Using one module local variable in another module in terraform

I am trying to access one module variable in another new module to get aws instance ids which are created in that module and use them in a module Cloud watch alerts which creates alarm in those instance ids . The structure is something like the below

**Amodule #here this is used for creating kafka aws instances*
 
     main.tf
     kafkainstancevariables.tf

Bmodule #here this is used for creating alarms in those kafka instances

     main.tf
     cloudwatchalertsforkafkainstancesVariables.tf

Outside modules terraform mainfile from where all modules are called main.tf variables.tf***

How to access the variables created in Amodule in Bmodule?

You can use outputs to accomplish this. In your kafka module, you could define an output that looks something like this:

output "instance_ids" {
  value = ["${aws_instance.kafka.*.id}"]
}

In another terraform file, let's assume you instantiated the module with something like:

module "kafka" {
  source = "./modules/kafka"
}

You can then access that output as follows:

instances = ["${module.kafka.instance_ids}"]

If your modules are isolated from each other (i.e. your cloudwatch module doesn't instantiate your kafka module), you can pass the outputs as variables between modules:

module "kafka" {
  source = "./modules/kafka"
}

module "cloudwatch" {
  source = "./modules/cloudwatch"
  instances = ["${module.kafka.instance_ids}"]
}

Of course, your "cloudwatch" module would have to declare the instances variable.

See https://www.terraform.io/docs/modules/usage.html#outputs for more information on using outputs in modules.