Messages & Queues in SQL Server

Enhance performance by using queues in SQL Server

Bio - Niels Berglund

Software Specialist - Derivco Software Specialist - lots of production dev. plus figuring out

ways to "use and abuse" existing and new technologies

Author - "First Look at SQL Server 2005 for Developers" Researcher / Instructor - DevelopMentor Speaker - TechEd, DevWeek, SQL Pass, etc. Longtime user of SQL Server

www.derivco.com niels.it.berglund@gmail.com @nielsberglund

9/14/2013 |

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Derivco

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Background

In certain scenarios beneficial to off-load work to other processes/threads.

In SQL Server we have no real way to control threading Historically difficult to create messaging applications in SQL

Server, due to queuing is highly concurrent. how do we ensure that multiple consumers does not block each other how do we de-queue without causing deadlocks etc.

SQL Server 2005 introduced new functionality which makes it easier to remove a de-queued row and return it to the caller in one atomic statement: DELETE TOP(x) OUTPUT statement

De-queuing old vs. new

--old wayDECLARE @tab TABLE(RowID bigint, MsgTypeID int, Msg varchar(500), EnqueueDate datetime2)INSERT INTO @tabSELECT TOP (1) * FROM dbo.tb_QueueTabClIdx WITH(UPDLOCK, READPAST)DELETE clix FROM dbo.tb_QueueTabClIdx clixJOIN @tab t ON clix.RowID = t.RowID

--new wayDECLARE @tab TABLE(RowID bigint, MsgTypeID int, Msg varchar(500), EnqueueDate datetime2)DELETE TOP(1) dbo.tb_QueueTabClIdx WITH(ROWLOCK, READPAST)OUTPUT deleted.RowID, deleted.MsgTypeID, deleted.Msg, deleted.EnqueueDateINTO @tab(RowID, MsgTypeID, Msg, EnqueueDate)

Queue types

Heap queue backed by heap table heap table is a table with no clustered index cannot guarantee order of en-queue, de-queue useful for specific queuing scenarios beware of side effects (data growth)

Ordered queue (FIFO, LIFO) First In First Out, Last In First Out in LIFO queues beware of hot-spots (inserting, deleting same group of

rows) backed by table with clustered index (can be non-unique or unique) de-queuing has to be done with an ORDER BY clause

SQL Server Service Broker Queue (SSB Queue) Bound queues (more later)

De-queuing ordered queues De-queue has to be done with an ORDER BY clause Unfortunately DELETE does not support ORDER BY By using a CTE we can however do it in a very efficient way

the query will execute as a DELETE, NOT SELECT followed by DELETE, nor as SELECT in context

of a DELETE SELECT automatically taking UPDATE lock

;WITH delCte AS ( SELECT TOP(1) RowID, Msg, EnqueueDate FROM dbo.tb_QueueTabClIdx WITH(ROWLOCK, READPAST) ORDER BY RowId)DELETE delCte OUTPUT deleted.*

SSB queues

SSB allows internal or external processes to send and receive guaranteed, asynchronous messages by using extensions to DML

It has the concept of queues, meant for use by SSB When using it in a "normal" queuing application, it has several

short-comings: convoluted way to en-queue messages rigid structure performance

One big plus though: Activation stored procedure that automatically is activated when a message is

en-queued on the SSB queue. can control concurrency

Performance I

Multiple producers running concurrently One consumer running in parallel Each producer en-queuing one record

running in a tight loop, 10,000 times

Consumer started together with the producers, de-queuing until no more data.

Performance II

Heap table Clustered index

Clustered unique index

Clustered primary

key

SSB0

200

400

600

800

1000

1200

En-queue (Msgs/Sec)De-queue (Msgs/Sec)

Bound queues

High volume inserts/deletes causes problems in a db: heap table unlimited growth statistics are never up to date auto updates of statistics constantly kicking in, creating

jagged CPU patterns and throughput new pages constantly allocated, de-allocated - stresses

internal allocation structures

Quite a few high performing queuing applications are using a bound queue in a ring-buffer scenario minimizes memory allocation, de-allocation

Table backing ring-buffer

CREATE TABLE dbo.tb_QueueTabRingBuffer ( MsgSlot BIGINT NOT NULL , MessageID BIGINT NULL , MsgTypeID int NULL , EnqueueDate datetime2 NOT NULL , Msg char(500) NOT NULL , ReferenceCount TINYINT NOT NULL )

Using a ring-buffer like table

After creation of table, prefill with data (keep track of how many rows inserted)

To enqueue: create a new message id find the next available slot update the message column and add one to the reference count

To de-queue find the smallest message id that has not yet been de-queued. read and return the message column decrement reference count, marking the slot as available.

We need to be able to extremely efficiently locate the next available slot

For this we use a sequence object

Sequence object

A high scale data structure to generate "next number" quite a few "neat" features

Introduced in SQL Server 2012 When used in conjunction with the modulo operator (%)

we can efficiently get next slot etc.

CREATE SEQUENCE dbo.seq_Test AS BIGINT START WITH 1 INCREMENT BY 1 CACHE 5000;GO

SELECT NEXT VALUE FOR dbo.seq_Test;

En-queuing

When en-queuing into a ring-buffer, the en-queue may block if no available slots.

Need to handle that in en-queue code (not shown below)DECLARE @enqueuSequence bigint;DECLARE @slot int;DECLARE @QueueSize int = 5000;DECLARE @msg char(500) = 'this is a new message';

--grab the next value, and slotSET @enqueuSequence = NEXT VALUE FOR dbo.seq_Enqueue;SET @Slot = @enqueuSequence % @QueueSize

--do the updateUPDATE dbo.tb_QueueTabRingBuffer WITH(ROWLOCK)SET EnqueueDate = SYSDATETIME(), Msg = @Msg, MessageID = @enqueuSequence, MsgTypeID = 2, ReferenceCount = 1 WHERE MsgSlot = @Slot AND ReferenceCount = 0

De-queuing

De-queuing is similar to en-queue. Need however to ensure that the de-queue sequence does not out of synch with the en-queue.

DECLARE @dequeuSequence bigint;DECLARE @slot int;DECLARE @queueSize int = 5000;

--grab the next MessageID, and slotSET @dequeuSequence = NEXT VALUE FOR dbo.seq_Dequeue;SET @Slot = @dequeuSequence % @QueueSize

UPDATE dbo.tb_QueueTabRingBuffer WITH(ROWLOCK)SET EnqueueDate = SYSDATETIME(), MessageID = NULL, MsgTypeID = NULL, Msg = 'dummy', ReferenceCount = 0OUTPUT deleted.*WHERE MsgSlot = @Slot AND MessageID = @dequeuSequence AND ReferenceCount = 1;

Performance

Ring buffer Heap table Clustered index Clustered unique index

Clustered primary key

0

200

400

600

800

1000

1200

1400

En-queue (Msgs/Sec)De-queue (Msgs/Sec)

Summary

Implementing efficient queuing in SQL Server is definitely doable SQL Server 2008R2

the best table structure for overall performance is a queue backed by clustered non-unique index

for certain specialized scenarios a heap-table can come into play (beware of data growth)

SQL Server 2012 a ring-buffer like structure using sequence objects are the absolutely

best performing table structure beware of more convoluted code to keep the en-queue and de-queue

in synch

SQL Server Service Broker too convoluted, not enough performance activation procedures its strong point