Hello Soham Actually it depends on which topic is getting the msg first. But if somehow both of your topics are getting msg at the same time then the callback of 1st object will be called first because it will be placed first in the queue callback of ros spin. But to avoid the case when sub2 topic is getting msg first, then you can guard the function of sub2 with variable update form sub1. This will ensure that once the variable gets updated from sub1 then only it will execute sub2. And if you are not using any parallel executors or threads your callbacks will always work in sequence one by one depending on the frequency of message.

@fusybots currently I'm guarding sub2 .

Nice!

Thanks