# How to define an event sourced command?

PREREQUISITE

Define a domain module
Define a stream module
Before defining the command, the event stream has to be defined in a .kstream file and the module dependency of the stream on command must be added.

Open .kdomain file, first define a command-set along with the association to the event-stream.

For example to define a command create lottery which will raise events defined in event-stream LotteryStream:
```
command-set[LotteryStream] Lottery{ ... }
```

Define the command inside the command-set with the following details:

specify the input (if any),
define one or more pre-conditions(if any)
specify the events that this command would raise(if any)
specify the output (if any)
define one or more post-condition (if any)

command-set Lottery {
  command create{
      input(lotteryName:String,amount:Int)
      pre{
        condition AmountMustBeGreaterThanZero => `input.amount>0` failing "Amount must be greater than 0"
      }
      event-raised(created:LotteryCreated)
      output(result:String)
      post{
       condition lotteryCreated => ...
      }
    }
}

In order to implement the command logic follow steps listed here.