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MQTT: topic rewrite on VerneMQ #2

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41 changes: 41 additions & 0 deletions mqtt/topic-rewrite-on-vernemq.md
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# Topic rewrite on VerneMQ

I want the MQTT client to subscribe to VerneMQ using a device ID based topic name, but to actually subscribe to a different topic.

For example, I want the MQTT client to being able to subscribe a topic named `/device/123` so it must only know its device ID (`123` in this example).

Since the device can chage over time I want the data to be published to the topic `/thing/456`.

VerneMQ has a nice feature called topic rewrite, available for both `SUBSCRIBE` and `PUBLISH` contro packets from the MQTT protocol.

The subscribe can be customized using the [auth_on_subscribe](https://docs.vernemq.com/plugindevelopment/subscribeflow#auth_on_subscribe-and-auth_on_subscribe_m5) hook.

The publish can be customized using the [auth_on_publish](https://docs.vernemq.com/v/master/plugin-development/publishflow#auth_on_publish-and-auth_on_publish_m5) hook.


```
┌──────────┐ ┌─────────────┐ ┌─────────────┐
│ client │ │ mqtt broker │ │ plugin │
└──────────┘ └─────────────┘ └─────────────┘
│ │ │
│ │ │
│ subscribe │ │
│ /device/123 │ │
│ ──────────────────────────▶ │ │
│ │ auth_on_su │
│ │ ──────bscribe───────▶ │
│ │ │
│ │ rewrite w/ │
│ │ ◀ ─ /thing/456─ ─ ─ ─ │
│ │ │
│ │ │
│ │ │
│ │ │
│ │ │
│ │ │
│ │ │
│ │ │
│ │ │
│ │ │
│ │
```