var (
	// ErrNotExist is the error returned when specified id does
	// not exist.
	ErrNotExist = errors.New("does not exist")
	// ErrAlreadyExist is the error returned when specified id already
	// exists.
	ErrAlreadyExist = errors.New("already exists")
)

type MetadataStore interface {
	// Create the metadata containing the passed in data with the
	// specified id.
	// Note:
	// * Create MUST return error if the id already exists.
	// * The id and data MUST be added in one transaction to the store.
	Create(string, []byte) error
	// Get the data by id.
	// Note that Get MUST return ErrNotExist if the id doesn't exist.
	Get(string) ([]byte, error)
	// Update the data by id.
	// Note:
	// * Update MUST return ErrNotExist is the id doesn't exist.
	// * The update MUST be applied in one transaction.
	Update(string, UpdateFunc) error
	// List returns entire array of data from the store.
	List() ([][]byte, error)
	// Delete the data by id.
	// Note:
	// * Delete should be idempotent, it MUST not return error if the id
	// doesn't exist or has been removed.
	// * The id and data MUST be deleted in one transaction.
	Delete(string) error
}

type UpdateFunc func([]byte) ([]byte, error)