There are a lot of old posts about multiplayer implementations on SE, so i'm looking for an up-to-date answer. What is the best server-client implementation of real-time multiplayer now? I have an Amazon MYSQL Server hosted by digitalOcean, I've made a REST web service that communicates with the Server and Android app, and I have my Android app running receiving and sending data.
I want to add a multiplayer minigame to what I have right now where 2 players would see the same screen and one user would receive location data (which i'm already storing in the server and sending to all devices with the app) from the other and would have an interface to interact with the data they're receiving (like building towers around the location and deploying units). The computer AI would need to be synced between both devices.
The matches would last maybe 3 minutes tops and the location data would have to be streamed constantly as well as the interaction from the other user. The computer AI would be streamed to both.
I've done a lot of research on the topic, and most of my experience was from a bluetooth P2P game I created in 2011 using sockets. So what are the most common implementations today? What are the most common libraries? I'm currently using Android Studio for the app and Notepad++ for the PHP webservice.
EDIT: I'm fine with answers promoting their own products, but explain how your product is better than Google's Game Services (which appears to be the best choice right now). I'm really looking for a way to implement the multiplayer with the server I'm already using.