Two nodemcu + relay + sensor

Hi everybody...
Two nodemcu are comunicating through Access point and Station mode without any problem using library available on IDE.

Next step is:

Server has a LDR sensor and once it reach a specific reading it should trigger a relay on the client.
My questions are:

  • How can my server with sensor can send command to client to trigger the relay?
    I have seen some examples where the client has a sensor attached and send get request to server (nodmcu with relay). I can't use this configuration bcz in the future I'll have more nodemcu with relay (as client) connected to my server.
    Could you guys point me out the best solution for this project without using MQTT?
    Please I am not asking for any code just give me some direction.

Many thanks

what code is the server running?
I would run a TCP or UDP server on one machine which communicated with the slave sensding commands to control it

Wifiaccesspoint.ino is the firmware running at the server.

Tks for your time.