could be routed to another server.

I'm not sure how it works when some libraries fallback to JSON-P tho

Christoffer Bo Petersen
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  • Few sources tell that every message sent by an open connection goes via the LB itself. LB checks the cookie to determine which server is serving this socket – ffff Jul 01 '15 at 19:43
  • What about TLS termination etc. Is it allowed to directly connect the client with any of ther Server Farm. – bharatj Dec 15 '20 at 16:53