Internal listeners are used for internal traffic and exchange of information between Apache Kafka brokers, whereas external listeners are used for external clients to be optionally enabled based on the relations created on particular charm endpoints. Each listener is characterized by a specific port, scope and protocol.

In the following table, we summarize the protocols, the port and the relation that each listener is bound to. Note that based on whether a certificates relation is present, one of two mutually exclusive types of listeners can be opened.

Listener name	Driving endpoints	Protocol	Port	Scope
SASL_INTERNAL	`cluster`	SASL_PLAINTEXT	`19092`	internal
SASL_INTERNAL	`cluster` + `certificates`	SASL_SSL	`19093`	internal
SASL_EXTERNAL	`kafka-client`	SASL_PLAINTEXT	`9092`	external
SASL_EXTERNAL	`kafka-client` + `certificates`	SASL_SSL	`9093`	external
SSL_EXTERNAL	`trusted-certificate` + `certificates`	SSL	`9094`	external
SSL_EXTERNAL	`trusted-ca` + `certificates`	SSL	`9094`	external

Since cluster is a peer relation, the SASL_INTERNAL listener is always enabled.

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