Error Code 1205 in Azure SQL Database

Error Code 1205 occurs in Azure SQL Database when a deadlock is detected. Deadlocks happen when two or more transactions are waiting on resources that the other holds, leading to a cycle of waiting that can only be resolved by terminating one of the transactions.

Summary Table

Background

Error Code 1205, commonly known as a deadlock error, occurs when two or more SQL processes are blocking each other by holding locks and waiting on resources the other process has already locked. SQL Server detects the deadlock and aborts one of the transactions to release the resources and allow the other transaction to continue.

Error Explanation

The error message associated with Error Code 1205 reads:

Error 1205: “Transaction (Process ID) was deadlocked on resources with another process and has been chosen as the deadlock victim. Rerun the transaction.”

This error means that SQL Server detected a deadlock, chose one transaction to terminate as the “victim” to break the deadlock, and prompted the user to re-run the affected transaction.

Common Causes

1. Simultaneous Access to the Same Resources: When transactions access resources in different orders, they can end up waiting on each other, resulting in a deadlock.
2. Long-running Transactions: Extended transactions keep locks active for longer, increasing the chances of a deadlock.
3. High Concurrency: A large number of concurrent transactions can increase lock contention.
4. Inefficient Queries: Poorly optimized queries might lock large data portions, increasing the risk of conflicts.

Steps to Troubleshoot and Resolve Error Code 1205

Step 1: Identify the Deadlock

Purpose: Locate the transaction causing the deadlock.

Example: Use Azure SQL Database’s Query Store or Query Performance Insights to identify deadlock details, including affected resources and SQL statements involved.

Step 2: Analyze the Deadlock Graph

Purpose: Understand the locking pattern that caused the deadlock.

Example:

-- Deadlock graph shows details like transaction IDs and wait resources.

Explanation: The graph highlights wait types, resources, and processes involved in the deadlock, aiding in identifying a solution.

Step 3: Shorten Transaction Duration

Purpose: Reduce lock-hold times to minimize deadlock likelihood.

Example: Simplify long-running queries or split complex transactions into smaller ones.

Step 4: Use a Consistent Locking Order Across Transactions

Purpose: Prevent circular waits by ensuring all transactions lock resources in a consistent order.

Example:

BEGIN TRANSACTION
-- Lock Resource A, then B for both Transaction A and Transaction B

Step 5: Use Aggregation Functions (if applicable)

Purpose: Use aggregation functions to limit the subquery result to a single value.

Example:

SELECT *
FROM Employees
WHERE DepartmentID = (SELECT MAX(DepartmentID) FROM Departments WHERE Location = 'New York');

Explanation: MAX or other functions can reduce multiple values to a single row, preventing deadlocks.

Step 6: Implement Retry Logic in Applications

Purpose: Automatically re-run deadlocked transactions.

Example:

int retryCount = 3;
while (retryCount > 0)
{
    try
    {
        // Execute SQL transaction
        break;
    }
    catch (SqlException ex) when (ex.Number == 1205)
    {
        retryCount--;
        if (retryCount == 0)
            throw; // rethrow if max retries are reached
    }
}

Workarounds

1. Consistent Locking Order: Ensure that all transactions acquire locks in the same order.
2. Shorten Transactions: Minimize lock duration by reducing transaction size.
3. Use Retry Logic: Re-attempt transactions automatically in the application.

Solutions

1. Rewrite Queries to Shorten Transaction Durations: Ensure queries are optimized to reduce lock duration.
2. Implement Retry Logic: Add retry mechanisms to handle deadlock retries automatically.
3. Apply Appropriate Indexing: Add indexes to reduce lock contention on frequently accessed columns.

Example Scenario

Suppose a transaction tries to update records in the Customer table and then Order table, while another transaction tries to update records in the Order table followed by the Customer table.

To resolve:

1. Consistent Order of Locks:
   • Re-order transactions to access tables in the same sequence to avoid circular waiting.
2. Retry Logic:
   • Add retry logic to the application, which will automatically retry transactions that encounter a deadlock error.
3. Indexing:
   • Add indexes on CustomerID and OrderID to minimize lock contention during queries.

By following these troubleshooting steps, you can effectively handle and resolve Error Code 1205 in Azure SQL Database.