💀 What is Deadlock?

A Deadlock is a condition in which two or more transactions are waiting for each other to release the lock on the data they need. This causes the transaction process to get "stuck" 🚫 and prevents any transaction from continuing.

📝 Example

Imagine there are two bank accounts, A and B, both with sufficient balance to make transfers. Here’s how a deadlock might happen:

1. Transaction 1:

   • 👤 User A wants to transfer money to account B.
   • 🔒 Transaction 1 acquires a lock on account A.
   • 🔄 Transaction 1 then tries to acquire a lock on account B, but B is already locked by Transaction 2.
   • ⏳ Transaction 1 must wait until Transaction 2 releases the lock on B.

2. Transaction 2:

   • 👤 User B wants to transfer money to account A.
   • 🔒 Transaction 2 acquires a lock on account B.
   • 🔄 Transaction 2 then tries to acquire a lock on account A, but A is already locked by Transaction 1.
   • ⏳ Transaction 2 must wait until Transaction 1 releases the lock on A.

⚠️ Deadlock Situation

• Both transactions wait for each other: Transaction 1 waits for Transaction 2 to release the lock on B, while Transaction 2 waits for Transaction 1 to release the lock on A.
• No progress: Because both transactions are waiting, neither can complete. The system appears to be in a "stuck" 💫 deadlock state.

❓ Why Does Deadlock Happen?

• Different order of data access: When transactions access data in different sequences, conflicts over locks can arise.
• Lack of deadlock prevention mechanisms: If the database lacks the ability to detect or prevent deadlocks, this condition can easily occur.

🔍 How To Prevent Deadlock?

• Assign a lock sequence: Define a strict order for acquiring locks on resources. For example, always lock accounts with smaller IDs first 🔢.
• Use timeout: Set a timeout ⏰ for waiting on locks. If a transaction doesn’t acquire a lock within the allotted time, it’s canceled and can retry later.
• Detect and resolve deadlocks: Enable the database system to automatically detect deadlock conditions and choose one transaction to cancel so others can proceed.

🚀 The Importance of Preventing Deadlock

Deadlocks can degrade the database’s performance 📉 and even lead to inconsistent data. Understanding the deadlock mechanism and applying preventive strategies is essential for reliable database management.