How Can I Efficiently Handle Concurrent Processing of a SQL Server Queue Table?

Concurrent Processing from a SQL Server Queue

When using a table as a queue, configuring it for concurrent processing by multiple clients can be challenging. Without proper configuration, multiple workers may attempt to process the same row, resulting in contention.

Optimistic Concurrency Control:

The query provided initially attempts to obtain a pessimistic lock on the row to be processed, allowing only one worker to access it. However, this approach can lead to extensive waiting and reduced concurrency.

Pessimistic Concurrency Control with Output Clause:

SQL Server 2005 introduced the OUTPUT clause, which allows for more efficient pessimistic concurrency:

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

This query finds the first unprocessed row using the READPAST hint, ignoring any locked rows. It then updates the row, setting PROCESSED to 1. The OUTPUT clause returns the updated row, which is processed by the client.

Table Configuration:

To optimize this approach, the table should be structured with the PROCESSED column as the first column of the clustered index. This ensures that rows are ordered by processing status.

Clustered and Non-Clustered Indexes:

While a non-clustered index on the ID column may be considered, it is generally not recommended for heavily used queues. This is because non-clustered indexes can introduce additional overhead and complexity.

Query Restrictions:

For optimal performance, querying the queue table should be limited to the Dequeue operation, avoiding Peek operations or using the table for both queueing and data storage purposes.

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