Why should you care?

Users see spinners; DBAs see head blockers. Finding the blocker is step one.

See it live — copy this example

Run in SQL Server Management Studio (SSMS) or Azure Data Studio.

-- Window A:
USE DataVerse;
BEGIN TRAN;
UPDATE dbo.Accounts SET Balance = Balance WHERE AccountId = 1;
-- do not commit yet

-- Window B:
USE DataVerse;
UPDATE dbo.Accounts SET Balance = Balance WHERE AccountId = 1;
-- waits

-- Investigate:
SELECT session_id, blocking_session_id, wait_type, wait_time
FROM sys.dm_exec_requests
WHERE blocking_session_id <> 0;

What happened?

• Window A holds the exclusive lock; B waits.
• dm_exec_requests shows who blocks whom.
• Commit or rollback A to free B.

Practice next

1. Reproduce with two windows.
2. Run the blocking query.
3. Commit A and confirm B finishes.
4. Set LOCK_TIMEOUT 3000 in B and watch error 1222.
5. Enable RCSI to reduce reader/writer blocking (lab).

Remember

Blocking = lock waits. Find blocking_session_id. Shorten or reschedule the owner work.

Black Friday checkout stall

A stuck DataVerse admin transaction blocked stock updates.

Outcome: Ops killed the blocker after confirming it was idle.