How Can SQL Server Efficiently Handle Concurrent Queue Processing Across Multiple Clients?

Using SQL Server as a DB Queue with Concurrent Multiple Client Processing

The capacity to efficiently process commands from a queue concurrently across multiple clients is essential when using SQL Server as a database queue. To achieve this, certain configurations and query optimizations must be implemented.

One approach involves using a pessimistic lock with a query like:

select top 1 COMMAND 
from EXAMPLE_TABLE 
with (UPDLOCK, ROWLOCK) 
where PROCESSED=false;

However, this strategy can lead to contention among multiple workers attempting to acquire the same row lock. A more effective solution is to leverage the OUTPUT clause, introduced in SQL Server 2005.

The OUTPUT clause allows for atomic dequeue operations. It facilitates finding the appropriate row, skipping locked rows, and marking the selected row as dequeued in a single operation:

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

For optimal performance, it's crucial to structure the table with the leftmost clustered index key on the PROCESSED column. If an ID exists as a primary key, it should be moved to the second column in the clustered key. Additionally, it's recommended to avoid using non-clustered indexes over queues.

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

Moreover, it's important to refrain from using the table for peek operations or dual-purpose as both a queue and a store. These practices can result in deadlocks and reduced throughput.

By adhering to the atomic dequeue process, utilizing the READPAST hint, and implementing a properly structured table, it's possible to achieve high throughput under extremely concurrent loads when using SQL Server as a DB queue with multiple clients.

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