Tutorial: Republishing an MQTT message

Open the Rules hub of the AWS IoT console.

In Rules, choose Create and start creating your new rule.

In the top part of Create a rule:

1. In Name, enter the rule's name. For this tutorial, name it republish_temp.

   Remember that a rule name must be unique within your Account and Region, and it can't have any spaces. We've used an underscore character in this name to separate the two words in the rule's name.

2. In Description, describe the rule.

   A meaningful description helps you remember what this rule does and why you created it. The description can be as long as needed, so be as detailed as possible.

In Rule query statement of Create a rule:

1. In Using SQL version, select 2016-03-23.

2. In the Rule query statement edit box, enter the statement:

   SELECT topic(2) as device_id, temperature FROM 'device/+/data'

   This statement:

   • Listens for MQTT messages with a topic that matches the device/+/data topic filter.

   • Selects the second element from the topic string and assigns it to the device_id field.

   • Selects the value temperature field from the message payload and assigns it to the temperature field.

In Set one or more actions:

1. To open up the list of rule actions for this rule, choose Add action.

2. In Select an action, choose Republish a message to an AWS IoT topic.

3. At the bottom of the action list, choose Configure action to open the selected action's configuration page.

In Configure action:

1. In Topic, enter device/data/temp. This is the MQTT topic of the message that this rule will publish.

2. In Quality of Service, choose 0 - The message is delivered zero or more times.

3. In Choose or create a role to grant AWS IoT access to perform this action:

   1. Choose Create Role. The Create a new role dialog box opens.

   2. Enter a name that describes the new role. In this tutorial, use republish_role.

      When you create a new role, the correct policies to perform the rule action are created and attached to the new role. If you change the topic of this rule action or use this role in another rule action, you must update the policy for that role to authorize the new topic or action. To update an existing role, choose Update role in this section.

   3. Choose Create Role to create the role and close the dialog box.

4. Choose Add action to add the action to the rule and return to the Create a rule page.

The Republish a message to an AWS IoT topic action is now listed in Set one or more actions.

In the new action's tile, below Republish a message to an AWS IoT topic, you can see the topic to which your republish action will publish.

This is the only rule action you'll add to this rule.

In Create a rule, scroll down to the bottom and choose Create rule to create the rule and complete this step.

In the MQTT client in the AWS IoT console, subscribe to the input topics, in this case, device/+/data.

1. In the MQTT client, under Subscriptions, choose Subscribe to a topic.

2. In Subscription topic, enter the topic of the input topic filter, device/+/data.

3. Keep the rest of the fields at their default settings.

4. Choose Subscribe to topic.

   In the Subscriptions column, under Publish to a topic, device/+/data appears.

Subscribe to the topic that your rule will publish: device/data/temp.

1. Under Subscriptions, choose Subscribe to a topic again, and in Subscription topic, enter the topic of the republished message, device/data/temp.

2. Keep the rest of the fields at their default settings.

3. Choose Subscribe to topic.

   In the Subscriptions column, under device/+/data, device/data/temp appears.

Publish a message to the input topic with a specific device ID, device/22/data. You can't publish to MQTT topics that contain wildcard characters.

1. In the MQTT client, under Subscriptions, choose Publish to topic.

2. In the Publish field, enter the input topic name, device/22/data.

3. Copy the sample data shown here and, in the edit box below the topic name, paste the sample data.

   {
     "temperature": 28,
     "humidity": 80,
     "barometer": 1013,
     "wind": {
       "velocity": 22,
       "bearing": 255
     }
   }

4. To send your MQTT message, choose Publish to topic.

Review the messages that were sent.

1. In the MQTT client, under Subscriptions, there is a green dot next to the two topics to which you subscribed earlier.

   The green dots indicate that one or more new messages have been received since the last time you looked at them.

2. Under Subscriptions, choose device/+/data to check that the message payload matches what you just published and looks like this:

   {
     "temperature": 28,
     "humidity": 80,
     "barometer": 1013,
     "wind": {
       "velocity": 22,
       "bearing": 255
     }
   }

3. Under Subscriptions, choose device/data/temp to check that your republished message payload looks like this:

   {
     "device_id": "22",  
     "temperature": 28
   }

   Notice that the device_id value is a quoted string and the temperature value is numeric. This is because the topic() function extracted the string from the input message's topic name while the temperature value uses the numeric value from the input message's payload.

   If you want to make the device_id value a numeric value, replace topic(2) in the rule query statement with:

   cast(topic(2) AS DECIMAL)

   Note that casting the topic(2) value to a numeric value will only work if that part of the topic contains only numeric characters.

If you see that the correct message was published to the device/data/temp topic, then your rule worked. See what more you can learn about the Republish rule action in the next section.

If you don't see that the correct message was published to either the device/+/data or device/data/temp topics, check the troubleshooting tips.