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 adding the listener.
Backend server	A backend server is cloud server added to a backend server group associated with a load balancer. When adding a listener to a load balancer, you create or select a backend server group to receive requests from the load balancer 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. When the listener settings are met, traffic is routed to the corresponding backend server group.
Health check	ELB periodically sends requests to associated backend servers to check their health results. This process is called health check, through which the ELB system decides whether backend servers are able to process requests. If a backend server is detected unhealthy, the load balancer stops routing requests to it, ensuring service reliability. After the backend server recovers, the load balancer resumes routing requests to it.
Redirect	HTTPS is an extension of HTTP. HTTPS encrypts data between a web server and a browser. Redirection allows requests to be redirected from HTTP to HTTPS.
Sticky session	Sticky sessions are a mechanism that ensures that requests from a client always get routed to the same 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 or receive data to or from each other, which is similar to Socket.
SNI	If an application provides multiple domain names and each domain name uses a different certificate, you can enable SNI when adding an HTTPS listener. SNI is an extension to TLS. It allows a server to present multiple certificates on the same IP address and TCP port number and hence allows multiple secure (HTTPS) websites (or any other service over TLS) to be served by the same IP address without requiring all those sites to use the same certificate. Before SNI, one server can use only one certificate. SNI allows the client to submit the domain name information while sending an SSL handshake request. Once receiving the request, the load balancer queries the right certificate based on the domain name and returns it 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 TCP connections initiated by clients and routed to backend servers by a load balancer per second.