You can view all retained messages in the EMQX on Retained Messages page by clicking Monitoring -> Retained Messages from the left navigation menu.
When a user publishes a retained message, EMQX will save this message in the system. The user can view this message on the Retained Messages page. When the user subscribes to the topic of this retained message, EMQX will publish this message to the topic, and users can receive this message immediately.
The retained message is never expired by default unless the user manually deletes this message.
Retained Messages List
The Retained Messages list includes the topic, QoS, client ID, and time when the retained message was published. In the list, you can click the Show Payload and Delete buttons to view the Payload of the retained message and delete a retained message. Click the Refresh button in the upper right corner of the list to refresh the current retained message list, and click the Settings button to jump to the retained message settings page.
Three types of system topics are saved as retained messages by default. If it is a cluster environment, it will keep different system topics' retained messages according to other node names. They are:
- $SYS/brokers/+/sysdescr - The system description of the current EMQX node
- $SYS/brokers/+/version - The version number of the current EMQX node
- $SYS/brokers - The number and name of all nodes of the current EMQX
Delete Retained Message
In most cases, you can delete retained messages in the client by publishing an empty message to the topic of the retained message. In addition, you can also delete retained messages in EMQX Dashboard. Click the Delete button on the Retained Messages list page to delete a retained message. You can also set the expiration time of the retained message on the Retained Messages configuration page. When the retained message expires, EMQX will automatically delete this retained message.
If you want to view the Payload of the retained message, you can click the Show Payload in Actions column of the retained message item.
On the pop-up window, you can click the Copy button at the lower right corner to copy the payload. You can also select the payload display format from the drop-down list at the lower left corner have more intuitive display for some special payload formats, such as JSON or Hex format.
By clicking the Settings button in the upper right corner of the Retained Messages page, you will be redirected to the Retainer tab on the Management -> MQTT Settings page where you can enable or disable the retained messages feature and also configure the settings for retained messages.
If you disable the retained messages feature by clicking the toggle switch, an Enable button will be displayed on the Retained Messages page. By clicking the button, you will be redirected to the Retainer tab.
Below are detailed descriptions of each field.
|Configuration item||Type||Optional value||Default value||Description|
|Storage Type||-||Built-in Database||-||-|
|Max Retained Messages||Integer||≥ 0||0 (Unlimited)||0: Unlimit. |
When you set a limit on the maximum number of retained messages, EMQX replaces existing messages once the limit is reached. However, you cannot store retained messages for new topics beyond the limit.
|Max Payload Size||Bytesize||1MB||Retain the maximum payload value of the message. If the payload value exceeds the maximum value, the EMQX will treat the retained reserved message as a normal message.|
|Message Expire Interval||Duration||Never Expire||The expiration time of the retained message, and 0 means never expire. If the message expiration interval is set in the PUBLISH packet, the message expiration interval in the PUBLISH packet shall prevail.|
|Message Clear Interval||Duration||Disabled||Interval to clean up expired messages.|
|Deliver Rate||Integer||≥ 0||1000||The maximum rate of delivering retain messages.|