How I Built Book Buddy's Digital Borrow System

The Problem

Traditional library systems are opaque. You don't know if a book is available until you physically show up. I wanted to eliminate all of that friction with Book Buddy.

Designing the Borrow State Machine

A book can be in one of four states: Available, Borrowed, Reserved, or Overdue. Each transition needs to be atomic to prevent double-borrowing.

async function borrowBook(userId, bookId) {
  const session = await mongoose.startSession()
  session.startTransaction()
  try {
    const book = await Book.findOneAndUpdate(
      { _id: bookId, status: 'available' },
      { status: 'borrowed', borrowedBy: userId, dueDate: getDueDate() },
      { new: true, session }
    )
    if (!book) throw new Error('Book unavailable')
    await session.commitTransaction()
    return book
  } catch (err) {
    await session.abortTransaction()
    throw err
  }
}

The findOneAndUpdate with the status check as a condition is an optimistic lock — if two users try to borrow the same book simultaneously, only one wins.

What I Learned

MongoDB transactions taught me a lot about atomicity. The pattern of checking a precondition inside the update query itself is something I now reach for automatically.

What I'd Do Differently

Add a reservation queue with WebSocket notifications so users get pinged the moment a book becomes available, rather than checking back manually.