We have several reasons to remove this feature:
1. The design does not make sense. A rule engine resource should
not have an impact on the system's messages directly after it is created.
This mqtt_sub actually conflicts with any design concept of the rules engine.
2. The implementation is incorrect. mqtt_sub uses a client pool to establish
a subscription relationship to an MQTT Broker. This causes a message to be
sent repeatedly to EMQ X. Unless a shared subscription is used,
or a Pool Size of 1 is configured.
3. The emqx-bridge-mqtt supports all the features of mqtt_sub.
This feature introduced by https://github.com/emqx/emqx-bridge-mqtt/pull/78.
And it released to v4.2.0 (NOT WORK), v4.2.1-v4.2.7 (FIXED)
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