Looking for real-time message solution recommendations

If this is not the right subreddit, I’m looking for the right subreddit. Please and thank you!

I’m working on a simple pub/sub(?), or notification/acknowledgment mobile app. I’m trying to design, or drop in a solution to handle messaging. It needs to be real-time.

The idea is basically: “I’m starting now,” and the recipient only needs to “acknowledge” the receipt (that they’ve seen it, and are actively responding as such.)

It should be able to scale up to several hundreds of users or more. I’ve recently seen Scaledrone, and am wondering what else I might be able to leverage. Alternatives.

I’m considering aws sns (edited: not sqs) as well, but I’ve not designed a system like this before. Thanks in advance!!