Deadlocks are one of the trickiest issues in databases. It is a situation where two or more operations put a stop to each other from going through. It’s like two people trying to cross a narrow space at the same time. Since both are unable to move, they are basically stuck there. That’s a database deadlock. 

Knowing what a deadlock is and how to steer clear of it can be very helpful. When you're aware of the reasons, you can put measures in place to stop them from happening again. In this blog, we will discuss how to avoid deadlock in database.

What are Deadlocks in Database

Deadlocks are situations that are often brought about by multiple queries that want to access the same resources simultaneously. Each query has control over a piece of data which the other query needs. If none of the queries can continue, they are in a wait state which can only be solved by the system.

Azure SQL Database has Read Committed Snapshot Isolation (RCSI) turned on by default. This allows queries that read data to run without interfering with queries that modify data. However, even with RCSI, deadlocks can still occur, especially when:

1. Queries run under isolation levels that increase blocking.
2. RCSI is disabled, and SELECT statements acquire shared locks.
3. Multiple transactions access resources in different orders.

An Example Scenario

Picture two sessions running updates:

• Session A updates a product table and holds a lock on a specific row.

• Session B updates a related description table but needs access to the row locked by Session A.

Session A now tries to update the description table but is blocked by Session B.

Neither session can continue until the system intervenes. Azure SQL Database’s deadlock monitor steps in and chooses one of the transactions as a victim. The chosen transaction is rolled back, while the other completes. The rolled-back transaction can then be retried.

Default Isolation and Its Role

Isolation levels control how data is accessed in concurrent transactions. RCSI changes how the read committed isolation level works. With RCSI enabled:

1. Reading data doesn’t block writes
2. Writing data doesn’t block reads

Snapshot isolation is also available and provides full transaction-level consistency using row versions.

Recognizing Deadlocks

When a deadlock occurs, the system emits a deadlock event. Applications see error 1205. This is exactly where setting up alerts can prove to be a good practice. These alerts allow database teams to investigate patterns and identify recurring problems. This way, you can plan prevention strategies. Monitoring deadlocks is essential to maintain performance and prevent repeated disruptions.

Preventing Deadlocks

Deadlocks can often be avoided with proper database design and query strategies. Here are some practical methods on how to avoid deadlock in database:

Index Tuning

Indexes help queries find data faster. Faster queries hold locks for shorter periods, reducing the chances of blocking others. Focus on:

• Non clustered indexes for frequently accessed columns.
• Clustered indexes are used to organize tables efficiently.
• Avoiding excessive indexing, which adds overhead and can slow down writes.

Even adjusting indexed views can make a difference. Sometimes dropping unused indexed views reduces extra locking during updates.

Query and Transaction Design

How queries are written affects deadlocks. Consider:

1. Accessing objects in a consistent order across transactions.
2. Breaking large transactions into smaller ones to reduce lock duration.
3. Avoiding user interaction during transactions. Waiting for input can hold locks for too long.
4. Using snapshot isolation for certain SELECT queries can reduce locking requirements, especially when read committed snapshot is disabled.

Forcing Plans in Query Store

Deadlocks sometimes occur only when the database engine chooses specific execution plans. Query Store allows you to force a strategy that avoids deadlocks without changing the query code. This is a safer approach than rewriting queries when you just need a different plan.

Adjusting Deadlock Priorities

When one transaction is more critical, you can adjust its deadlock priority. This doesn’t prevent deadlocks, but it makes the most important transaction less likely to be rolled back.

Monitoring Deadlocks

Azure SQL Database provides tools to analyze deadlocks. Deadlock graphs show:

• The victim transaction.
• All processes that were involved.
• The resources they own and wait for.

Using these graphs, database teams can identify patterns. For example:

1. Table scans that lock many rows.
2. Foreign key constraints that require multiple checks.
3. Lock hints that increase the number of locks.

Understanding these patterns allows targeted changes, like adding indexes or rewriting queries to reduce conflict.

Best Practices Summary

To keep deadlocks under control:

• Tune indexes regularly.
• Keep transactions short and consistent.
• Monitor for recurring deadlocks.
• Use snapshot isolation for read-heavy operations.
• Adjust priorities if needed.
• Consider using Query Store to enforce safer execution plans.

Conclusion

A common question for many database operators is: what deadlocks are in the database, and how can they be reduced? Deadlocks can happen naturally in any busy database. They are problematic, but reducing and preventing them is very simple. By tuning indexes, carefully designing transactions, and regularly monitoring, you can reduce their impact. If you check the database regularly and make adjustments, deadlocks can be easily prevented. 

GeoPITS specializes in helping businesses optimize databases and prevent deadlocks. Their team ensures your Azure SQL Database or SQL database in Fabric runs smoothly, with minimal downtime and better performance. Contact GeoPITS today to make your database faster, safer, and deadlock-free.

FAQs

1. What is a database deadlock?

A deadlock happens when two or more transactions block each other from accessing resources. Neither can proceed until the system rolls back one of them.

2. How to avoid deadlock in a database?

Deadlocks are eliminable by means of short transactions, consistent ordering of locked tables and avoidance of long waits. In case these means are not sufficient, using indexes in order to lessen scans, dividing big operations into smaller ones and automatic retry of failed transactions will be helpful.

3. Can deadlocks be prevented completely?

No system can guarantee zero deadlocks, but careful indexing can help you out a lot. You can even invest in transaction design and monitoring to drastically reduce their frequency and impact.