Adventures in Machine Learning

Mastering SQL Server Deadlock: Examples and Simulations

SQL Server Deadlock: Understanding the Concurrency ProblemWhen working with databases, concurrency problems can arise that can cause havoc in the system. One of these issues is SQL Server deadlock, which occurs when two or more sessions are blocked, and each session is waiting for a resource locked by the other.

This article will delve deeper into SQL Server deadlock to give you a comprehensive understanding of the problem.

Definition of SQL Server Deadlock

A SQL Server deadlock is a concurrency problem that occurs when two or more sessions lock resources in a way that each session tries to acquire the other’s locked resource, resulting in a never-ending wait. The result is a freeze in the system, and the only solution is for SQL Server to automatically terminate one of the sessions, called the deadlock victim.

Example of Creating a SQL Server Deadlock

To create a deadlock scenario, let’s look at two tables, the invoices table and invoice_items table. Suppose two sessions want to update the same invoice but with different items at the same time.

Session A wants to update item A1 in invoice I1, while session B wants to update item A2 in invoice I1. These sessions lock the respective invoice_items rows that need to be updated.

Now, session A needs to update the invoice I1 row, which session B has locked. At the same time, session B also needs to update the same invoice row I1, which session A has locked.

Here, both sessions are blocking each other, waiting for the other’s resource to be released, resulting in a deadlock.

Locks and Resources in SQL Server Deadlock

Resource Locking in SQL Server Deadlock

Resource locking is the process of preventing concurrent access to shared resources, which creates consistency in the database. In a SQL Server deadlock, two sessions are locking the same resource, resulting in a dependency cycle where each session is blocking the other’s resource.

The two resources in a deadlock are the victim and the proceeding process.

Session Locks in SQL Server Deadlock

When a session locks a resource, it acquires a lock on that resource. SQL Server has four types of locks: shared locks, exclusive locks, update locks, and intent locks.

In a deadlock scenario, one or both sessions have an exclusive or update lock on the locked resource.

Deadlock Victim in SQL Server Deadlock

A deadlock victim is a session that SQL Server terminates automatically to resolve a deadlock scenario. SQL Server considers the terminated session as the victim.

SQL Server chooses the victim based on the least costly victim. The cost of a victim is based on the resources locked by the session and the time it has to complete its transactions.

Conclusion

SQL Server deadlock is a severe concurrency problem that can cause system freezes. It’s essential to understand how it occurs to prevent it from happening.

In this article, we have discussed the definition of SQL Server deadlock and provided an example of creating a deadlock scenario. We’ve also explored resource locking and session locks in SQL Server deadlock and talked about the deadlock victim.

By understanding these concepts, you can proactively design your database and applications to avoid SQL Server deadlock issues. Simulating a SQL Server Deadlock: Creating Tables and Executing Transactions

In our previous article, we discussed how SQL Server deadlock occurs and provided an example of creating a deadlock scenario.

Now, let’s simulate a SQL Server deadlock by creating tables and executing transactions using two sessions.

Creating Tables for SQL Server Deadlock Simulation

We will simulate the scenario by creating two tables, the invoices table and invoice_items table. The invoices table will contain the invoice_id and customer_name columns, while the invoice_items table will contain the item_id, item_name, qty, and invoice_id columns.

To create the tables, execute the following SQL command:

“`

CREATE TABLE dbo.invoices (

invoice_id INT PRIMARY KEY,

customer_name VARCHAR(50)

);

CREATE TABLE dbo.invoice_items (

item_id INT PRIMARY KEY,

item_name VARCHAR(50),

qty INT,

invoice_id INT FOREIGN KEY REFERENCES dbo.invoices(invoice_id)

);

“`

Executing Transactions for SQL Server Deadlock Simulation

Next, we will execute transactions using two different sessions to update the same invoice item, causing a deadlock scenario. Suppose session A wants to update item A1 in invoice I1, while session B wants to update item A2 in invoice I1.

To execute transactions with two sessions, you’ll need to connect to the database twice, creating two different sessions.

Session A

“`

— Connect to the database with session A

USE [DatabaseName];

GO

BEGIN TRANSACTION;

— Lock the invoice item rows that need to be updated

SELECT * FROM dbo.invoice_items

WHERE item_name = ‘A1’ AND invoice_id = ‘I1’

WITH (UPDLOCK, ROWLOCK);

— Wait for a few seconds to simulate session B’s transaction

WAITFOR DELAY ’00:00:05′;

— Update invoice item A1

UPDATE dbo.invoice_items

SET qty = 10

WHERE item_name = ‘A1’ AND invoice_id = ‘I1’;

COMMIT TRANSACTION;

“`

Session B

“`

— Connect to the database with session B

USE [DatabaseName];

GO

BEGIN TRANSACTION;

— Lock the invoice item rows that need to be updated

SELECT * FROM dbo.invoice_items

WHERE item_name = ‘A2’ AND invoice_id = ‘I1’

WITH (UPDLOCK, ROWLOCK);

— Wait for a few seconds to simulate session A’s transaction

WAITFOR DELAY ’00:00:05′;

— Update invoice item A2

UPDATE dbo.invoice_items

SET qty = 20

WHERE item_name = ‘A2’ AND invoice_id = ‘I1’;

COMMIT TRANSACTION;

“`

The two sessions, A and B, are now executing transactions to update different invoice items but in the same invoice, I1. The SELECT statement with UPDLOCK and ROWLOCK locks the rows that need to be updated and holds that lock until the transaction commits.

The WAITFOR DELAY statement simulates the execution delay before starting the update operation.

Deadlock Occurrence

After running the two sessions, you’ll find that the transactions are blocked, waiting for each other to release the locked resources. As both sessions hold locks on the invoice_items table, they are waiting for the other to release the locked rows.

At this point, SQL Server detects the deadlock and chooses one of the sessions to terminate to resolve the deadlock. SQL Server considers the terminated session as the victim and automatically rolls it back.

To check the deadlock occurrence log, you can execute the following SQL statement:

“`

SELECT *

FROM sys.dm_tran_locks

WHERE request_session_id IN (

SELECT blocker_session_id

FROM sys.dm_os_waiting_tasks

WHERE session_id = ‘YourSessionID’

);

“`

Replace ‘YourSessionID’ with the session ID of one of the two sessions you are running. This SQL statement will show the deadlock victim and the transaction details causing the deadlock.

Conclusion

By simulating a SQL Server deadlock, you can understand the impact of concurrent transactions in a database and how sessions are blocked, waiting for each other to release the locked resources. You can also see how SQL Server resolves the deadlock by automatically terminating one of the sessions.

Understanding SQL Server deadlock is a vital step in designing database applications to avoid or mitigate this type of issue. In this article, we discussed SQL Server deadlock, a severe concurrency problem that can cause system freezes.

We talked about the definition of SQL Server deadlock, provided an example of creating a deadlock scenario, and how to simulate it. We also explored resource locking, session locks, and the deadlock victim in SQL Server deadlock.

By understanding these concepts, you can proactively design your database and applications to avoid SQL Server deadlock issues. The key takeaway from this article is that understanding SQL Server deadlock is crucial for maintaining database integrity, and by simulating it, we can gain a deeper understanding of how to avoid or mitigate these types of problems.

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