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What Is a Listener?

Updated at: May 10, 2019 16:53

You need to add at least one listener to a load balancer. A listener is a process that checks for requests from clients using the protocol and port you configure for connections between clients and the load balancer, and routes the requests to backend servers using the load balancing algorithm you select.

A listener also defines the health check configuration, through which the load balancer automatically checks running statuses of backend servers. If a backend server becomes faulty, the load balancer automatically forwards traffic to other healthy ones. Traffic forwarding to this server will resume once it recovers.

Supported Protocols

ELB provides load balancing at both Layer 4 and Layer 7. Select a protocol that can meet your needs based on the application scenario.

Table 1 Protocols supported by ELB



Application Scenario

Layer 4


  • Source IP address–based sticky sessions
  • Fast data transfer
  • Scenarios that require high reliability and data accuracy, such as file transfer, email sending and receiving, and remote login
  • Web applications with a number of concurrent connections or that require high performance

Layer 4


  • Low reliability
  • Fast data transfer

Scenarios that focus on timeliness rather than reliability, such as video chat, game, and real-time financial market information push

Layer 7


  • Cookie-based sticky sessions
  • X-Forward-For request header

Applications in which the data content needs to be identified, such as web applications and mobile games

Layer 7


  • An extension of HTTP for encrypted data transmission that can prevent unauthorized access
  • Encryption and decryption performed on load balancers to reduce the workload of backend servers

Applications that require encrypted transmission

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