# Defining Commands

A command describes what someone wants to do. Define it as a struct and implement two functions: `Subject`, which returns the subject the command acts on, and `Preconditions`, which returns the conditions under which its events may be written.

This makes the struct a `Command`:

```go
type AcquireBook struct {
  BookID string
  Title  string
  Author string
  ISBN   string
}

func (c AcquireBook) Subject() string {
  return "/books/" + c.BookID
}

func (c AcquireBook) Preconditions() []architecturekit.Precondition {
  return []architecturekit.Precondition{
    architecturekit.OnStateRead(),
  }
}

type BorrowBook struct {
  BookID        string
  ReaderID      string
  BorrowedUntil string
}

func (c BorrowBook) Subject() string {
  return "/books/" + c.BookID
}

func (c BorrowBook) Preconditions() []architecturekit.Precondition {
  return []architecturekit.Precondition{
    architecturekit.OnStateRead(),
  }
}

type ReturnBook struct {
  BookID string
}

func (c ReturnBook) Subject() string {
  return "/books/" + c.BookID
}

func (c ReturnBook) Preconditions() []architecturekit.Precondition {
  return []architecturekit.Precondition{
    architecturekit.OnStateRead(),
  }
}
```

`OnStateRead` makes sure that the events are only written if the state the command is decided on still holds. For the other preconditions, see **[Using Preconditions](/docs/architecturekit/v1.0/using-preconditions)**.