Transaction (Process ID) was deadlocked

 The error message "Transaction (Process ID) was deadlocked" typically occurs in a database system, such as SQL Server, when two or more transactions are trying to access the same resources simultaneously, but in a way that leads to a deadlock. This means each transaction is waiting for the other to release a resource, creating a situation where none of them can proceed.

Here are steps to troubleshoot and resolve this issue:

1. Identify the Deadlock:

• SQL Server Profiler: Use SQL Server Profiler to trace deadlocks. You can configure it to capture deadlock graphs.
• Extended Events: Use Extended Events in SQL Server to capture deadlock information. This can be more efficient than using the Profiler.
• System Health Session: SQL Server includes a default system health session that captures deadlock information. You can query it to find details about deadlocks.

2. Analyze the Deadlock Graph:

• Look at the deadlock graph to understand which transactions and resources are involved.
• Determine the sequence of locks that lead to the deadlock.

3. Review and Optimize Queries:

• Indexes: Ensure proper indexing to reduce lock contention.
• Query Design: Optimize queries to be more efficient and reduce the duration of locks.
• Transaction Scope: Minimize the scope and duration of transactions to lower the likelihood of deadlocks.

4. Lock Hints and Isolation Levels:

• Lock Hints: Use lock hints (e.g., NOLOCK, ROWLOCK) where appropriate to control locking behavior.
• Isolation Levels: Adjust transaction isolation levels. For instance, using READ COMMITTED SNAPSHOT can reduce locking and blocking.

5. Retry Logic:

• Implement retry logic in your application to handle deadlocks gracefully. When a deadlock occurs, retry the transaction after a short delay.

6. Database Design:

• Ensure the database schema is designed to minimize locking conflicts. For example, properly normalizing tables and using appropriate data types.

Example: Implementing Retry Logic in T-SQL

DECLARE @retry INT = 0;
DECLARE @max_retries INT = 3;
DECLARE @delay INT = 500; -- milliseconds

WHILE @retry < @max_retries
BEGIN
    BEGIN TRY
        BEGIN TRANSACTION;

        -- Your SQL operations here

        COMMIT TRANSACTION;
        BREAK; -- Exit the loop if the transaction is successful
    END TRY
    BEGIN CATCH
        IF ERROR_NUMBER() = 1205 -- Deadlock error number
        BEGIN
            SET @retry = @retry + 1;
            IF @retry < @max_retries
            BEGIN
                WAITFOR DELAY '00:00:00.500'; -- Wait for 500 milliseconds before retrying
            END
            ELSE
            BEGIN
                THROW; -- Raise the error if max retries reached
            END
        END
        ELSE
        BEGIN
            THROW; -- Raise the error if it's not a deadlock
        END
    END CATCH
END

Example: Adjusting Isolation Levels

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
BEGIN TRANSACTION;

-- Your SQL operations here

COMMIT TRANSACTION;

Example: Using Extended Events to Capture Deadlocks

CREATE EVENT SESSION [CaptureDeadlocks] ON SERVER
ADD EVENT sqlserver.lock_deadlock
ADD TARGET package0.event_file(SET filename=N'C:\Path\To\Your\File\DeadlockCapture.xel', max_file_size=(5))
WITH (MAX_MEMORY=4096 KB, EVENT_RETENTION_MODE=ALLOW_SINGLE_EVENT_LOSS, MAX_DISPATCH_LATENCY=30 SECONDS, MAX_EVENT_SIZE=0 KB, MEMORY_PARTITION_MODE=NONE, TRACK_CAUSALITY=OFF, STARTUP_STATE=OFF);
GO

ALTER EVENT SESSION [CaptureDeadlocks] ON SERVER STATE = START;
GO

By following these steps and using the appropriate tools and techniques, you can effectively troubleshoot and resolve deadlock issues in your database system.