# Hierarchical MQTT Eventing ## User Story & Use Case >As AAS Components user >I want hierarchical MQTT topics in the AAS Registry Component >so that I can subscribe precisely to the topics of interest without having to filter on client side MQTT supports hierarchical topics with [wildcard subscription support](https://www.hivemq.com/blog/mqtt-essentials-part-5-mqtt-topics-best-practices/). For example, if the topics *a/b/x* and *a/b/y* exist, subscribing to *a/b/+* subscribes to all events broadcasted on both topics. ## Feature Overview The following hierarchical topics with their respective payloads are implemented: * aas-registry//shells/created * aas-registry//shells/updated * Payload: New AAS-Descriptor * aas-registry//shells/removed * payload: Old AAS-Descriptor * aas-registry//submodels/added * aas-registry//submodels/updated * Payload: New SM-Descriptor * aas-registry//submodels/deleted * Payload: Old SM-Descriptor * aas-registry//shells//submodels/added * aas-registry//shells//submodels/deleted * Payload: New SM-Descriptor * /aas-registry//shells//submodels/deleted * Payload: Old SM-Descriptor For AAS-Identifier encoding, [Base64URL encoding](https://www.base64url.com) and [URL encoding](https://en.wikipedia.org/wiki/Percent-encoding) are available. ## Feature Configuration The feature can be configured in the *registry.properties* file by setting *registry.events = MQTTV2* for Base64URL encoding or *registry.events = MQTTV2_SIMPLE_ENCODING* for URL encoding. Additionally, the Registry id needs to be configured via *registry.id = .* An example configuration for this feature could be: ``` registry.events = MQTTV2 registry.id = product-aas-registry ``` The MQTT broker connectivity is configured via [mqtt.properties](../../general_configuration/mqtt.md)