listener.md

Listener implementation

Socket handling

Envoy uses the following procedure for creating sockets and assigning them to workers.

1. When a listener is created, a socket is pre-created for every worker on the main thread. This allows most errors to be caught early on in the listener creation process (e.g., bad socket option, unable to bind, etc.).
   • If using reuse_port, a unique socket is created for every worker.
   • If not using reuse_port, a unique socket is created for worker 0, and then that socket is duplicated for all other workers.
2. In the case of a listener update in which the address/socket is to be shared (i.e., a named listener that has configuration settings changed but listens on the same address will share sockets with the draining listener), all sockets are duplicated regardless of whether using reuse_port or not. This keeps the logic equivalent, and also makes sure that when using reuse_port no TCP connections are dropped during the drain process (in the race condition between assigning a queue and closing the listening socket on the old listener).
3. In the case of hot restart, all sockets are requested by worker index. If the old process has a match, the socket is duplicated and sent to the new process. Thus, hot restart and a listener update effectively share the same update process. This is so that no connections are dropped when using reuse_port.
4. A consequence of every listener having dedicated sockets (whether duplicated or not) is that a listener can close() its sockets when removed without concern for other listeners.

For more information, see ListenSocketFactoryImpl and its use within ListenerImpl and ListenerManagerImpl.