# Basic Concepts

Table 1 Some concepts about ELB
Term | Definition
--- | ---
Load balancer | A load balancer distributes incoming traffic across backend servers.
Listener | A listener listens on requests from clients and routes the requests to backend servers based on the settings that you configure when you add the listener.
Backend server | A backend server is a cloud server added to a backend server group associated with a load balancer. When you add a listener to a load balancer, you can create or select a backend server group to receive requests from the load balancer by using the port and protocol you specify for the backend server group and the load balancing algorithm you select.
Backend server group | A backend server group is a collection of cloud servers that have same features. When you add a listener, you select a load balancing algorithm and create or select a backend server group. Incoming traffic is routed to the corresponding backend server group based on the listener's configuration.
Health check | ELB periodically sends requests to backend servers to check whether they can process requests. If a backend server is detected as unhealthy, the load balancer stops routing requests to it. After the backend server recovers, the load balancer will resume routing requests to it.
Redirect | HTTPS is an extension of HTTP. HTTPS encrypts data between a web server and a browser.
Sticky session | Sticky sessions ensure that requests from a client always get routed to the same backend server before a session elapses.

WebSocket

WebSocket is a new HTML5 protocol that provides full-duplex communication between the browser and the server. WebSocket saves server resources and bandwidth, and enables real-time communication. Both WebSocket and HTTP depend on TCP to transmit data. A handshake connection is required between the browser and server, so that they can communicate with each other only after the connection is established. However, as a bidirectional communication protocol, WebSocket is different from HTTP. After the handshake succeeds, both the server and browser (or client agent) can actively send data to or receive data from each other.

SNI

SNI is an extension to TLS and enables a server to present multiple certificates on the same IP address and TCP port number. This allows multiple secure (HTTPS) websites (or any other service over TLS) to be served by the same IP address without requiring all websites to use the same certificate. SNI allows the client to submit the domain name information while sending an SSL handshake request. Once the load balancer receives the request, it queries the right certificate based on the domain name and returns the corresponding certificate to the client. If no certificate is found, the load balancer will return a default certificate.

Persistent connection | A persistent connection allows multiple data packets to be sent continuously over a TCP connection. If no data packet is sent during the connection, the client and server send link detection packets to each other to maintain the connection.
Short connection | A short connection is a connection established when data is exchanged between the client and server and immediately closed after the data is sent.
Concurrent connection | Concurrent connections are total number of TCP connections initiated by clients and routed to backend servers by a load balancer per second.

Parent topic: Product Concepts