Using SQL Server as a Concurrent Queue with Multiple Clients

In a scenario where a table serves as a queue, it's crucial to configure it and query it in a way that allows multiple clients to process queue items concurrently.

When using pessimistic row locking with UPDLOCK and ROWLOCK, only one worker can acquire the lock and process a row. To resolve this issue and enable concurrent processing, consider the following approach:

Queue Implementation Using OUTPUT Clause

The OUTPUT clause provides a mechanism to atomically retrieve and modify a row. Here's how to implement a queue using the OUTPUT clause:

with CTE as (
  SELECT TOP(1) COMMAND, PROCESSED
  FROM TABLE WITH (READPAST)
  WHERE PROCESSED = 0)
UPDATE CTE
  SET PROCESSED = 1
  OUTPUT INSERTED.*;

This query performs the following steps atomically:

1. Uses the WITH (READPAST) hint to skip any locked rows.
2. Selects the top row with PROCESSED=0.
3. Updates the selected row to mark it as processed (PROCESSED=1).
4. Outputs the updated row, which can be used by the worker to process.

Clustered Index Optimization

To optimize performance further, it's crucial to create a clustered index on the PROCESSED column. This ensures that data is stored in order of processing.

CREATE CLUSTERED INDEX cdxTable on TABLE(PROCESSED, ID);

Avoid Non-Standard Queries

For optimal throughput, it's essential to avoid querying the queue table using methods other than the dequeue operation described above. Attempting to peek or use the table for additional purposes can introduce deadlocks and performance degradation.