In this section the publish flow is described. VerneMQ provides multiple hooks throughout the flow of a message. The most important ones are the auth_on_publish and auth_on_publish_m5 hooks which acts as an application level firewall granting or rejecting a publish message.
The auth_on_publish and auth_on_publish_m5 hooks allow your plugin to grant or reject publish requests sent by a client. It also enables to rewrite the publish topic, payload, qos, or retain flag and in the case of auth_on_publish_m5 properties. The auth_on_publish hook is specified in the Erlang behaviour auth_on_publish_hook and the auth_on_publish_m5 hook in the auth_on_publish_m5_hook behaviour available in the vernemq_dev repo.
Every plugin that implements the auth_on_publish or auth_on_publish_m5 hooks are part of a conditional plugin chain. For this reason we allow the hook to return different values. In case the plugin can't validate the publish message it is best to return next as this would allow subsequent plugins in the chain to validate the request. If no plugin is able to validate the request it gets automatically rejected.
The on_publish and on_publish_m5 hooks allow your plugin to get informed about an authorized publish message. The hook is specified in the Erlang behaviour on_publish_hook and the on_publish_m5 hook in the on_publish_m5_hook behaviour available in the vernemq_dev repo.
The on_offline_message hook allows your plugin to get notified about a new a queued message for a client that is currently offline. The hook is specified in the Erlang behaviour on_offline_message_hook available in the vernemq_dev repo.
The on_deliver and on_deliver_m5 hooks allow your plugin to get informed about outgoing publish messages, but also allows you to rewrite topic and payload of the outgoing message. The hook is specified in the Erlang behaviour on_deliver_hook and the on_deliver_m5 hook in the on_deliver_m5_hook behaviour available in the vernemq_dev repo.
Every plugin that implements the on_deliver or on_deliver_m5 hooks are part of a conditional plugin chain, although NO verdict is required in this case. The message gets delivered in any case. If your plugin uses this hook to rewrite the message the plugin system stops evaluating subsequent plugins in the chain.