# Listener discovery service (LDS)¶

The listener discovery service (LDS) is an optional API that Envoy will call to dynamically fetch listeners. Envoy will reconcile the API response and add, modify, or remove known listeners depending on what is required.

The semantics of listener updates are as follows:

• Every listener must have a unique name. If a name is not provided, Envoy will create a UUID. Listeners that are to be dynamically updated should have a unique name supplied by the management server.

• When a listener is added, it will be “warmed” before taking traffic. For example, if the listener references an RDS configuration, that configuration will be resolved and fetched before the listener is moved to “active.”

• Listeners are effectively constant once created. Thus, when a listener is updated, an entirely new listener is created (with the same listen socket). This listener goes through the same warming process described above for a newly added listener.

• When a listener is removed, the old listener will be placed into a “draining” state much like when the entire server is drained for restart. Connections owned by the listener will be gracefully closed (if possible) for some period of time before the listener is removed and any remaining connections are closed. The drain time is set via the --drain-time-s option.

• When a tcp listener is updated, if the new listener contains a subset of filter chains in the old listener, the connections owned by these overlapping filter chains remain open. Only the connections owned by the removed filter chains will be drained following the above pattern. Note that if any global listener attributes are changed, the entire listener (and all filter chains) are drained similar to removal above. See filter chain only update for detailed rules to reason about the impacted filter chains.

Note

Any listeners that are statically defined within the Envoy configuration cannot be modified or removed via the LDS API.

## Statistics¶

LDS has a statistics tree rooted at listener_manager.lds.