Number of clients connected to AWS IoT MQTT topic

Question

We developed an iOS application and configured the app to listen to all the messages published on a particular MQTT topic on AWS IoT.

All I want to know is, is there a way where I can pull a list of connected devices who are listening on that topic ? (I checked the cloudWatch logs and I couldn't find the detailed info like the MAC address of the device connected etc).

Also, I want to know if there is limit on the no. of devices that can be subscribed to a topic.

Answer 1

There is no limit to the number of devices on a single topic. One of the major pros for the service - it scales virtually infinitely. Here's a full list of all the AWS IoT Limits.

There's no easy way to query the number of devices connected directly from AWS IoT. But there is a way to detect when any device subscribes and unsubscribes from a topic using the message data from AWS Reserved Topics - specifically these topics below:

$aws/events/subscriptions/subscribed/{clientId}
$aws/events/subscriptions/unsubscribed/{clientId}

You could create an IoT rule to forward all subscribed and unsubscribed events from those reserved topics to S3, DynamoDB or RDS to be queryable. A simple Lambda function to increment on subscribes and decrement on unsubscribes within DynamoDB would work really well too. (DynamoDB Atomic Counter)

Also here's what the message body looks like - from Subscribe/Unsubscribe Events:

{
    "clientId": "186b5",
    "timestamp": 1460065214626,
    "eventType": "subscribed" | "unsubscribed",
    "sessionIdentifier": "00000000-0000-0000-0000-000000000000",
    "principalIdentifier": "000000000000/ABCDEFGHIJKLMNOPQRSTU:some-user/ABCDEFGHIJKLMNOPQRSTU:some-user"
    "topics" : \["foo/bar","device/data","dog/cat"\]
}