# Defining Commands

A command describes what someone wants to do. Define it as a struct and implement the `Command` trait with two methods: `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`:

```rust
#[derive(Clone)]
struct AcquireBook {
  book: Subject,
  title: String,
  author: String,
  isbn: String,
}

impl Command for AcquireBook {
  fn subject(&self) -> Subject {
    self.book.clone()
  }

  fn preconditions(&self) -> Vec<Precondition> {
    vec![Precondition::on_state_read()]
  }
}

#[derive(Clone)]
struct BorrowBook {
  book: Subject,
  reader_id: String,
  borrowed_until: String,
}

impl Command for BorrowBook {
  fn subject(&self) -> Subject {
    self.book.clone()
  }

  fn preconditions(&self) -> Vec<Precondition> {
    vec![Precondition::on_state_read()]
  }
}

#[derive(Clone)]
struct ReturnBook {
  book: Subject,
}

impl Command for ReturnBook {
  fn subject(&self) -> Subject {
    self.book.clone()
  }

  fn preconditions(&self) -> Vec<Precondition> {
    vec![Precondition::on_state_read()]
  }
}
```

`on_state_read` 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/rust/using-preconditions)**.

The subject is a `Subject`, which is a path that EventSourcingDB accepts, such as `/books/42`. **A value that is no such path can not be a `Subject` at all**, so the command holds its subject as one, and a value that can not be part of a subject is refused when the command is built, not when it is executed. `Subject::new` panics for such a value, which fits a subject written in the code, while `parse` and `SubjectScheme::try_build` refuse it with an error, which fits one that comes from a request (see **[Composing Subjects](/docs/architecturekit/rust/composing-subjects)**).

*Note that a command derives `Clone`, since executing it may decide on it once more after a conflict, which takes the command anew (see **[Guarding Against Concurrent Changes](/docs/architecturekit/rust/using-preconditions#guarding-against-concurrent-changes)**).*