mqtt.go | Yunba Go SDK - Please redirect to yunba-go-sdk | SDK library

As per the comments - the issue was due to another client using the same client id. The easiest way to check for this is to read the broker logs (from the clients perspective the connection is just dropped without warning).

The MQTT spec states:

The Server MUST process a second CONNECT Packet sent from a Client as a protocol violation and disconnect the Client [MQTT-3.1.0-2]. See section 4.8 for information about handling errors.

This is a fairly common issue (and is the first thing mentioned in the common problems section of the project readme).

If you want to have a struct available throughout your code, you might want to use a Singleton Pattern [1].

In Go you basically define an exported global variable in a package which will be available by all code that imports the package.

You can have client live in payload package (or whichever works for you, this is just an example):

You can not just arbitrarily change the domain name if you are just stream proxying, it needs to match the one presented in the certificate by the remote broker or as you have seen it will not validate.

You can force the client to not validate the server name by setting client.tls_insecure_set(True) but this is a VERY bad idea and should only be used for testing and never in production.

From the docs:

As thrashed out in the comments

Messages will only be queued for an offline client that has subscribed at greater than QOS 0

More details can be found here

I realized that when I was creating the project and registry on the Google Console, I actually mistyped the name I was intending (I thought it was "testmqtt" but it was actually "tesmqtt").

So if you're having an issue similar to this, I'd suggest trying the follwing:

• Make sure your you've spelled everything right. Make sure the project title is correct, the registry title, etc. It sounds dumb but these types of mistakes happen, so it doesn't hurt to check them first. Otherwise you'll overthink things like I did.
• Check out this this page for troubleshooting. There's two parts of this troubleshooting page that might really help you. The first is trying to see if you can actually connect to the cloud at all. You can test if you're able to make a connection by issuing a command like openssl s_client -connect mqtt.googleapis.com:8883 on the command line. You'll need to have openssl downloaded in order to issue that command, however. You can see the page I just linked for more details about testing your connection. The second thing you can do is check to see if you have authentication by running a gcloud command using Google's sdk. The troubleshooting page I linked also has more details in this regard.
• This quickstart guide is also particularly helpful. It might be confusing to navigate at first but understanding it will be your best bet.
• Google's github repository also has some good examples of how to make an mqtt connection to the cloud IoT core.

DavidC in the comments below helped me find some of these helpful links, so credit should go to him.

paho.mqtt.golang only supports MQTT 3/3.1. If you want properties, which were introduced in v5, take a look at paho.golang which is a total rewrite that supports MQTT v5 (and v5 only). Support for properties is demonstrated in the chat example:

To publish at QOS 2 is a multi step process so the most likely reason is that you are disconnecting the publishing client before all the messages are actually finishing publishing to the broker. You should probably do that publishing in a loop and using the returned token from the call to client.publish() to wait until it has completed before disconnecting the client.

e.g. as shown in the example:

Publish is defined as follows: